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Old 04-10-2009, 09:30 AM   #11 (permalink)
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Join Date: Apr 2009
Location: Kansas City
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This looks like a good place to pose my question!

I have a stock 2001 Ford Focus ZX3 with tires that are near requiring replacement. I am hoping to improve my fuel mileage a bit when I get new ones. The stock tire size (currently on the vehicle) is 205/50R16. I'm running them at 50 PSI and driving carefully. I usually get between 31-33 MPG for my commute.

My car has the 5 speed manual and turns around 3000 RPM at 70 MPH. It has a high power to weight ratio and I think could use some gear ratio adjustment, with a tire change being the easiest method possible.

I had considered swapping wheels to get a skinnier tire, and also considered just going up on my tire diameter by going with a 205/55, 205/60, or even 205/65R16 size tire. Calculations via this site:

Wheel / tire size calculator / comparer - BIGCUSTOMWHEELS

would indicate that going to a 205/65 would drop RPMs by 9.8% for a given speed, which I would expect to help my fuel economy significantly. I do mostly highway driving, but increasing the moment of inertia with larger tires has me a bit concerned about acceleration losses.

What would be the best move in terms of increasing efficiency by a tire change?

a) upsize the tire modestly (to a 205/55 or 205/60)
b) upsize the maximum (205/65R16, which I think is the largest I could fit in my wheel wells)
c) change to a different wheel--get a skinnier tire on a 14"x5.5" old steel wheel, for example, which would (I think) fit the car

I've read the other threads I could find regarding rolling resistance, and would also appreciate any help I could get in identifying the best tires to get for options A-C above, subject to frugality!

Thanks for any guidance--


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