89dieselkong

Just like the title says, if I throw 35's on my dually will I see a big drop in MPG? I hav 235/85R16's on there now, and would put 315/75R16's on there. Most of driving is highway and the rear axle is a 4:10. Anyone done this and have reports? Whaddya all think?

1320

Depends on how fast you drive. I bet your mpg average goes up if you indeed spend a lot on the hiway. I run 235-85's on mine with 3.54 gears. I also have a nv5600 swap which od in 6th is a tad more then the oem 4500 trans. I just need speed rated tires, and 7th gear.

IDhorsehauler

My mpg has gone up 2-3% going from stock 235/85/16 to 255/85/16. 315s are about 1.5" larger in diameter than my 255s so I would expect you to gain even more unless there's some law of diminishing returns regarding diameter vs mpg. You probably already know this but you're definitely going to have to get some spacers for the rear wheels if you go with 315/75s as they will definitely rub.

Purplezr2

In a way there is, at some point it will be hard to turn the weight of the tires due to gear ratio's and there is more contact patch with a 35x12.5 tire then a 255(about 2.5 inches more) this increase rolling resistance.

DIESELWRKS

another thing to consider when you do changes like this your speedometor will be off I have a smarty that I recalibrate mine which isnt 100% acc but I tuned this with my GPS to get close as I can this way you will know how fast your running. this will help you hand calculate your mpg better. just my 3 cents.

edcasey

I noticed in another post that you are thinking of converting to SRW. When I converted my 2000 3500 to SRW I went from 15.5mpg to 16.9 running the exact same tires.