Can You Modify A Car To Save Fuel? - Fifth Gear 

Anyone who has ever ridden a bicycle on wet roads knows it takes more energy to ride on wet road than dry. The pumping action to push the water aside isn't free.

The Fifth Gear technicians were wrong. Rain dramatically increases your fuel use. The density of water is like 1000 times higher than air. You are constantly pushing against dense stuff in the rain. Let alone the tyres pushing the dense stuff away from your grippy parts and splashing it meters high into the air. Lastly the windscreen wipers take a tiny bit of extra energy.

“Fuel prices have risen by record levels this year” *2022: 😏*

"We're giving away free money today." "Oh great!" "Here's 50p" "-_-"

Couple things: 1) Pumping the tank and putting in exactly 1 gallon does not take into consideration the amount of fuel you missed siphoning or the fuel left in the lines. 2) Lapping a track to negate the effects of wind does not work because drag increases exponentially with speed instead of linearly. 3) Rain increases drag on the car both in terms of modifying the aerodynamic shape of the car as well as the momentum transfer when the car hits each of those rain drops. 4) A wet surface reduces static friction, which you are not taking into consideration at all with MPG runs. You're interested in rolling resistance with MPG runs. 5) Added electrical usage (running the wipers and headlights) increases the resistance on the alternator. Similarly, if you happened to have the defog on, that would run the A/C compressor. 6) Advancing the timing only really helps in high throttle applications when you have a really big, long explosion. When you have just a little explosion just to maintain speed, you want it where the crankshaft is at the most efficient spot, which is at 90 degrees from TDC. You also increase the risk of pinging and blowing a hole in the motor. That was not scientific at all and, frankly, a very poor test.

If this were scientific the mods would have been done one at a time. It is likely that the new chip's profile was not optimised for driving steadily at 60 and so masked any benefit of other mods

They didn't take the fuel left in the fuel lines into account in the first run. They only vacuumed the fuel left in the fuel tank but ignored the rest of it before setting the base figures. Then they drove the car until it cuts out after using all the fuel left in the tank (1 gal), plus the amount left in the fuel lines; which was not present when they made the refill before the second run. That amount of fuel could easily help them make more miles which could affect the final score.

Skip to 2:30 where the video actually starts

Test was wrong. the engine had not run out of fuel on the first test, this means you have extra fuel in the pipe.s injector rail ect. had you of refilled the car started it then drained it, and put 1 gallon in you may have seen an increase in mpg on 2nd test.

It's the Car S.O.S. guy!

The problem is the brand new tyres. Fuel efficient ones or not, you should allow the tyres to settle.

Don't mess with the chips! A proper test would be to reduce the weight and test on the track. Change the tires and test on the track, and so on. Keep up vehicle maintenance, maintain proper tire pressure, accelerate smoothly, use cruise control on dry level roads, release the gas pedal when you see a stop light or stop sign coming ahead to coast part of the way before braking (also conserved brake pads), plan your route with one that has the least amount of stops and traffic, if running multiple errands go to the stop furthest away first and work your way back home.

After you ran out of petrol the first time, it would of drained you fuel line so really you got further because of the fuel in your lines that you did take out firstly

Best fuel saving tip: don't drive.

If only he knew what gas prices would become

I believe the difference comes from the fact that on the first run, the car only had it's fuel tank drained, but on the second run the car had it's complete system drained before the fuel was poured in

Hold on what a load of nonsense. Driving in the rain is LESS efficient as your tires have to pump water AWAY to prevent hydroplaning.

Fuel is cheaper than bottled water?! Today in 2018: -In UK the averate price is 65 pence for 1 litre of bottled water. -In UK average fuel price of fuel is 140 pence per litre. In 2017 fuel was really cheaper than 65p per litre?

Do this with a V8 Muscle Car and change just the Gear Ratio for each test. The results would be astonishing.

the video looks like 480p even tough i set it to 1080p

Love how people are so interested in the fuel price, this episode was from 2009!

