Great video found the info really helpful. Out of interest what size wheels and tyres are you running as planning to do something similar, however reconsidering now if economy is that bad.
The fuel used will not be as bad as you think due to the Speedo mileage being out, you will be travelling a greater distance than what your Speedo mileage is displaying due to the extra circumference of the wheels. I see you stated Mpg measured at fill ups fuel used vs miles travelled not just looking at the computer, yet another RU-vidr UrbanArkOverland did an experiment in a transporter with swamper wheels fitted and got completely different results. I've got swamper wheels fitted to my van to and not noticed that much difference, I would say it's knocked my speedo out by about 10%, so have to stick to about 63\64 mph according to my clocks due to actually doing 70 mph. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-PQCNxVYKjMg.html
Really good point and something that had crossed my mind before working out the last mpg test. The mileage variation from our trip to Scotland was quite a lot so we used the mileage travelled for the whole trip from the more accurate newer van rather than the mileage that this van recorded. The result was still 33mpg compared to to previous tests on the original road wheels which were more accurate on the speedo. These road wheels regularly achieved around and slightly over 40mpg.
