Tyre Size Myth Busters: Does the Wrong Tyre Size Ruin Your Fuel Economy and Suspension?

 

I. Introduction: The Temptation of Bigger Tyres 

In Kenya, there's a strong temptation to fit larger, wider tyres or higher profile tyres (e.g., changing from a $15 \text{ inch}$ rim to a $17 \text{ inch}$ rim) to improve a car's look or better handle rough roads. While a higher profile tyre can help absorb potholes, choosing the wrong size is a costly mistake that directly impacts your car's fuel economy, suspension life, and even your speedometer's accuracy.

This article dispels common myths and shows you, using simple calculations, the precise mechanical and financial damage caused by incorrect tyre sizing on your Japanese import.

II. The Mechanical and Financial Damage of Incorrect Tyres 

The only correct tyre size is the one specified by the manufacturer. Deviating from it, especially by increasing the Overall Diameter, causes a cascading list of problems.

1. Ruined Fuel Economy

• The Problem: A larger overall diameter means the engine has to turn a heavier object. For every revolution of the axle, the car travels a longer distance. This requires more Torque and forces the engine (and transmission) to work harder to maintain speed.

• The Cost: This hard work translates directly into a $5\%$ to $10\%$ increase in fuel consumption. Over a year, this can be tens of thousands of shillings in wasted petrol.

2. Damaged Suspension and Steering Components

• The Problem: Wider tyres and heavier rims place massive, unintended strain on the suspension system, especially on the hub bearings, ball joints, and steering rack.

• The Cost: In Kenya, where suspension parts already suffer from bad roads, oversized tyres accelerate wear, leading to early failure of expensive parts like the steering rack and shock absorbers, often costing KES 30,000+ to repair.

3. Inaccurate Speedometer and Odometer

• The Problem: The car's computer is calibrated to the Original Equipment (OE) tyre's overall diameter to calculate speed and distance. If you use a tyre that is $5\%$ larger in diameter, your speedometer will read $5\%$ too slow.

• The Risk: You will unknowingly be driving faster than you think, risking a speeding ticket or accident. Your odometer will also record a lower mileage than you actually drove, which can be an issue during servicing.

4. Rubbing and Body Damage

• The Problem: Tyres that are too wide or too tall can rub against the inner fender lining or the suspension components during tight turns or when the car hits a dip.

• The Risk: This rubbing can quickly destroy the tyre side wall, leading to a catastrophic blowout.

III. Simple Math: How to Change Tyre Size Safely 

If you must change the size (e.g., upgrading from a $14 \text{ inch}$ to a $15 \text{ inch}$ rim), you must ensure the Overall Diameter remains almost exactly the same—known as Plus Sizing.

Tyre Size Breakdown: $195/65\text{R}15$

• 195: Tyre Width in $\text{mm}$

• 65: Aspect Ratio (Sidewall Height is $65\%$ of the $195 \text{ mm}$ width)

• 15: Rim Diameter in inches

The Safe Change Principle (Keep the Diameter Constant)

To maintain the same overall diameter when you increase the rim size, you must reduce the Aspect Ratio (the middle number).

Old Size (Example)	Rim Size	Aspect Ratio	Tyre Width
$205/65\text{R}15$	$15 \text{ inch}$	$65$	$205 \text{ mm}$
Safe Change	$16 \text{ inch}$	$55$ (Reduced)	$205 \text{ mm}$ (Constant)
Safe Change	$17 \text{ inch}$	$45$ (Further Reduced)	$205 \text{ mm}$ (Constant)

• The Sidewall Trade-Off: While going to a smaller aspect ratio (like a 45) allows you to fit a larger rim, it gives you a thinner rubber sidewall. On Kenyan roads, a thinner sidewall is more prone to rim damage from potholes.

Conclusion: For the sake of safety, fuel economy, and lower maintenance costs in Kenya, stick as close as possible to the manufacturer's original tyre size. If you want a slightly higher sidewall for comfort, ensure the overall diameter is within $\pm 3\%$ of the original.