Unique Comparative Tyre Calculator

Now you can play about with wheel and tyre combinations to see which will work before you rush out and buy.

This calculator lets you compare wheel and tyre combinations to see how they will affect a variety of factors shown below.

The amount of effect will be proportional to the amount of difference to "standard" and whether the new wheel/tyre combination has a larger or smaller overall diameter than "standard".

A word of caution: Car manufactures spend millions on getting things right for Mr/Mrs Average. Major changes to rolling diameter are rarely a good thing. It is usually best when choosing new wheels and tyres to try and end up with the same overall diameter. In the Calculator, this is achieved when % Difference is zero.

Also, don't forget, this won't tell you if those extra wide tyres will fit in the wheel arch!

The features listed below will be marginal if the % Difference is in single figures and you may not notice them.

Acceleration and

Fuel economy

Smaller overall diameter will give better acceleration. It will mean higher revs, more fuel, possibly more noise and wear. Top speed may be reduced if max revs are reached too soon.

Larger overall diameter will give poorer acceleration. It may make for fuel efficient, relaxed cruising, but you may need to change down to overtake. Top speed may be higher, or lower if the drag overcomes the reduced torque.


Smaller overall diameter will give a reading which is too high. You will appear to be going faster than you are.

Larger overall diameter will give the opposite effect.

Traction control

My BMW 8 Series has different front and rear tyre sizes as standard.

If your car has traction control and you get your front and rear comparative sizes wrong you can really upset the traction control (or whatever your manufacturer calls it). Some systems can detect very small changes, so you want a % difference as small as possible.


How to use the Comparative Tyre Calculator:
(Make sure your browser has not blocked sites from running script)

You have original tyres which are 225/50x17. This information will be on the tyres.

This means they are 225mm wide, have an aspect ratio of 50% and fit wheels which are 17 inches in diameter.

Enter the 225, 50 and 17 in that order into the spaces in the top row of the table.
This forms the "standard" to which any other tyre sizes you try will be measured against.

Then, in the next row of empty spaces, try out different sizes.
Press the "Calculate" button to update the results.
Always enter the width, followed by aspect ratio and then wheel diameter.

You can compare several combinations at once to make clear comparisons and judgements.

Widths vary in 5mm steps and Aspect Ratio varies in 5% steps. Not all manufactures make all sizes in all speed ratings.


What do the numbers mean?

% Diff - The percentage difference in terms of rolling circumference of the tyre compared to the original. A minus tells you the tyre has smaller diameter and will turn faster at a given speed.

"70"mph - The figure here is what your speedometer will tell you when you are going at 70 mph. A bigger number tells you the tyres have smaller diameters and go round faster than normal. A smaller number tells you the tyres are taller and go round more slowly than normal. The ideal number to get here is 70mph - unless you are are deliberately trying to alter the "gearing" of the car.

Comparative Tyre Calculator - remember to press the "calculate" button below the table!