BlueLghtning

Hey guys. Would I see an increase in fuel mileage (taking speedo error into account) going up in tire size just a bit? This would be for mostly empty highway driving and very light towing. (Less then 2500 lbs if that). I don't own anything that big yet to tow.

My truck currently has the stock tire sizes on it right now. (245/75R16). My front tires were basically brand new when I bought the truck, but I knew I'd need new rear tires soon. A gentleman near me is selling some used tires for a good price. He has two 245's for the rear which would work fine and get me two decent tires for cheap. He also has 'four' 265/75 R16's for a good price also. I'm tempted to do this as it would get me a bit higher geared which I think would help for fuel mileage and is my main concern right now since I'm not towing much. On the other hand it's kind of wasteful as I'd be taking off 2 prefectly good tires, but I would get my truck geared a bit higher if it helps overall. I might be able to sell the good tires to re-coup some of the loss too. I'm thinking it might take something like 285's to really see a difference though in fuel mileage? I'm sure there's a point though where you get too big and you start to hurt your mileage, and I'm sure towing anything substantial for sure would hurt mileage once you get bigger on tires.

All tires are Load Range "E"

FYI, my speedo is dead on accuurate right now verified by a GPS. I do not have any device to change it at the moment. I've plugged the numbers into the gearing calc and it looks like I'd be lowering my speedo and odo by 3.73% Effectively, I'd be changing my 3.55 rear end to a 3.42. This doesn't even seem like that big of a change especially when you compare to going to 285's or even 305's or bigger. I would imagine I should be fine on basically an empty truck. It looks like at 2000 RPM's, I'd be going from 68mph to 71mph. (Of course the stock speedo wouldn't reflect that until it was reconfigured for the larger tires).

Note: This is a 1998 Dodge Cummins 2500 24V stock truck with no mods. 3.55 rear end 5 speed manual.

DL1221

Wow, your very first post, welcome to the forum!

I don't think going from 245s to 265s will make any noticeable difference in your fuel mileage. I think the rolling radius is only like one inch taller or something like that. I have 265s on my 96, but I've never had 245s so I can't compare the two. Theoretically your fuel millage should improve, but I really don't think you'll see any noticeable difference, unless you do a before-and-after controlled experiment.

Anyway, the very first thing I would do on your truck is install a fuel pressure gauge. Fuel pressure is something that you really need to monitor on any 24-valve engine. Read up on it in the 24-valve engine forum, if you don't know why. Good luck.

BlueLghtning

Thanks for the reply. I was afraid it wouldn't be much of a difference. I didn't realize this was my first post here. I guess I forgot to post my intro. I've been lurking on this forum and several others for a few weeks now. I'll go back and add my intro.

I do have the fuel pressure guage and just hooked up an EGT guage.

wcbcruzer

I didn't read the whole thing but basically, if you want an increase in mileage get 235/85/16's. 265's will lower your fuel economy cause they're heavier and wider imo.

BlueLghtning

Cool thanks for the info.

Unfortunately, he had sold the 265's by the time I got there, so I just ended up with the 245's he had.

TIMMY22

Bigger tires=more leverage on front end components. Can get pricey.