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Guest Message by DevFuse
 

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tire size, gas mileage, stress...


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2 replies to this topic

#1 bjwirth

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Posted 05 July 2004 - 08:30 PM

What do you think of this idea...

the other day I was driving with my wife in separate cars (2000 forester s) and noticed there was a discrepancy in speedometer readings. My car said we were doing about 70 mph, while her forester was doing 74. So either my speedo is reading too low, or hers is reading too high. Lets assume the latter for a minute (perhaps incorrectly).

What if the overall diameter (or circumference) of the tire was bigger, leading to fewer revolutions of the wheel per mile covered. would this correspond to higer gas mileage, since I'm getting more distance traveled per RPM (assuming same gear)? and perhaps a correction to the speedo? I don't know anything about differentials or stress or torque, but from my simplistic point of view, it would be like I had taller gears in my transmission... yes? no?

SO i've been reading some past threads, and some posted that you could put 205/75/15 on a forester L- according to my crude calculations, I could put a 65 series tire on my wifes car on the 16" rim. could I go taller without scaping? are there any drawbacks? would I see an improvement in MPG?

ONe last thing, when we bought the car used, the dealer put on a set of S-rated tires. I don't care one way or another, except I noticed in the side panel, the car specs an H-rated tire. does a forester really need this kind of tire? Beleive me, we don't go tearing through the twisties in this car (nor do we go 130 mph).

thanks for any help,
Brian

#2 Ranger83

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Posted 06 July 2004 - 10:01 AM

Most tire manufacturers will have the rolling circumference of your particular tire model/size on their websites.

Most car speedometers read a little low, for the obvious reason that they don't want to be sued by people getting tickets. When I check my speedo with GPS, 72 indicated is about 69-70 actual.

There are many variables to car mileage other than rolling diameter of the tire. It was said back in the 70's that enforcing proper tire inflation would have saved more fuel than the 55 mph speed limit did.

#3 Esteban32696

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Posted 11 July 2004 - 06:37 AM

Go to this site, http://www.miata.net...e/tirecalc.html & type in the different tire sizes. Look at the bottom, & you will see the differences in Speedo reading, etc.


Most tire manufacturers will have the rolling circumference of your particular tire model/size on their websites.

Most car speedometers read a little low, for the obvious reason that they don't want to be sued by people getting tickets. When I check my speedo with GPS, 72 indicated is about 69-70 actual.

There are many variables to car mileage other than rolling diameter of the tire. It was said back in the 70's that enforcing proper tire inflation would have saved more fuel than the 55 mph speed limit did.






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