Simon_CSK Posted May 5, 2019 Share Posted May 5, 2019 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? Quote Link to comment Share on other sites More sharing options...
Eightpot Posted May 5, 2019 Share Posted May 5, 2019 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 🤓 Quote Link to comment Share on other sites More sharing options...
Bull Bar Cowboy Posted May 6, 2019 Share Posted May 6, 2019 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 Quote Link to comment Share on other sites More sharing options...
Escape Posted May 6, 2019 Share Posted May 6, 2019 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. Quote Link to comment Share on other sites More sharing options...
Simon_CSK Posted May 6, 2019 Author Share Posted May 6, 2019 Thanks guys. The speedo can be as much as 5mph over especially at higher speeds. Quote Link to comment Share on other sites More sharing options...
Escape Posted May 6, 2019 Share Posted May 6, 2019 The difference in speedo reading is proportional. 3% at 30mph will be barely noticeable (1mph), at 60mph the difference of 2mph should show. Quote Link to comment Share on other sites More sharing options...
neil110 Posted May 12, 2019 Share Posted May 12, 2019 Worth having a look here as well. https://www.etyres.co.uk/how-to-change-your-tyre-size/ Quote Link to comment Share on other sites More sharing options...
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