The cheapest and easiest fuel saver I know of is to simply place a tennis ball behind the accelerator pedal. I've found that it's not so much the car that uses a lot of fuel, but rather my driving style. At the moment I'm using about 10 liters per 100km, but if I concentrate on saving fuel, I bring this down to 8 liters per 100km. I drive very short distances, and mostly start stop and hills.

new tyres have more rubber on them, so per wheel rotation, you go a bit further... The odometer showed a little bit less distance traveled, but maybe it was wrong due to the tyres being slightly larger !

decrease tire width

With simple inexpensive modifications to weight and aerodynamics I brought my 1992 Geo Metro 3 cylinder hatchback up from 45 MPG highway, to 68.2 MPG. The first mod is to install an Econo-gauge so that you know what your fuel consumption is at any given time.

I've set the resolution to 720p but looks like 360p, you obly had one job... ONE JOB!

Logic #1: keep your RPMs the lower possible. As others pointed out, you can go faster on an higher gear and save more fuel than going slower on a lower one. The higher the RPMs the faster the engine is working and therefore burning more fuel. Logic #2: downshift to slow down. If you brake a little bit and lower the gear, the engine itself will force the car to go slower. This can save fuel and make your brakes last longer, however my parents say it's not useful because it'll make your clutch wear out faster - I tend to disagree though, there's just as much clutch friction as there would be when shifting.

Maintain full is also great for emergency situations. Thanks for the content and look forward to more.

Best fuel saving tip: be in the correct gear so you're not pushing the gas pedal down (without stalling). I know loads of people who never use their 5th gear because they feel it's slower to accelerate but once you reach the speed limit you should be cruising in your top gear with lowest revs to save fuel.

Tire rolling resistance falls as the treads wear down. So those new tires might have reduced MPG over the old ones, even though the new ones are LRR.

I suspect new eco tyres have more rolling resistance than part worn normal tyres. But I agree that rain was probably the big factor here. In my own driving I notice the wind as the biggest factor. A strong headwind knocks 10mpg off. Even on an oval track I suspect the wind would be a factor.

Although rain makes the surface more slippery, it also adds the fact that the tyres need to push that water out of the way, therefore using more fuel.

46 to a gallon is amazing for a 20+ year old car

Tap into Oxygen sensors wires , install a variable low voltage knob which will allow you to control fuel dosage. You can make the car run like a lawnmower or you can set it to rich fuel setting where it will be more powerful.

Drain the fuel lines etc as well, or drive it till it runs out before first test. The fuel pump, the "in" fuel line and return line, plus if it has a regulator bowl could easily make a 2 mile diff.

the easiest way to improve the fuel economy is by installing a flat floor under the car to make it more aerodynamic. this mod is actually quite cheap to make on your own and you actually save a couple of percent of fuel

Before the first test there was still fuels in the engine but the second there was not. I better more practical test would've been to fill it up and drive a certain number of laps/ miles and see home much fuel you used for each. Thats what everyone else does

and this polo according to dvla is still drivind around and has MOT untill 25 August 2017 and tax until 01 June 2017

That woman who said "it's not quite worth it, but thank you". The nerve of some people

I have an 1999 SUV, I did see an MPG increase with new tires, new S belt, modern spark plugs, high flow air filter, modern fuel injectors, new coil pack, removing the full size spare and lithium ion battery and driving like I'm early to an audit. It was still only a marginal increase.

coast (in gear) as far as you can up to a junction or round about...especially with common rail diesel engines.

"What can you do to achieve more MPG from your vehicle?" would be a good title...

It's funny because my 2006 f150 4x4 heavy duty payload package which comes with 4.10 gears a heavier frame/springs, all steel body, steel wheels, towing of 9900lbs, was rated at 13 city 17 highway. I changed 3 things. 1. Replaced the air box restrictor pipe going into fender that reduced from 3 to 2 inches with a 3 inch straight thru pipe. 2. Replaced the factory muffler with magnaflow. 3. Bought a gryphon programmer that was tuned for those changes. The result not only gave me much improved performance and drive ability like better shift points and better throttle response but also gave me a 2-3 mpg increase across the board to 15-16 city and 18 to 19+ (depending on speed ) mpg. I have 35 gallon tank and would max out at 400 miles per tank before and 500 miles per tank after. I have a 4 speed automatic with 285/70/17 TOYO all terrain tires. The truck is 12 years old with none of the fancy technology the new trucks have and I'm getting basically the same mpg. The auto companies are scamming people with suppressed mpg. Period.

needs to run on 4 of those spacesavers :D

you can't really use a stock fuel tank to test accurate volume of fuel. Thats why almost every scientific test uses a dedicated 1 gallon secondary tank.

