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      07-30-2011, 10:21 AM   #1
LarThaL
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Does the speedometer calculate speed off the front wheels or the rear?

Basically, I'm trying to figure out which tire diameter is crucial to keep speedometer readings accurate.

Also, how much can you change the relative tire diameters from stock before it messes up the stability functions, brake anti-lock, etc.

The stock rear is 668mm and the stock front is 654mm. 14mm or 2.5% difference.
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      07-30-2011, 11:37 AM   #2
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      07-30-2011, 11:46 AM   #3
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Well the speed that the speedometer is read is measured by revolutions the speed sensor in the transmission picks up coming from the output shaft... That is all i know haha but you can use this chart to measure overall diameter between tire sizes!

http://www.miata.net/garage/tirecalc.html
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      07-30-2011, 12:07 PM   #4
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Quote:
Originally Posted by Rob@StrasseForged View Post
Well the speed that the speedometer is read is measured by revolutions the speed sensor in the transmission picks up coming from the output shaft... That is all i know haha but you can use this chart to measure overall diameter between tire sizes!

http://www.miata.net/garage/tirecalc.html
Great calculator.
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      07-30-2011, 12:08 PM   #5
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I would think it HAS to be the rear because the speedometer moves on a dyno (if memory serves me) and it would do that unless the speed is measured from the rear.
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      07-30-2011, 03:15 PM   #6
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should be from the rear, dyno explains it
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      07-30-2011, 05:51 PM   #7
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Now I've got to recalculate.
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      07-30-2011, 07:03 PM   #8
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Speed sensor is in the tranny indeed; therefore rear tires affect speedo. Just ordered 255/275-35-19 tires, so error should decrease a little after that .
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      07-30-2011, 07:29 PM   #9
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The wheels are not the same size.
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      07-30-2011, 07:58 PM   #10
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It has to be rear because when you burn out the speedo goes up pretty fast
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      07-30-2011, 09:28 PM   #11
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As others have said it's measured at the transmission not at the wheels. When it comes to changes due to wheel diameters only the rear tire diameters matter because they're the drive wheels.
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      07-30-2011, 09:36 PM   #12
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[quote=Mace;10131128]As others have said it's measured at the transmission not at the wheels. When it comes to changes due to wheel diameters only the rear tire diameters matter because they're the drive wheels.[/QUOTE

+1 there is a speed sensor on the tranny.
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