WIDER OR LARGER TIRES ?

One method of improving tire grip is to increase the contact area between the tire and the road. This can be accomplished by fitting a tire with a larger circumference (i.e. a larger or oversized tire) or fitting a tire with a wider thread area. A tire with a larger circumference has a larger longitudinal contact area, which will provide better straight line stability but will affect overall gearing and will increase the ride height and the car's center of gravity, and will also increase the rolling mass the bakes will have to overcome. There is nothing that can be done to rectify the increased ride height and center of gravity; but the drivetrain gearing can be rectified by fitting a differential with a higher diff ratio. The increased rolling mass that the brakes need to overcome can be rectified by upgrading the brakes. However, even though a larger tire may be able to fit under the wheel arches when the car is static, it may knock against the body or suspension components when suspension travel occurs and during cornering. Stiffer suspension springs with less travel and stiff shock absorbers may compensate for the tire knocking against the body during suspension travel but will result in a harder ride.

Wider tires, on the other hand, have a wider contact area but will need to have a smaller aspect ratio to retain the overall circumference of the wheel. Thus going from a 185/60 R 14 91H tire to a tire that is 215 mm wide will require a tire with an aspect ratio of 50% as a 185/60 R 14 91H tire has a sidewall height of 111 mm (60% of 185 mm) while a 215/50 R 14 91H will have a sidewall height of 107.5 mm. A tire with a similar overall circumference to the standard tire will not interfere with the body or suspension parts during suspension travel but will be slightly heavier, and will increase the rolling mass that the brakes need to overcome, but not to the same extent that larger tires do. However, wider tires are also prone to knocking against the body or suspension parts during cornering. This can be overcome by fitting a wider wheel arch and fitting a spacer on the wheel hubs to allow the wheels to be mounted further away from the suspension parts. This will, however, place greater stress on the front wheel bearings and on the steering mechanism.

Source: http://www.custom-car.us/safety/tires.aspx

Tire Upgrades

Tires and shock absorbers play an important part in road grip, and it is road grip that ultimately allows the car to be accelerated and decelerated while also providing predictable handling and stability. As such road grip in an important element in car safety. Upgrading your brakes, for example, is a useless endeavor if your car has little grip due to worn tires or worn shock absorbers. This is obvious as worn tires with low grip may lock up and skid when you apply brakes, and the wheels need to be able to rotate to slow the car down. Similarly when the tires are bouncing through the air when you brake over bumps, road grip is lost and braking becomes ineffective.

A simple safety precaution would be to check your tires' thread depth and tire wear, but on modified cars with extra horse power, the grip provided by standard tires may not be sufficient even if the tires are in good condition. On modified cars you may want to fit larger or wider tires that provide a larger contact area with the road, or low profile tires that have shorter, less flexible sidewalls and provide more precise handling. You may also want to fit a tire that has a higher maximum speed rating and higher load rating. If you do opt for larger or wider tires, you will need to upgrade your shock absorbers (or dampers as they are also called) and possibly your springs to handle the increased unsprung weight.

But first you need to know what tires your car has, what tires are suitable for your car, and the effect of changing your tires.

Source: http://www.custom-car.us/safety/tires.aspx