BigToe wrote:
Thanks PNichols.
If you have an E-450, then you must have 4.56 gearing?
What transmission does your rig have? How many speeds?
Did you notice any difference in shift points on grades with the taller tires?
I "think" that our E450 has a 4.56 rear differential, but am not absolutely sure as some E450's may have been available with a 4.30 rear differential. I tried researching this using the codes on the door sticker but was never able to determine the rear differential gearing for sure.
Our 2005 Itasca Class C's E450 chassis has the 5-speed 5R110W transmission. As I understand it, this transmission actually has 6 distinct internal gear ratios, with the computer invoking one of the two internal ratios for one of the transmission's gears - depending upon (outside air ambient?) temperature.
I have not noticed any difference in shift points with the taller tires, but any effect purely due to tire size may be masked because I mainly travel in Tow/Haul mode.
The speedometer reads a bit on the "too-slow" side with the taller tires, but this is of no concern to us (a dealer could maybe tweak this). We cruise with a speedometer reading in the 58-60 MPH range, with the tach reading around 2200 RPM ... and hopefully our mileage being in the 9-10 MPG range (but I don't keep records on this).
Of course the larger diameter tires somewhat compensate for the high 4.56 rear differential by moving the overall drive train's ratio lower. However we do prefer the E450's overall drive train's ratio - even with our taller tires - being higher than that of a stock E350's because we prefer the improved low speed pulling power during occasional offroad travel.
