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Old 10-13-2020, 12:57 PM   #13
Senior Member
Brand: Still Looking
State: Texas
Posts: 4,696
THOR #2121
Originally Posted by jannnda View Post
2600 rpm at 65mph? On my 31Z (V10) at 65mph I am just a little over 2000 rpm. I also have never gotten 8.5 mpg.
Really nice looking rig BTW.

The RPM difference is likely not 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. Id bet Ford still uses same axle gearing for different GVWR, although I havent found 2021 detailed specs yet.

In that light, that hes 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.
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