In 2007, I bought a new Kia Sportage AWD 2,7 V6. The vehicle was deliver-ed with 235/60 R16 tyres on 16-inch wheels. The cost of fuel at that stage was approximately R7,00 a litre. The vehicle is not driven off-road and occasionally tows a light luggage trailer. It seldom carries more than two people and the speeds I travel are legal. I currently achieve an average fuel consumption figure of 12,0 litres/100 km. With the current cost of petrol, I am considering options to improve my vehicle’s fuel efficiency. The idea is to fit 225/65 R17 tyres on 17-inch wheels to increase the overall diameter by a small percentage. Please evaluate my proposal and advise whether it will be:
• Physically possible to fit the larger-circumference tyres into the vehicle’s wheel wells;
• Compatible with the engine-management system of the vehicle;
• And compatible with the automatic transmission?
Also, what speedometer correction will be necessary and how will the Kia’s power and torque outputs be affected?
The vehicle has performed admirably and is a pleasure to drive. New vehicles are prohibitively expensive and therefore I am hoping that my compact SUV will serve me well into retirement.
PETER SWANEPOEL, Via email
Answer: Changing the wheel diameter is not guaranteed to save you fuel (it might actually increase the fuel consumption) and therefore there is no point in discussing the possible impact on the vehicle. What you will essentially do is alter the gearing of the vehicle (making the gearing longer). If you change gears at the same r/min as before, you will be doing greater speeds or, when you cruise at 120 km/h, your engine speed will be lower.
Remember that the engine will still have to propel the same mass as before, therefore delivering the same power. The only way you will save fuel is if the power is delivered more efficiently at lower revs (assuming you drive at the same speeds as before).
This is not a foregone conclusion, however. When engineers calculate the gearing of a new vehicle, they take the required performance requirements into consideration as well as the brake-specific fuel-consumption (BSFC) map for the engine. This map looks at the specific fuel usage of the engine versus engine speed and load. This displays areas of constant fuel consumption as contours on a map. The idea is to keep the operating point of the engine close to the optimum region (least fuel being used).
By altering the gearing, you might fall outside this optimum region and use more fuel.
In our opinion, the extra costs of the wheels, recalibration of the speedo, effect on the braking system, spatial constraints and so on will not be worth the effort. Remember, driving style has the greatest influence on fuel economy. Whenever you need to replace your vehicle, consider a smaller, lighter model.