If you are thinking of changing your wheel and/or tyre sizes, it is required to maintain the same diameter (rolling circumference) that is as close as possible to the original. This is so that the gearing of the vehicle is not adversely affected. If you can do this it will also ensure that the ride height will not change significantly and that your speedo will remain accurate. However if you would like to increase your gearing, because of a low geared vehicle and to increase your fuel economy, you can increase the profile of your tyres. Just ensure that you have enough room for your new tyres in the wheel arches.
Just remember that your speedo will not remain accurate, you can get them recalibrated or be aware that you may be driving at a higher speed.
Please check out New Tyres Quick tyre size conversion chart for more details on which tyres will covert.
You will need to check...
on the outside of your wheel arch with the suspension fully dropped
the inside of your wheel arch,
on full lock when driving
The latter two can be resolved by wheel spacers by bringing your wheels out a little. Longer bolts may be required.