Changing wheel size

In message , Paul writes

If you lower the aspect ratio of the tyres then there will be minimal change to the rolling radius. If you don't change the aspect ratio then there will be negative effects of gearing and speedo reading.

More info here:-

Cheers Paul, particularly for the link.

If you fitted larger wheels and pro rata tyres you'd raise the gearing. Generally this results in better economy. But difficult to check because the odometer would now give an incorrect reading.

Thanks Dave, you set me thinking. The last time I changed some wheels on a car, I went from 13" to 15" and it was the first time since I had owned the car that the speedo reading tallied with my satnav, previously the car was always showing about 4mph faster.

As my Megane speedo shows a similar difference, can I expect the same result?

Thanks,

I've used my Tomtom in several cars and it never agrees with the MPH. The car always reads 2-3 MPH faster. I'll bet the Tomtom is correct.

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Don't forget to check your insurance. Changing wheels may be regarded as a 'modification'.

Cic.

Be easy enough to work out if you can find out the rolling radius of the various wheel tyre combinations.

even if the speedo reads correctly after fitting different wheels the mileometer won't read right.

It won't read the same, but surely it may be closer to reality?

Chris

The mileometer is usually quite accurate because it is directly pulse or cable driven, whereas the average analogue speedo has a flappy needle which is safely adjusted to overread. so putting big wheels on will often make the speedo closer but put the (previously accurate) mileometer out.

I'll take your word for it in general but it's not universal. BMWs over read in both speedo and odometer. Buy roughly the same amount.

Yep, that's what prompted my question; both the Focuses I have had were

Chris

not what I have seen, so obviously whatever is done it will be worth checking on a motorway journey with a satnav as to accuracy. and in this case YMMV !!

Look....The easiest way to change the overall gear ratio is to change the gears in the differential. In some cases, aftermarket suppliers offer the necessary parts and often the vehicles' manufacturer has an excellent selection available. If you are on a budget, a little research will probably reveal that your vehicle was available with a number of differential ratios....

Nah, bigger circumference wheels/tyres are easier :)