Originally Posted by Chance
The RPM difference is likely ďnotĒ due to V8 versus V10. For many years Ford specifications called for lower gearing on higher-capacity motorhome chassis.
A quick look at motorhome specs indicate yours is on 18,000-pound GVWR while his is on 22,000-pound GVWR chassis. That GVWR difference in the past changed the rear axle gearing by 25%, which would explain his higher RPM compared to yours. Iíd bet Ford still uses same axle gearing for different GVWR, although I havenít found 2021 detailed specs yet.
In that light, that heís getting 8.5 MPG at 65 MPH on a heavier motorhome while running 25% higher RPM speaks well of 7.3L V8 fuel economy compared to V10.
It will be interesting to see what the V8 does on 16,000- or 18,000-pound GVWR chassis with taller gearing. It should do a little higher MPG under similar conditions.
I was wondering if it was the gearing. Makes sense. I didnít buy it for the gas mileage.......but if I got 8.5 mpg that would be great. If I pull the toad I get around 7mpg. If I donít pull the toad I get in the high 7mpg. I have never hit 8mpg.