Calculating speed with larger rims
Discussion
You need to work out the circumference of your new rear wheels and the old ones. The ratio (% difference) between the two will be the multiplier that you need to apply.
Remember on a low profile tyre the height of the tyre is the percentage of the profile multiplied by the width. ie 205/60 tyre = 123mm high.
The end result will give you the difference between your two setups, however it won't tell you if your speedo was accurate, as it probably wasn't anyway.
If you sit down with the gear ratios and the final drive ratio, then with the above figures you can work out the mpg per 1000rpm. You rev counter is likely to be more accurate than your speedo (but not necessarily so!)
This isn't the be all and end all as for full accuracy you will need to take into tyre inflation, account tread depth and tyre growth (which happens with high speed running)
But at least its a start.
davidy
Remember on a low profile tyre the height of the tyre is the percentage of the profile multiplied by the width. ie 205/60 tyre = 123mm high.
The end result will give you the difference between your two setups, however it won't tell you if your speedo was accurate, as it probably wasn't anyway.
If you sit down with the gear ratios and the final drive ratio, then with the above figures you can work out the mpg per 1000rpm. You rev counter is likely to be more accurate than your speedo (but not necessarily so!)
This isn't the be all and end all as for full accuracy you will need to take into tyre inflation, account tread depth and tyre growth (which happens with high speed running)
But at least its a start.
davidy
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