Get an lpg installation. That's what I did. It works.

@1:47 the fuel was £1.195 per litre!!! August 2022 and the price is £1.95!!!!

First off they should have tested each configuration multiple times to check for consistency, first run still had fuel in the line unless they burned it out off camera. Secondly you can most definitely mod for fuel economy, especially with a diesel vehicle. Aside from weight reduction and low rolling resistance tires the best thing to do (with diesel at least) is increase the size of your injector nozzles as much as possible but set the injection quantity lower, get a power tune, add a large turbo and make sure it's breathing as good as possible. You'll want to set your injection timing more advanced. The idea is to inject the fuel instantly at the most efficient time. Basically aside from setting your large injectors for a lower lower fuel setting you want to mod for power. Power = more efficient and as long as you exercise restraint from enjoying the additional power you will see a vast improvement in fuel economy. I've tuned 40-45 mpg VW TDI s into consistent 60+ mpg with this method, actually my personal Jetta is looking at 70 mpg and a lot of power on tap if needed.

Change to a lower ratio final drive!

I have no idea what that giving money away segment proves.Should have spent it on upgrading that potato used to film/edit that or some training for the person.

30% of the overall car drag come from the wheels alone. The wheels are the most inefficient part drag wise on any car. Choose the smallest and narrowest wheels which are legal for your car and close off the biggest portion of the rims with something flat or with foil. This mod alone can increase your efficiency by about 10%-20% depending on the wheels you had before. Next is the, rear box, rear mirror, windshield wipers, door handles and front air inlets. You can optimize all of those with little mods like. - detach windshield wipers and put them in the trunk for later us if needed. - install small vortex generators besides the mirror to increase attached air flow (effect can be measured with small strings attached to the car, easy and simple) - tape off the door handles if not needed. especially in the rear, say you usually ride alone and can open them from the inside, tape them off permenantely - close certain sections of the front air inlets (monitor the temp. usually cars are road tested at 45°C in the desert, if you live in the UK you don´t need that) - You can add a small spoiler to the rear which is curved 12° deg inwards to decrease drag. You can save probably an other 5% -10% or so with those mods.

How does reduced friction on the road surface from rain help? Isn't it static friction between tires and road meaning more friction is good?

If you buy a Chip from a bangladesh-brit you car will lose even HP...

thats too much to pay. I would rather not have a car

Here's one mod that works (assuming the computer doesn't freak out, in which case you'd have to retune it): Just make the intake and (especially) the exhaust as free-flowing as you can. It takes energy to suck the air in, and it takes energy to push the exhaust gasses out. Backpressure in the exhaust (like a stock silencer, or a cat (removing that is obviously illegal in many places)) means the engine must work harder to get the exhaust gasses out, therefore giving you less usable power per amount of fuel used. But of course there is also the matter of scavenging (using the correct size pipe, which people mistakenly use as justification for backpressure - these are different things), so don't overdo the pipe diameter. And yes, in my experience playing with the spark timing will improve economy if you drive right. It makes the engine more efficient so you cna have the same power with less fuel, or more power with the same amount of fuel.

Please think about the fuel in the fuel filter and fuel lines, could be half a liter or more of fuel in those on top, also wet road cause more resistance so could have lost a couple of mpg there

Wouldn't rain add weight and make it worse?

I bet it was the ECU

Depends on the car. I have converted my clutch fan with dual electric fans in my 1998 Ford Expedition and there was a great increase in fuel efficiency. Next I will change the manifolds with shorty headers for more torque (less gas for speed), change the fuel injectors, fuel pump and fuel filter, spark plugs and install an open air intake.

