odometer with 33s
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What is the original tyre size? Work out the circumference and you'll have the percentage.
Tyre circumference = 3.1416 x (rim size in inches x 25.4 + 2 x tyre width x tyre profile / 100)
When you have the figures for both tyres divide the larger one by the other and make it a percentage by removing the whole number and multiplying the result by 100. This percentage directly relates to the error in the odometer, so if your tyres are 5% larger than standard, the wheels rotate 5% less than expected and thus reads 5% fewer km than actually driven, so to get true km multiply your distance apparently travelled by (1 + percentage) - see below for an example.
Example:
2009 D40 standard tyre = 255/70R16 = 3.1416 * (16 * 25.4 + 2 * 255 * 70 / 100) = 2398mm
Slight oversize tyre = 265/75R16 = 3.1416 * (16 * 25.4 + 2 * 265 * 70 / 100) = 2442mm
Percentage = 2442/2398 = 1.018 = 1.8% larger (but remember the 1.018 for below)
If you've travelled 48,500km on these tyres then your odometer is out and you've actually done 48500 * 1.018 = 49,373km.
Using your current tyre (can't do percentage of course) your tyre has a circumference of:
3.1416 * (16 * 25.4 + 2 * 285 * 75/100) = 2619mm
Note that this calculation must be done for each tyre size you've had for the life of the car if you want it to be really accurate. However, even with 5% variation the difference between services isn't a killer.
Tyre circumference = 3.1416 x (rim size in inches x 25.4 + 2 x tyre width x tyre profile / 100)
When you have the figures for both tyres divide the larger one by the other and make it a percentage by removing the whole number and multiplying the result by 100. This percentage directly relates to the error in the odometer, so if your tyres are 5% larger than standard, the wheels rotate 5% less than expected and thus reads 5% fewer km than actually driven, so to get true km multiply your distance apparently travelled by (1 + percentage) - see below for an example.
Example:
2009 D40 standard tyre = 255/70R16 = 3.1416 * (16 * 25.4 + 2 * 255 * 70 / 100) = 2398mm
Slight oversize tyre = 265/75R16 = 3.1416 * (16 * 25.4 + 2 * 265 * 70 / 100) = 2442mm
Percentage = 2442/2398 = 1.018 = 1.8% larger (but remember the 1.018 for below)
If you've travelled 48,500km on these tyres then your odometer is out and you've actually done 48500 * 1.018 = 49,373km.
Using your current tyre (can't do percentage of course) your tyre has a circumference of:
3.1416 * (16 * 25.4 + 2 * 285 * 75/100) = 2619mm
Note that this calculation must be done for each tyre size you've had for the life of the car if you want it to be really accurate. However, even with 5% variation the difference between services isn't a killer.
Thanks old tony. Standard size tyres are 265/70/16 so they have a circumference of 2442mm, 285/75/16 are 2619mm
So 2619÷2442= 1.072 = 1.07% larger
So if iv travelled 600km i multiply that by 1.07
600×1.07= 642km
Is that correct?
So 2619÷2442= 1.072 = 1.07% larger
So if iv travelled 600km i multiply that by 1.07
600×1.07= 642km
Is that correct?
bods
Member
I'd agree with that. 7% further than the odometer reads. Mine with 32s on it travels 3% further than the odometer reads...
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