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Old 01-20-2006, 10:16 AM   #1
Scooby Specialist
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Join Date: Jan 2005
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2003 Impreza WRX
Platinum Silver

Default curious

i have been reading threads about people uping the size of there rims and wider tires and there speedometer is going to be off. and rpms at different speeds will be different than stock tires/ rims. what is that all about?
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Old 01-20-2006, 10:34 AM   #2
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'03 325iT


Um, different sized tires will have different circumferences which will change the number of revolutions required to cover one mile.

Here's a calculator that will help with determining tire diameter.

Actually, I like this one better as it will show the speedometer difference.

Last edited by VpointVick; 01-20-2006 at 10:48 AM.
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Old 01-20-2006, 11:01 AM   #3
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What the heck are you trying to say??? Please reformat that in readable English.
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Old 01-20-2006, 12:04 PM   #4
Pete Holt
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The closest sized tires to go with in 17" would be a 225/45.(The speedo would only be off by about .3 mph) In 18" you would want a 225/40.
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Old 01-20-2006, 01:40 PM   #5
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The width has nothing to do with it. It's the overall circumference (or diameter) that affects the speedo and odo.

The speedo/odo system is like any other system of gears. When you change the size of one of the gears, the speed at which the other gears turn changes. Increasing or decreasing the size of the wheel is like changing the size of a gear, making the gauges reading the gears further up the line read differently.

Most car's speedo/odo's are geared to run a little fast (you think you are going faster than you actually are) with brand new OEM tires. My TS came with 195/60r15's which are about 24.1" in diameter. My speedo and odo were about 4% off. When my speedo read 100kmph I was actually doing 96kmph (or at a reading 50mph I was doing 48.5mph). For every 1000km my odo said I was putting on the car I was actually only travelling 960km. When I switched to 205/55r16's, which are 24.9" in diameter my speedo and odo were much closer to being correct.
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