Larger than stock tires?

[Sean]

Just put a set of BStone Dualer AT's 215/75/R15 on my S-10, the stock tigger paws were 205/75/R15. My question is this.....How much affect do you think that larger tires will have over the stock? I have put about 300 miles on them so far and my odometer seems to be reading off and the gas milege has changed.

Thanks

[jeff]

Your old 205/75R15 tires measured 27.1" in diameter. The new 215/75R15 tires measure 27.7" in diameter. This is a pretty small difference in diameter and should not really be affecting the mileage. Having only 300 miles of driving is not enough to say that overall mileage is worse. Your speedometer is now reading about 2% slower. Again, not that much. If it's reading 55mph on the speedometer you are really traveling about 56mph. That's hardly noticeable to the driver and the factory speedo probably has that calibration error regardless of what tire size you run.

The height increase of the tire should not have caused any big change in your mileage. The rolling resistance could've increased with the new tires and that could be a more likely explanation of the mileage reduction. Check the tire pressure.

Aloha

[Sean]

Your old 205/75R15 tires measured 27.1" in diameter. The new 215/75R15 tires measure 27.7" in diameter. This is a pretty small difference in diameter and should not really be affecting the mileage. Having only 300 miles of driving is not enough to say that overall mileage is worse. Your speedometer is now reading about 2% slower. Again, not that much. If it's reading 55mph on the speedometer you are really traveling about 56mph. That's hardly noticeable to the driver and the factory speedo probably has that calibration error regardless of what tire size you run.

The height increase of the tire should not have caused any big change in your mileage. The rolling resistance could've increased with the new tires and that could be a more likely explanation of the mileage reduction. Check the tire pressure.

Aloha




Cool, thanks......i did check the tire pressure and its at 40psi. Maybe i need to clean my air filter.

The main problem is the GM gas gauge, that thing moves more than a flag when the wind blows.

[CRAIGHALL]

Remember also you may of went from wore down old size to new taller size

[grantdcol]

Larger diameter tires have a larger circumference, therefore they make fewer revolutions for any given speed than a smaller tire. Because the drivetrain turns (slightly) slower, your odometer will register fewer miles that you actually travel, and when you divide the incorrect number of miles by the amount of gas you used, you'll get fewer miles per gallon. You need to find out what percentage you're off, and compensate for mileage.

For example, I know that my odometer registers 9 miles for every 10 I travel. So when I calculate fuel milage, I multiply the number of miles from my odometer times 10/9 (1.111...) and then divide by the fuel I used. Also, when my speedometer shows 45, I'm going 50, 54->60, 63->70, etc.

I doubt you're actually loosing much 'real' fuel economy.