Speedos are set to show a higher speed than you are actually travelling. By increasing the wheel size, your speedo will show a slightly lower value than before which will, in fact, be more accurate than what it is currently showing. So I would recommend that you dont calibrate anything and enjoy the increased accuracy.



If you do decide to have it calibrated anyway, please let me know where you finally ended up having it done.

Originally Posted by Grease Monk

Speedos are set to show a higher speed than you are actually travelling. By increasing the wheel size, your speedo will show a slightly lower value than before which will, in fact, be more accurate than what it is currently showing. So I would recommend that you dont calibrate anything and enjoy the increased accuracy.





If you do decide to have it calibrated anyway, please let me know where you finally ended up having it done.

14 to 15" is a big change in tyre perimeter.

So overal ODO will be wrong only. Total distance travelled will be much higher than the ODO. So I think this must be corrected.

And the error in Speedo and ODO is normal.

You have the science all wrong.



When you increase from 14" to 15", the tyre profile reduces(or it should reduce, if you upsize properly). So, the rolling circumference of the tyre remains the same anyway.

For argument's sake, I'm ignoring tireboi's valid comment.



The change in speed due to the diameter increase is proportional to the ratio of the diameters. D2\D1 = 15/14 = 1.0714



This is a 7% increase in speed for the same RPM of the tire\engine.



Stock ODOs have an inbuilt error of around 5-10% (possibly even higher in bikes). The tire change will, therefore, show you a more accurate speedo reading.

Originally Posted by Grease Monk

For argument's sake, I'm ignoring tireboi's valid comment.

Sorry to be pedantic, but it's TYREBOI. We're followers of the Queens English, right?

Originally Posted by Nikhil

You have the science all wrong.



When you increase from 14" to 15", the tyre profile reduces(or it should reduce, if you upsize properly). So, the rolling circumference of the tyre remains the same anyway.

I am not good at Auto stuff. What I thought is if we change wheel size, then the tyre size also increases.

So here, even the rim size increases, the tyre radiaus remain the same? If yes, why somebody increase the rim size?

Check out this Tyre size calculator!



It shows ODO and speedo errors depending upon the upgrade from stock wheels - Tire Size Calculator - tire & wheel plus sizing



Some other "Must Read" info - Car Bibles : The Wheel and Tyre Bible Page 1 of 2

Originally Posted by anandkrishnantc

So here, even the rim size increases, the tyre radiaus remain the same? If yes, why somebody increase the rim size?

Yes, as Nikhil said if upsized properly the overall circumference stays similar. If the difference was as big as you thought, you will have more worries than just the odo error.



Reasons for increasing rim size - looks and performance. In India only the former is valid

The best way out is to put a GPS in your car and then write down the real values of speed difference in the GPS and the cars speedo and odo.

then make a mental note of it and you will be fine after some time. it will get into your habit.

There is a tyre size calculator here



www.tyrebibles.com



Plan your upgrade accordingly

Originally Posted by sohail99

Check out this Tyre size calculator!



It shows ODO and speedo errors depending upon the upgrade from stock wheels - Tire Size Calculator - tire & wheel plus sizing



Some other "Must Read" info - Car Bibles : The Wheel and Tyre Bible Page 1 of 2

Nice infor over there...thanks.

I decided not to change the tyre. The steel will be replaced with alloys.

But again there is some complications. alignment of the hub and the alloy centre postions......Normally this will be equal to existing steel rim?

Originally Posted by Nikhil

Sorry to be pedantic, but it's TYREBOI. We're followers of the Queens English, right?

Forgive my insolence m'lord