Mileage/Tire question

[zdkaiser]

So, I increased my tire size from 205/55/-16 (the stocks) to 215/50/-17. So the difference in diameter according to the tire calculator is 2.4%. Does that mean, my speedometer is off by 2.4%? My understanding is, with the bigger tires, I will be covering more ground with each revolution. So when I am doing 70mph, according to my speedometer, I am actually doing around 71.68mph. So then, my odometer would be off as well. I would need to adjust my mileage by 2.4% as well? 250 miles would actually be 256.I am curious as I noticed a decrease in my mileage with the new/heavier tires/wheels. This was expected. But I didn't think I would be jumping from 35mph to 30mph. I think I just failed to calculate correctly with the 2.4% adjustment. Any further insight.....?

[DrunkenMonster]

Yes your speedometer will be a difference since your going to a smaller tire.

[joatmon]

speedometers in Vibes may or may not be accurate, so your best bet is to try to verify it with a GPS or perhaps timing calibrated road markings.Quote, originally posted by zdkaiser »I am curious as I noticed a decrease in my mileage with the new/heavier tires/wheels. This was expected. But I didn't think I would be jumping from 35mph to 30mph. I think I just failed to calculate correctly with the 2.4% adjustment. 35 to 30 mph is too big a change to be explained in just the difference in tire sizes. You said mph, did you mean mpg? Either way, the percentage change would ideally be the same as the speedometer and odometer. Since the measured distance would drop 2.4%, the mpgs (calculated on the uncorrected odometer reading) would also 2.4%, all other things being equal, mpgs would drop from 35 to about 34. However, tires have many other factors that affect efficiency, and some of them may be playing a part in the mpg difference. I know that when I went from Bridgestone Turanza LSTs to Goodyear TripleTreds, my mpgs dropped about 8%, both sets of tires being the stock 205/55R15edited to correct a stupid mistake I made using the tire size calculator

[djb383]

Quote, originally posted by DrunkenDixie »Yes your speedometer will be a difference since your going to a smaller tire.Isn't he going to a larger tire.....both taller and wider? A taller tire also places more load on the motor thus a taller tire is harder for the engine to turn. This will have a greater effect on a small displacement/low torque engine like what's in the Vibe vs a big cube/high torque engine.

[zdkaiser]

yes, i meant 35-30 mpg.so correct me if i am wrong, but to calculate my new mpg i should add another 2.4% onto my odometer mileage reading because the new tires are that much larger?

[star_deceiver]

Ya, your odometer won't read right either. Were you "exercsing" your new tires to see how they handled and burned the extra fuel having fun?

[DrunkenMonster]

My bad didn't see that the wheel size was different. 205/55/-16 (the stocks) to 215/50/-17I knew I should have just kept my mouth shut..

[vibolista]

Quote, originally posted by zdkaiser »yes, i meant 35-30 mpg.so correct me if i am wrong, but to calculate my new mpg i should add another 2.4% onto my odometer mileage reading because the new tires are that much larger?Yup add 2.4% to your miles and you will come up with a more accurate mileage number. The slightly taller tire you are using should not bring your mileage down that much, unless it has a higher rolling resistance or you haven't inflated it enough. I'm running on 225/50-17s and keep them at 41 psi front and 39 rear. Also note that if you start out gently, it will increase your mileage quite a bit. A taller tire takes a little more power and torque to get rotating. At moderate highway speeds you should actually increase your mileage by turning the engine at a slightly lower rpm. My summer MPG hit 40 and now that it's a little colder I'm seeing mostly 36 to 37 mpg. Now that it's getting colder out, you will also note decreases in MPG figures. Winter gasoline formulations are different from summer gas. With a little patience and the right tire inflation, you should be able to improve or at least maximize your gas mileage. Also make sure that you're running the least amount of weight possible in the car. Don't store stuff in the car that you don't need... cause it's got weight and you are dragging it everywhere you go... burning extra fuel to move it!Check your tire inflation.