fuel mileage and tire size

[Execut1ve]

nice looking truck there, is that an extra bottle of oil next to the battery? lol

I like the foam insulation there, what is it supposed to do for the engine?

I see you're up north, that truck is conspicuously rust free. I have the typical rust on the rear fenders, cab corners, and rocker panels that I"ll have to deal with sometime how do you keep yours looking so fresh?

[Frank Lee]

I like to keep a quart of the same brand and weight of oil onboard as what's in the engine- just another idiosyncrasy of mine. The truck doesn't use much oil.

That's it's engine blanket; there's a thread on it here.

The rockers under the x-cab are starting to go bad but the rest of it is pretty clean. Chalk it up to obsessive-compulsive washing in the winter, and knowing where on the truck to wash.

[Execut1ve]

yeah rust repair is definitely on the list, although it's the last thing on the list... down there below new radiator, replace wheel bearing, axle pivot brackets or camber bushings, rear main seal, oil pan gasket... I could go on

[brucey]

I've had terrible luck with changing tires, upsizing or downsizing. It seems that unless you change the gearing as well mileage almost always will go down. I currently have 3% smaller/20mm narrower tires on and am seeing at least a 10% drop in mileage from them. Although that's going from a LRR to a dedicated snow tire

[mcrews]

Originally Posted by d0sitmatr

Ive gone through this several times, the larger diameter helps lower RPM, that is true, but you have to temper it with something that isnt going to make your truck work harder to get, and maintain, speed.

my ranger actually gets better mpg now with the slightly smaller tires I put on recently due to the previous tires size making the truck work much harder just to maintain speeds, and I could really feel the truck working a lot harder to get up to speed.
the biggest thing I noticed is going over highway overpasses.
with the previous tires, my truck would down shift 1, sometimes 2 gears. now it doesnt downshift at all.

unfortunately this post has no relavance to the question because: we don't know what the origanial size was. Maybe your slightly smallers now are actually only about 4% LARGER than the factory size? who knows?? but you can't make a blanket statment that smaller is better when in fact it is not.

[mcrews]

Originally Posted by Execut1ve

I'm in the fantasy stage of putting bigger tires on my truck. I'm considering the interaction between tire size and fuel mileage. Currently on the truck the tires are about 29" and I would be stepping up to 31's or 33's if I went bigger.

Pros: lower RPM at cruising speed

Cons: tires weigh more and have greater moment of inertia

Not sure: larger tires act like taller gears, so I would probably experience reduced power and acceleration from a stop. unsure how that would influence fuel econ

Anyone else want to weigh in? Any thoughts on what the overall effect on fuel econ would be? Thanks!

What is the stock size? Are you over or under that?
1. where is the sweet spot in RPM for your best highway mileage?
how much of your driving is highway?
2. how fast are you going mph at that point?
3. go to a tire site and get the diameter on your current tire and your dream tire.
4. remember, your just trying to go taller not wider.
5. Look at the wieght difference?
6. Do you go thru brake pads right now? if so don't add too much wieght

It is a trade off. but you have to really concider the queastions first.

I run 255/45/18 instaed of 245/45/18. It's 3.4% larger diameter. I do 60% freeway driving. My rpms are at 1800 at 57mph. THe increase in mileage was probably 8-9% ( i changed tire size long before I got to this site)

It worked for me. But if I was just a city driver, I don't think there would have been a decernable difference.

[Execut1ve]

stock is 235/75/15, about 29". I would be going up to a 31x10.5x15 or 33x10.5x15. possibly a 9.5" wide variant if I can find it. driving is a pretty even mix between city and highway. in the truck at 65mph I'm at roughly 2k rpm

[CapriRacer]

Originally Posted by Execut1ve

I'm in the fantasy stage of putting bigger tires on my truck. I'm considering the interaction between tire size and fuel mileage. Currently on the truck the tires are about 29" and I would be stepping up to 31's or 33's if I went bigger.

Pros: lower RPM at cruising speed

Cons: tires weigh more and have greater moment of inertia

Not sure: larger tires act like taller gears, so I would probably experience reduced power and acceleration from a stop. unsure how that would influence fuel econ

Anyone else want to weigh in? Any thoughts on what the overall effect on fuel econ would be? Thanks!

Probably the only data you are going to see that even hints at the question you are asking is some data Smithers showed to the California Energy Commission. I analyzed that data here:

Barry's Tire Tech

Basically the data says that larger is better. Just be aware that the data is only about RR and not the effect a larger diameter tire has on the engine RPM, etc.

But the data says the effects of tire size are small compared to the effect careful tire selection has. Also, this data is only about passenger car sized tires. Your truck likely came with tires designed to the same system and it would be tempting to go even larger by going to LT metric tires. That is likely the wrong direction as LT metric tires are designed to a different set of standards and part of what comes along is rubber that can withstand higher loading. That rubber is less flexible and not conducive for better RR.

If you are considering going larger, stay with a passenger car size tire and carefully select a tire where LRR is listed as a feature. The problem will be determining if the LRR label is being applied because the tire truly has LRR or that it is just better than comparable tires - meaning same UTQG treadwear rating.

[Execut1ve]

so, in general less flexible sidewalls -> higher rolling resistance? and more flexible sidewalls -> lower rolling resistance?

[Execut1ve]

by the way, an interesting read the link you provided capri