Many motorists are tempted to change wheel and tyre sizes, but it is often done for the wrong reason. It’s only worthwhile if you want to improve a car’s appearance. It will seldom affect a car’s performance or fuel consumption because the percentage change in the overall gear ratios is usually very small...
Furthermore, if your car is equipped with ABS brakes or 4-wheel drive such a change is not recommended. An ABS-enhanced braking system is calibrated to respond to wheel speed information obtained from a sensor at each wheel. This implies that any deviation from the factory-equipped tyre size and aspect ratio will upset the calibration. In fact, it is even possible that a badly worn tyre will upset the calibration and render the ABS ineffective.
With a 4-wheel-drive vehicle, another problem arises. Non-standard tyre sizes and worn tyres cause the small gears inside the differentials to work a lot harder than normal. The viscous couplings or multi-plate clutches on some part-time 4WD vehicles are especially at risk because they’re not designed to slip all the time.
On most less exotic vehicles a tyre and wheel size change is perfectly feasible, provided you...
Keep the following aspects in mind:
1. A number of tyre companies make a range of tyres that enable you to change in one-inch steps from 13 to 17-inch diameter wheels while keeping the running circumference of the tyres more or less the same. This is done by changing the aspect ratio.
2. Dividing the section width (the widest part of the tyre) into the section height (the distance from the rim to the tread) will reveal the tyre's aspect ratio. For example, if the height is 120.25 mm and the width is 185 mm then the aspect ratio is 120,25/185 = 0.65. This is usually expressed as a 65-profile tyre, or a 65% aspect ratio tyre. A smaller ratio implies a shorter sidewall and a wider tread. Tyre designers increase the width on low profile tyres to maintain a suitable contact area.
3. Aspect ratios have changed over the years from 60 to 50 and now even 45, 40 and 35. Shorter ratios lower the ride height and widen the tyre so that there is less flexing during cornering. This is supposed to improve tyre grip, but the tyre is less able to absorb road shocks.
4. Tyre circumference determines the speedometer and odometer (distance travelled) readings. Any change will affect these readings as well as the shift points on automatic transmissions, so that it makes sense to change the circumference as little as possible.
5. Larger diameter tyres will rotate at a slower speed compared to the vehicle speed, so that the speedometer and odometer will under-read. A smaller diameter tyre will have the opposite effect.
6. Any proposed tyre/wheel change must be planned in consultation with a tyre dealer. He will have a chart that will enable suitable wheel and tyre combinations to be planned, and he will also steer you way from changes that will cause the bigger tyres to interfere with the bodywork or suspension.
Watch the following video for more information: