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Tyre sizes vs Speedo reading error


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So I'm more or less just thinking out loud here, trying to figure out if my thinking and calculations are correct. Someone else may have already thought about this as well.. who knows.

 

Driving my car, when the Speedo reads 100, my phones GPS actually gives me 93kph. The current wheels & tyres are different to the factory options, and when I use this calculator: http://www.tyresizecalculator.com/tyre-wheel-calculators/speedo-error-calculator

 

And I enter the factory wheel / tyre specs vs what's on the car now, it says it now reads 99 when it should be 100 so the current difference is negligible. So for all intents and purposes, I'm using 100kph and it would still be underreading by 6kph instead of 7.

 

So, I want to try and figure out what tyres I should use on my new car, to try and fix the speedo difference and get a more accurate reading. When I'm using the above calculator, there is this sentence: "Speedometer reads xxx, but the actual speed is xxx"

 

So, when I'm wanting to compare different size tyres from the original to new, I should ACTUALLY put 93 into the "Speedo reading" box, rather than 100. Correct? Then it will give me a comparison reading based on the GPS'd 93kph / Speedo 100kph.

 

An example:

 

If current tyres 215/50/17 gives me a reading of 93kph, then comparing that to a 245/40/19 gives me a new reading of 98kph.

 

So if my current speedo at 100 gives me a GPS speed of 93, by changing to the above new tyre it would then give me a GPS speed of 98.

 

Or changing to a 245/45/19 gives me a new reading of 101kph.

 

Does that stack up? Comments?

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  • General Member

Used to be able to use a wee Jaycar device to correct speedo's but yeh not sure if the tire size calculator here : still works

 

but yeah built in 'safety feature' cars since the 70's have been Govt controlled factory 10% out (lower)

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Most manufactures program in a small under reading of the speedo head ( so there is no comeback on them for speeding offences) most are 10% under. 

If you get the speed reading from what the ecu sees you will find it is pretty smack on with your gps reads ( as long as you are running a similar size tyre to factory) 

 

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