Dear All,

I bought a set of SSIII Rims off a friend a few years ago, they are 215/60R14.

From what i gather, i can fit 225/70R14 tires on my 14x6 Rims.

I was shocked when i look at the comparison. My revolutions per mile would decrease by almost 10%. Does this mean by simply changing my tires, i could decrease my fuel consumption by 10%?

If i DO buy these new tires, should i be worried that my power that i put down to the pavement will decrease by 10%, Right now i have 2.73 highway gears, and on full throttle my tires chirp nicely from 1st to 2nd.




225/70-14215/60-14DifferenceDiameter inches (mm)26.4 (670.6)24.16 (613.6)-2.24 (-57) -8.5%Width inches (mm)8.86 (225)8.46 (215)-0.39 (-10) -4.4%Circum. inches (mm)82.94 (2106.75)75.89 (1927.68)-7.05 (-179.07) -8.5%Sidewall Height inches (mm)6.2 (157.5)5.08 (129)-1.12 (-28.5) -18.1%Revolutions per mile (km)763.9 (474.66)834.86 (518.76)70.96 (44.09) 9.3%

 

No, 10% decrease in rpm does not equal 10% increase in mileage. It will depend on your rear end ratio and if you're driving hiway vs city.
I did the experiment on hiway driving with a 700R4 driving in 3rd 1:1 and 4th 0.7 OD. Best case scenario, all hiway the 30% reduction in rpm gave almost 10% increase in mileage.

 

You also have to compare the rolling resistance of the tire, wider tire=more resistance. You won't notice much of an improvement.

 

Gas mileage wise any improvement may be modest but looks wise the taller tire should look much better. 215/60 doesn't fill up the wheel well too much.

 

X2

 

Here's a picture of what it looks like now.

Sorry the babes are blocking the rear wheelwell. I tend to not take pictures with dudes.

 

I think the 225/70R/14 is about the best fit you can get with a 14" wheel. You sure the current tires are 60 series? 215/70R/14's are pretty common and only a little shorter than the 225's, not really enough to notice in performance or mileage.

 

Wider tires (all else being equal - particularly including rolling radius) will hurt mileage. Only by a tiny percentage point though.
Raising your overall gearing normally trades acceleration for mileage, but if you overdo it you end up hurting both mileage and performance. If you have to plant the gas pedal every time you hit a slope, or have to downshift constantly your gears are too tall.
What is ideal for your particular case depends on many variables, is it hilly or flat where you live, are you at a high altitude location, is your car often fully laden or used for towing, do you mainly drive in town or on open highways?. Whatever you end up with it will always be a compromise between driveability and economy. Perhaps that's why modern upscale cars have so many ratios in their transmissions.
For years Oldsmobiles managed fine with only three, and the four speed lock-up in my current H bodies have all the gears I need for my requirements, including town driving, high speed cruising and towing. GM seemed to get that right at least.

Roger.