I bet the weather changed enough that the wind and rain blew on the car and slowed it down. Apart from that, most cars are geared too low for the best fuel economy on a steady speed of 60. Sports cars are usually geared for the highest speed and power peak of the engine, but of course when you are puttering along at more sedate speed limits, the wind isn't blowing as hard against the car and you could actually use a higher gear if the car had it. A big form of resistance comes from the internals of the engine and gas flow within it. It's useful to control your speed down a hill but when you don't need to slow down you're just putting more fuel in to fight the forces. You can simulate a higher gear by repeatedly shifting into neutral to gradually slow down, then giving it a squirt of accelerator to get back up to speed in the highest gear. It's a pain in the arse though and probably wears your clutch more quickly. Drum brakes are more efficient than disc brakes, because in most cars, the pads rub against the discs like a junky old bicycle - wasting fuel. It's not a lot but a couple of kilos of force on each wheel adds up. I jacked up a car and found the wheels with drums spun freely but the wheels with disc brakes were a bit stiffer, requiring me to keep pushing the alloy round with my fingers. My Cooper S is a bit of a brake drag machine - making the car noticeably harder to push. There are also people of forums like ecomodder who reshaped the aerodynamics of their car too.

46 mpg?? why Polo is not sold in N.America? oh wait, VW is not considered green anymore?? ;-)

tldr: impossible...

If you're reliant on the cars own odometer reading for this test then changing the old, worn tyres for brand-new max radius tyres is always going to affect the results? The odometer only reads final drive RPM?

Apart from all other factors mentioned in the posts. New tyres has a slightly bigger circumference than worn tyres, thus the speedometer reading will be different between new and used tyres and on a 45 mile trip it could be different by a mile or two. That would mean that although the clock shows 60mph he might be going 1 or 2 mph faster than the previous test and the distance would show as shorter. If they used external equipment to measure speed and distance it would not matter off course.

turbo it.

Move to Saudi Arabia and pay only 0.26$ US per liter :)

Best modification for increased MPG Guaranteed, restrict the fuel filter. I've done this twice by accident on two separate cars. 1996 Civic EX Coupe manual went from 40mpg to 30 mpg after installing a new fuel filter. Vtec engaged and I had a sports car again. I shelved that new fuel filter and put the old one back on netting me 40 mpg again. You gotta love the OBD-II ECU's!

maybe upgrading to improved injectors, a service and s professional engine flush will improve mpg

looks like VW did a good job

Just install an electric motor and you save fuel!

smaller rims, thinner tires in the 155-185 width sizes, run higher psi, set up a hot air intake system, delete your spare/jack/toolsets, lightweight battery

in every car show that was filmed in the uk and had some science involved there was always a factor of rain that fucked it up completely

god i hate british money

All things being equal, removing the weight from the rear should have helped, and possibly the tires would have too. I do wonder if the ECU tune had some adverse effect to offset these potential savings. Also as noted by another reply the first test would have still had some fuel in fuel lines etc, but was completely emptied after the first test. Even differences in temperature and weather can affect this. It's difficult to be accurate with things like this and if you are accurate (ie in a lab), then it's not really representative of real life. The only sure fire way to improve economy is to reduce weight and drag (eg close windows, remove that aftermarket spoiler). The tires may help but there are so many variables I think it would be hard to quantify/prove

While fuel price rises in the UK it's been nice and low here in the US :)

The only thing I've seen tested in real world conditions that's increased fuel millage with any noticeable improvement was when mythbusters did the test where they covered a car in clay and cut golf ball dimples into it. I want to try to find some plastic or fiberglass wall panels with similar sized dimples in it that I can cut and custom mount to a car or my truck to try it myself, but I haven't been able to find any.

driving in rain is like pushing a wall in front of you. Where I would do 4.5L/100km in dry weather I do in high 5s low 6s when raining. That's what cost you the distance. wet surface might have less rolling resistance, but rain slows you down BIG time.

