I have a vehicle that came standard with 16" wheels but now has older 15" wheels. What if any difference will be in fuel economy because of this?

You should be fine. If I remember correctly + or – 3% for tires. But if your not sure go to a tire shop and ask them.

@shego yes that’s about what I was guessing. For those who don’t know, the market has all but dried up in 15” used tires as most new vehicles nowadays don’t use them. The dealer initially put on a set of dogs that were illegal as hell. They didn’t want to spend the $ on brand new tires. I complained enough so the solution at the time was to mount new 15” tires only get bigger ones to offset the smaller rim. There was no way they were going to buy new rims and tires. But the other day I got the original spare out from under the truck to compare and the original looks to be quite a bit larger in diameter than the new ones. The V-6 isn’t exactly a powerhouse so maybe the slightly smaller wheel will make a tad more HP so the trans won’t shift back and forth between overdrive as much?

It makes virtually no difference. Fuel consumption is dominated by the drag coefficient of the vehicle and the mass.
HP will be the same but the smaller tires will give you more slightly more torque while cutting your top speed a fraction.
Don’t worry about it.

The speedometer and the gizmo to tell you how far you’ve gone won’t be as accurate though.

Watch your speed !! the speedometer on vehicles are calibrated from the factory with the wheel size that came on it. going from a 16” wheel to a 15” will dramatcally change the accuracy of your trucks speedometer.

I have stopped many people for speeding, when they swear they were not. their speedometer was not accurate, because they bought bigger or smaller tires for their vehicle. have your truck’s speedometer checked. go on the interstate and have someone follow you at 50 mph. if their 50 mph is not your 50 mph, then calculate the difference and allow for this difference, while driving. a helpful tip from your local Metro Police Officer.

@john65pennington Yeah that crossed my mind too. The way I think about this is if the wheels are slightly smaller the veh will be actually moving slower than the speedo is showing so I have a built in speeding ticket preventer? it probably wont be a lot though. I can see where the guys riding in those donks will be fooled using those huge wheels. The truck is 10 yrs old so the slightly incorrect odo reading won’t be a factor as far as trade in value goes. I usually hang on to a veh so long it won’t matter. I put maybe 6–7 k a year on the work truck so those tires will be on there close to 6 yrs at least. Maybe then I will find original wheels when it comes time for new tires. These ones were free.

Saving or losing gas mileage from smaller tires would depend on the type of driving. If you do a lot of highway driving, then smaller tires may be a loss. When up to speed, your engine would not have to work as hard since the larger wheels/tires are covering more ground. On the other hand, if you are doing a lot of city driving, the smaller tires may lead to a gain in fuel economy (More mpg). The engine would not have to work as hard to move smaller, lighter, rims and tires. So, the smaller tires may improve your gas mileage if you are moving slowly and making a lot of stops. But, if you are worried about saving gas, there are other things you can do, that would affect your fuel consumption more than the switch to 15” wheels. Being easy on the gas, and keeping a proper following distance (no: break, gas, break, gas,etc) In fact, aggressive driving can lower your gas mileage by 33 percent at highway speeds and by 5 percent around town. So, drive sensibly and save gas.

@Afos22 thanks there well this is enlightening. I drive pretty much an even split between highway and city so i will get a decent average for mpg. And it’s still 100 degrees every day here so the a/c is going to be in play for some time. As soon as we get out of that I will check the mileage without the a/c to hopefully see the mpg go up a little. If I accelerate gradually the trans stays in o.d. better. I’m still glad I have the v-6 as the v-8 in that model pickup really does suck down the gas no matter what.

This calculator is quite handy for that sort of thing.

For those particular sizes, the differences are less than 2% in any respect, so the only real difference is handling; the 15” tires will have more sidewall flex and feel more “wobbly” under hard cornering conditions. MPG, acceleration, top speed, and speedometer accuracy won’t be affected enough to really be worth caring about.

@john65pennington Normally true, but in this case the taller sidewalls and smaller rim balance out. Now, it the OP were running 225/75R15 then the speedometer would read higher than the actual speed whereas many people get a wider tire thinking it will improve grip and handling while keeping the profile the same, resulting in exactly the scenario you describe.