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Discussion Starter #1
I assume the speedo gets its speed information / signal from the front wheels, like most cars. Does anyone know for sure?

The reason I am interested - I recently replaced the standard 245/40/R17 rears with 255/40/R17. If the speedo reads from the rears, my speedo will under-read by about 1.5%, given the slightly bigger radius of the new rear tyres. If it reads from the front as I suspect, it of course is not a problem.

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roadsta
 

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The pulse sensor is usually on the tranny. It will read from the front on a front drive car and rear for a rear drive car. The quickest way to check for speedo error is to use a GPS at steady speed.
 

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Discussion Starter #3
The pulse sensor is usually on the tranny. It will read from the front on a front drive car and rear for a rear drive car. The quickest way to check for speedo error is to use a GPS at steady speed.
Thanks mate, I was just beginning to worry about this forum...

This makes sense, and I will test with a GPS.

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roadsta
 

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The reason I am interested - I recently replaced the standard 245/40/R17 rears with 255/40/R17. If the speedo reads from the rears, my speedo will under-read by about 1.5%, given the slightly bigger radius of the new rear tyres. If it reads from the front as I suspect, it of course is not a problem.

roadsta
Would changing the tire width have any effect on the the speedo? I can understand if you changed the wheel size or apect ratio, but the width?
 

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Discussion Starter #5 (Edited)
Would changing the tire width have any effect on the the speedo? I can understand if you changed the wheel size or apect ratio, but the width?
Because 40% of 255 is more than 40% of 245, the height of the sidewall increases by a small amount, giving the overall tyre (or tire if you wish!) a slightly bigger radius, and therefore the speedo will be affected. (That is what the '40' means, it is the sidewall height expressed as a % of the width, hence 'aspect ratio'.)

The stock 245 tire is about 24.7 in diameter, the new one is about 25 in. This gives 831 rev/mile for the new vs 841 rev/mile for the stock.

Try this calculator

Tire Size Calculator - tire & wheel plus sizing

And that is about what I know about this subject!

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roadsta
 

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I have a Excel SS for calculating tire sizes for all my cars.

As you know, increasing tire width doesn't mean you will always get better traction. This is because the weight of the car is the same. A larger patch means lower PSI between the wider tire and the road surface. Rubber compound and groove pattern plays key roles too.
 
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