Tire Size (width) impact on MPG - Fuel Economy, Hypermiling, EcoModding News and Forum

SirKeats · 10-31-2009, 12:51 AM

So I have my Metro's for commuting and they're great. This question isn't about them. It's about my beast of a truck I use for camping, towing, etc.

I'm thinking getting a slightly wider tire and am trying to find something that will tell me how much it will affect my fuel economy. It's already bad enough in the truck, I'd like to impact it as little as possible since it's our only truely reliable rig for long-distance travel (the geo's are just for in-town commuting).

Anyway, I'm thinking of jumping from 245/75R17's to 265/70R17's... so the diameter will be the same but the width would be about .5" more.

Any thoughts on how great an impact this would have on fuel economy?

dwtaylorpdx · 10-31-2009, 12:59 AM

LT285/70R16 to LT235/85R16 Made a 5MPG difference on my YJ. Both 32" tall. 11.4 to 9.4 Wide. At higher stupid speeds its almost 10 different.(over 75)

These were Toyo Open Country M/T's

Dave

SirKeats · 10-31-2009, 01:16 AM

wow. that's a lot. but that's a full 2" wider. i'm only talking a half inch. if it were a direct conversion... i'd be looking at what, like a 2.5mpg difference??? but i suspect it wouldn't work like that. no?

dwtaylorpdx · 10-31-2009, 01:41 AM

I bet its tread dependent, I know my dad was really happy with the latest set of Michelins on his Dodge Diesel, He's getting a mile or so better mileage, and they roll way nicer, don't bounce and they use like 85 PSI on the Alcoa wheels. They are also wearing very well. Were NOT cheap though!

Dave

Frank Lee · 10-31-2009, 02:41 AM

I think there are too many variables to connect fe to just width. Even tires within the same "family" have differing r.r.s depending on what size they are.

Were I to guess I'd say 1/2" isn't enough width change to really affect fe... supposedly lower profile tires have r.r. advantages so it would cancel out?

I've seen where tires have substantial dimension variation anyway- they aren't as dimensionally stable as a lot of other things that pop out of molds.

I've run several different widths but then on mine they were also several different manufacturers, several different weights, several different diameters so I personally can't draw conclusions from experience.

I have seen Michelin make oblique reference to "square" contact patches having lowest r.r.. I tried a tire pressure/contact patch geometry experiment to see what it would take to get "square" patches on my car but I didn't really reach a solid conclusion. If anything it appears many stock tires are too narrow for the "square" patch theory.

Daox · 10-31-2009, 08:33 AM

Darin posted this a while back.

tire width vs. drag (Cd)