I'm looking to put 315/70-17 tires on an 03-3500 turbo diesel. I want lower rpms at 70 mph. I'm thinking lower rpms equal better fuel mileage, but not sure how the added weight will offset this. Has anyone done this without a lift kit? My tape measure says it will work as long as I don't get too twisted up off road. Hope to hear from someone that has done this, and maybe what to do, what not to do. Thanks much!

 

You'd be best off asking this question in the Cummins Diesel Section...which is part of the Dodge Ram section. I'm betting there's someone there that's already running this tire on their CTD.

Right now I want to say that unless your gear ratio is 4.10 or higher, you are going to see more of a loss than a gain in running that tire size.

 

The 315/70's are a good bit taller, probably 8-10% depending on what you have on the truck right now. This would definitely lower the RPM at highway speed. The extra width would add air resistance and rolling resistance, and around town, the effectively taller gearing and heavier weight would hurt your acceleration and gas mileage. I would definitely want to hear from someone who had done it before I tried to fit that size without a lift.

 

I run the BFG's 315/70/17 on my 04 QC Hemi 1500. I love the look and ride...it really fills out the space in the wheelwells. However, it does have a negative impact on both the gas milage and performance. You can tell that these tires are heavy!!! I get about 2mpg less and usually have to leave the tow/haul mode to keep the tranny from constantly downshifting. I'm thinking the extra torque of the Cummins would make the hit a little less noticeable (at least I'm hoping cause I'm probably gonna trade up to the deisel soon).

My truck is completely stock with NO torsion bar adjustments and I've never had any rubbing issues with this tire...but, I don't go off road. I did get the ECU flashed to account for the oversized tires though.

Hope this helps,
-Dave

 

I run BFG 315/70/17 tires on my 2006 3500 diesel. They fit and work well off road, i live and work on a ranch. However they definately have a negative impact on fuel economy. The best i can get on the highway is around 15.5-16, everyone esle in the Cummins universe gets 19-20.
dwd

 

I have 315/70/17 on my 03, 2500. I just got it so I don't know too much about the gear ratio but I do know that the tires to rub on sharp turns.