Size matters - Changing from stock tyre has consequences

Kevin Baxter, Contributor

Included in our exploration of the tyre last week was tyre identification, where we sought to make sense of its alphanumeric code. This week, we will aim to put this information into sharper focus.

Knowing the recommended tyre size for your vehicle is very important and cannot be overstated. Most motorists may choose to downsize or upsize their tyres. Any alteration in recommended tyre size will result in dynamic variations in your vehicle's fuel economy, mileage, suspension/ride, steerability/stability or even tyre

failure.

There is usually an aesthetic appeal for lower aspect ratio and wider tyres. These low profile tyres will improve high-speed steering response and overall handling, but they reduce ride comfort. A higher aspect ratio with a taller sidewall will be able to absorb more road bumps, providing a better ride, but it is inhibited by a lessened steering response.

Our Jamaican road profile is noted for pothole defects, irregular contouring and sharp corners, hence these factors, coupled with your driving style, must be considered in your tyre selection.

CONSIDER YOUR WHEEL DIAMETER

Another factor to be noted when deviating from your stock/Original Equipment Manufacturer (OEM) recommendation is your wheel diameter. Upsizing your rim size has always been a modification fashion that gets a lot of traction among car enthusiasts. Twinned with this upsizing will be the need for bigger tyres, which increases the overall wheel (rim and tyre) diameter.

Governing the operation of modern vehicles are highly computerised engine management systems which are dependent on input data such as speed, load and temperature. Maintaining accurate speed data to your vehicle's computer determines its information accuracy. Simply put, your stock wheel's diameter is an important factor in the processing of information equation.

Your car manufacturer would have already calculated and predesigned your vehicle's speed output and torque capacity and your overall wheel diameter is a vital component of that design. Considering gear ratios, strictly speaking, one rotation of the engine equals one rotation of your road wheel.

A bigger wheel rotation would be less, with that same single rotation of the engine. Your engine now would have to work harder to rotate the bigger wheel which provides a larger contact point. This extra effort translates into additional fuel needed to propel the vehicle along the road surface. This is usually noticeable in round town stop and go traffic where the vehicle is intermittently moving from a position of rest.

However, if your driving style remains the same, then upsizing wheel diameter will increase your mileage by enabling you to cover more distance, especially at top speeds.

Conversely, if you downsize then acceleration from rest will increase but expect a drop-off in top speed, which will decrease fuel efficiency.

Another factor is that your speedometer display may not be reflecting your actual speed as you may well be travelling at greater or lesser speeds. This may be a point of contention when arguing a possible speed ticket. If the police radar instrument determines excess speed of your approaching vehicle and records it, your speedometer may have misled you into thinking that you were driving within the speed limit.

Load capacity of the tyre cannot be discounted, especially if you carry a lot of weight over long distances. The weight of the vehicle must be supported by the tyre and if you exceed its maximum permissible limit then you must be prepared for any eventuality. That may be tyre damage, personal injury or possible death.

INFLATION ISSUES

Inflation pressures must be keenly monitored, because driving with tyre pressures above or below the recommended value will impact the quality of the ride and fuel economy, as well as compromise tyre life. Some vehicles carry a tyre pressure monitoring system on your dashboard display that alerts of imbalances in tyre pressure.

A normal tyre can lose one psi (pound per square inch) per month. This is important to note, especially as an underinflated tyre generates more heat, wears on its outer edges and vehicle weight is now supported by the sidewall of that tyre rather than the centre of the threads. If that tyre should bump against a curb or road imperfection, the result could be hazardous, particularly at high speeds.

When you check your inflation pressure with a tyre-pressure gauge (do not depend on its appearance) it is best to do it when the tyre is cold (parked for over 3 hours). Your tyre pressure increases when the tyre gets hot, or is driven for over long distances. If tyre pressure is checked when the tyre is hot, it is difficult to reach the recommended cold-tyre pressure and you risk overinflation or underinflation, which may have dire consequences for the tyre or the occupants of that vehicle.

Finally, always inspect and examine your tyre threads, as it will often tell a tale about your tyre's performance.

Kevin Baxter is acting manager of the HEART Trust/NTA Jamaican-German Automotive School.