How they should have done it: Bump pressure up to sidewall, borrow set of WORN IN lrr tires, remove side mirrors and the stupid antennae, make a grille block, undertray, kammback/boattail, properly designed tire spats, wheel covers, rear wheel skirts, meaningful weight reduction (remove seats), power steering delete (assuming it has P/S), NO CHIP, and most importantly, don't drive in the rain and pretend its the same. Rain may reduce the friction of the tires, but now you have to push the water out of the way just like air.

put more air in tires. so the presure is bigger.. for example i have e36 316i sedan 1,6 l with tire presure front 2,6bar rear 2,8 bar. 6,5l/100km avrige. used to be 7,5 - 8l

Even with petrol priced at £1.74, like it currently is, that £550 he spent on that chip and those tyres is a false economy.

Interesting point at the start about proportional fuel taxes - it's not like the environmental impact, road maintenance, etc. go up and down with the price of oil.

They should have put on narrower eco tires. Cut down RR and Areo both. They also did not break the tires in, new tires will always have more RR than old used ones. They also started out with a little bit more fuel in the first run with it being in the lines. Rain added a ton more resistance to the run as well as more humid air to deal with in the A/F ratio.

hey, great info! do you have a facebook page?

*the track the grand tour used Cool!*

Only replacing the original air filter box for a high performance direct intake , in a saving driving helps you save 0.5l/100km. The reason is, more air pressure and less throttle for get the same engine performance.

I like all these car tests that these guys do, but i sure wish they would put more thought and scientific measures in place into these tests.

in the second test the wipers were on which is much less aerodynamic, also the car has to drive through rain and air and not just air so there was more resistance so the car needed to work harder and use more fuel

Your fuel line was not empty on the first test. Do the test test every time you introduce a change in the vehicle configuration to test each modification impact on mileage. Barometric air pressure have HUGE impact on both performance and mileage. So you should test all changes the same day. In a gasoline engine, CDI ignitions burn fuel more effectively. Also reducing engine drag with some additives like militec1 can help mileage. Do the test in more serious way...

Wet air is more dense, and thus will give worse mpg due to air resistance which is significant at 60mph.. Furthermore, atmospheric pressure greatly affects the volume of air naturally aspirated motors (an air pump) ingest, so fuel mapping would inject less fuel and the motor would make less HP during low pressure events. So weather conditions were likely the source of the differences.

"Fuel prices have risen my record levels this year"

use lightweight crankpulley and lightened flywheel might works?

The test may have been flawed. The first drive was after only emptying the fuel tank. The fuel lines, pump, filter, and rails all hold some gas. If you run the car until it dies it empties at least the fuel pump and probably some of the fuel in the lines. All the subsequent tests I assume were with a completely empty fuel system. I didn't see you mention if the following tests were done with the same conditions (tank had gas in it and you emptied it and put in exactly a gallon). Humidity and barometric pressure could also cause a non trivial difference in consumption.

Maybe a bit too late now to say, but it's not scientific to test all those things together. That chip would clearly be the weakest link. LRR tires and pumping up your psi to just beneath the max on the tire wall is about as much as you can get. Well that and cleaning the airfilter and changing to a slightly lighter oil.

When your engine conks you have literally no fuel , in your fuel lines or in your injectors. But when you started the test you said the tank was drained not the fuel lines; so on the second test you where actually starting with less fuel. Also rain doesn't help your miles per gallon but will actually damage it so as wet damp air is alot denser and the rolling resistance at 60 is a negligible difference as the majority of drag is caused by the air.

Fifthgear.. pls try the pilipino improve invention.. KLR power box by randy palmado.. not only fuel efficient but also add to horse power.they even try it to dyno machine

Holy shit, it's in HD!

try to do the same test without the Rain! another trick is to put a bit more pressure in the tires. i am astonished by the fact of the chip didn't make the diference! good work! you need to come to Portugal to give some gas money to People! :-)

Take out the spare tyre and have the correct tyre pressures. That's all you can do, other than servicing.

When I was going to tech school for my automotive papers my instructor told us that the wind resistance doubles in a car between 90 and 100 kph. now this was 25 years ago and I'm certain his info on that was out of date. But depending on the aerodynamics of the car I can see there being truth to it. I wonder what altering the speed would actually do in this case?

Well done guys! Blowing up myths is fun, isn't it? I noticed the energy saving tyres all roll harder and therefore less comfortable. Constant small bumps are coming through, which I can't feel with a set of comfort oriented tyres.