Thor Chateau Class C are 99” compared to Axis/Vegas at 94” wide, plus Class Cs of that shape are assumed to have a higher coefficient of drag. Weight differences plays a much smaller role on fuel economy on the highway.
E-350 and E-450 are essentially the same except for the rear axle and springs, which adds a tiny bit of weight and “may” have different axle gear ratio. All E-450s are listed with 4.56 gears, whereas some E-350s (particularly older ones) may have 4.10 gears. If both have 4.56, MPG should be about the same when comparing equal units.
In my opinion all of these minor differences are still not going to make a huge difference even when combined in one unit. My previous Class C was narrow at 94”, light, and had 4.10 gears. Even then, I only got ~ 8 MPG on Interstates at 70 MPH. I only got 10 MPG twice while driving slowly out west sightseeing. Driving speed is as important as anything short of significant downsizing — it pays to slow down.
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