Tires: Tires & Wheels, stock tire size, dodge ram 3500

Question
QUESTION: I have a 2003 Dodge Ram 3500 with factory sized tires and wheels. They are 265/70/17 on steel wheels. Can I improve fuel economy by switching to 245/70/17? I know the smaller tire is shorter and my RPM would increase by approx 25, but the tires are 8 lbs. lighter. Will less rotating mass more than offset a 25 RPM gain? Also I am looking at aluminum rims from various mfg. However most make a 8.5 inch wide rim and a 245 tire is not recomended beyond a 8 inch rim. There are mfg who make 8 inch aluminum rims but not with the correct offset that I know of.

ANSWER: Ron,,

Believe it or not, that would be going the wrong direction.  You want to go bigger - both diameter and load carrying capacity.

But the biggest thing you can do is to replace your tires with low rolling resistance tires - which really don't come in sizes for your vehicle.  I am afraid that you are stuck with the fuel economy you have.

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QUESTION: I have used worn Michellin ribed tires neting 30.3 MPG vs 27or28 on Toyo mud and snow. Both tires are the same size, but different weights 8 or 9 lbs more for the Toyo. (as weighed on a bathroom scale) This makes me think unsprung weight is important and the further away from center the rotating weight adds up exponentially. My truck is a short box and I never am near maxed out even while towing, I mostly use it as a car. Do you think I should use 17.5 wheels and commercial duty tires to reduce rolling resistance, is rolling resistance more important than unsprung weight? Am I barking up the wrong tree, and should I stick with the stock tire size and use a 8.5 proper fitting rim?

Answer
Ron,

It isn't so much "unsprung" weight as it is "mass in the tread area".  I cover that in more detail here:

http://www.barrystiretech.com/rrandfe.html

Short version:  All Season tires have less mass than All Terrain tires - and that means the All Season tires will get better fuel economy. But All Terrain tires have to endure harsher conditions, so the rubber is more chip and chunk resistant - and that also mean lower fuel economy. (the rubber characteristics part of the picture.)

I think going to 17.5" rims is not the way to go, but an 18" might work for you.  But, I'm sorry.  I can't tell you what tire size will work as there aren't charts available that tell where the fenders, suspension pieces and the frames are located for the literally thousands of different vehicles that are out there.  Without that info, I can't give you a reliable guess.

But you have a vehicle .......  Don't forget, the worst case is with the suspension fully compressed  - AND - with the steering wheel fully turned.