Ford Class C with 6 speed transmission

Gjac wrote:

pnichols wrote:
I think that I've improved my motorhome's V10 fuel economy over what is possible with it's 5-speed 5R110W transmission by retrofitting it with non-stock tires that are larger in diameter. Going to a larger diameter tire makes all gears "taller" in any vehicle for improved fuel economy when cruising.

Since our Class C motorhome is a relatively small one for it's E450 chassis, I don't need the extra pulling power that the E450's 4:56 ratio differential provides, so I may as well get some improved mileage through use of larger diameter tires.

An E350 chassis has a 4:12 rear differential ratio, so larger diameter tires on it under a motorhome might make it lug down on hills a bit.

Hi Phil, what was you MPG before and after the tire change?

I don't know exactly, Gary, as I don't track gas mileage precisely. I'm assuming that in the long run taller overall gearing (as larger diameter tires provide) will improve cruise gas mileage somewhat - it only makes sense that that would be the case.

My speedometer reads about 1.5 MPH lower due to the non-stock tire diameter, but that is of no concern to me. We cruise very conservatively at only 57-60 MPH and in overdrive this results in around 2000-2100 RPM engine spin speeds.

My taller tires are just a bit narrower, but since the stock Ford rims are rated for a range of tire widths we have not experienced any problems with bead leaks. The slightly narrower tires also cause improved air flow cooling between the tires in the rear dual tire sets when traveling in high ambient temperatures. Tire tread wear and sidewall heat stress are also improved a bit because larger diameter tires turn less rotations per mile.

However, the main reason we went to a larger diameter was to improve ground clearance for all undercarriage components during occasional travel on rough roads. RV trip flexibility is very important to us because we cannot predict what situations we may be presented with.