bigger tires on a diesel effect on mpg?

[ryanG]

I have an 08 ram 2500 diesel. Does anyone know how the mpg is effected by 33s or 35s. I know its gotta be worse, but how much? I get around 20.5 on the highway right now. thanks for any help.

[jforce]

Tires affect the final drive ratio,so my bet is the taller tires will help a bunch,but to a point.Like if you ran 44" mudders,the added rolling resistance and air compression would negate any decrease in ratio. That truck is not broken in yet,the cummins has a silly long break-in period,like it ain't broke in till 30k ! then the mileage will go up.But 20 hwy is still good

[charger R/T]

like stated the tire would effect final drive ratio. A taller tire would drop your RPM at a set speed. It should help some. It would also make your speedo go off. I would guess the new trucks have the same thing in the computer to allow you to change the tire size to get the speedo correct with different tire sizes. It use to be called pinion factor on the scanner screen.
Tim

[detobias]

if you switched to an identical tire, only taller, then your "effective" FDR would decrease which should improve mpg

if you switch to a different tire then its mpg effect is unknown

tires have a certain "rolling resistance" which is measured on a tire dyno usually at 50 mph ; and this info usually isnt made available to consumers

a "mudder" or Winter "M+S" tire will typically have a much higher rolling resistance than a "normal" or OE tire ; thus lower MPG, all else being equal

[ryanG]

Thanks for the help guys

[john doe]

You also have to take into account that a larger tire will weight more and could reduce mileage. An other variable is that in city driving having taller gearing can have a weird effect on mileage because you aren't always in the highest gear.

[detobias]

Quote:

Originally Posted by john doe You also have to take into account that a larger tire will weight more and could reduce mileage. An other variable is that in city driving having taller gearing can have a weird effect on mileage because you aren't always in the highest gear.

both points are true

the inertial effects of a longer tire will be more pronounced in city driving and virtually negligible on steady speed cruizin

[jforce]

You can look into the smarty programmer from mads electronics.It can correct the speedo,and there are huge performance gains too.it's $700 though.But's it's a hardcore piece.totally worth it in my opinion