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Simon_CSK

Tyre diametre question

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My Discovery came with 255 55 18 tyres however along the way I have aquired and now use 255 60 18's I noticed that the speed on the speedo is less than the speed on my GPS and I am more likely to beleive my GPS.

This got me thinking when I refuel, I always run full to empty and always rest my trip, if my speed is out them my mileage must also be out. Given that the rolling diametre of a 55 is less than a 60 I must be covering more ground. The question is how much more? 

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Circumference = Pi x D

You would need to get the diameter of both tyres to compare 

Or put a string round the tyre and measure how long it is 🤓

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2 hours ago, Simon_CSK said:

My Discovery came with 255 55 18 tyres however along the way I have aquired and now use 255 60 18's I noticed that the speed on the speedo is less than the speed on my GPS and I am more likely to beleive my GPS.

This got me thinking when I refuel, I always run full to empty and always rest my trip, if my speed is out them my mileage must also be out. Given that the rolling diametre of a 55 is less than a 60 I must be covering more ground. The question is how much more? 

 

255/55R18 = circumference of 91.2inches and does 695 revolutions per mile

255/60R18 = circumference of 94.3inches and does 672 revolutions per mile

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Meaning your speed is 3% higher than indicated. Assuming it was spot on to begin with, usually when fitting slightly larger tyres you'll find the speedo is actually more accurate than standard, because they all tend to read high.

Likewise, economy will be 3% better than what you calculated

Filip.

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Thanks guys. 

The speedo can be as much as 5mph over especially at higher speeds.

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The difference in speedo reading is proportional. 3% at 30mph will be barely noticeable (1mph), at 60mph the difference of 2mph should show.

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