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Author Topic:   Correlation between tire size & speedo reading?
Paul
unregistered
posted June 28, 2000 10:29 AM           Edit/Delete Message Reply w/Quote
If I upsize to a larger diameter tire, will that make the speedometer read faster than actual speed or slower?

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MCJ
unregistered
posted June 28, 2000 04:44 PM           Edit/Delete Message Reply w/Quote
Go to the Tire Rack website and read up on "plus sizing." You can increase the diameter of your tire and not effect the speedo if you go with the appropriately lower aspect ratio (sidewall height). If you keep the same diameter tire and change the sidewall height, it will also affect your speedo. Changing widths one or two sizes will have minimal impact on your speedo. Also consider going the manufacturers' websites and view the detail specs on the tires under consideration. They will show revs/mile which is what you need to see.

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Paul
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posted July 01, 2000 09:49 AM           Edit/Delete Message Reply w/Quote
Thanks, I know about width, aspect ratio, diameter, etc, and how "plus sizing" works.

My question is "If I increase the overall diameter, by going with a wider tire with the same aspect ratio and rim (its an SUV), will that cause my speedometer to read higher than my actual speed or lower than my actual speed?"

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Daveb
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posted July 01, 2000 11:15 AM           Edit/Delete Message Reply w/Quote
The speedo will read lower than you are actually going with a larger diameter tire. The tire isn't turning as fast to cover the same distance.

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donfromnaples
unregistered
posted July 17, 2000 09:23 AM           Edit/Delete Message Reply w/Quote
quote:
Originally posted by Paul:
Thanks, I know about width, aspect ratio, diameter, etc, and how "plus sizing" works.

My question is "If I increase the overall diameter, by going with a wider tire with the same aspect ratio and rim (its an SUV), will that cause my speedometer to read higher than my actual speed or lower than my actual speed?"



Paul, taller tires will actually slow you down a little, give you better gas mileage a bit, and provide a softer ride with a little less cornering ability. powerdog.com will tell you how much difference there is old size and new. Don't go more than 10% difference or new cars will show computer error messages,ABS will not work as effectively, and tires may rub. Hope this helped.

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Paul
unregistered
posted July 17, 2000 09:30 PM           Edit/Delete Message Reply w/Quote
Hey Don, Thanks for the post. That was a helpful link! Now that I know my new tires are 2.1% taller, does that mean my speedometer is reading 2.1% slower?

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Eric
unregistered
posted July 18, 2000 01:33 PM           Edit/Delete Message Reply w/Quote
It's not a simple as 2% bigger = 2% speed difference. Your speedo takes a reading from the axle. if you go to a BIGGER diameter, and if the axle spins the same rate, the car would travel a greater distance with the same axle revolution. Therefore your speedo would read slower than you are actually going because the axle is still revolving at the same rate, but the tire is covering more ground. To find how much your speedo is off, you need to find out how many turns your stock tire made to do 1 mile. then revolve your new tires the same amount and see how much ground they cover. If this is confusing you, ill try to explain better. a 2.1% bigger diamete tire would probably have VERY LITTLE effect on your speedo accuracy.
-eric

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