Diesel Economy

Tyre size and gear choice

Words by Andrew Leimroth

Diesel isn’t getting any cheaper and travelling isn’t getting any lighter, so it’s more important than ever to drive economically. Two simple things have quite dramatic effects on engine power – tyre size and the gear used to tow under heavy loads.

Tyres size and tread pattern play an important role in economy. We have proven on our dyno that even a simple tyre change on a LandCruiser from a 265/70-16 to a 275/75-16 can wipe off 10% power at the rubber wheels.

For example, we need 200 NM of torque to maintain 90km/h on a road at 20% engine load. After installing the only slightly bigger tyres, we lost 10% power at the rubber wheels and produced 180NM of torque for the same 20% engine load. So, to maintain 90km/h with 200NM torque, we put down the foot a little more on the throttle, to offset the loss of power at the wheels, now equating to 30% engine load.

Gear choice when towing is another important factor. Diesel engine torque is magnified by gears. The lower the gear, the higher the torque and slower the wheels spin. The higher the gear, the lower the torque and the faster the wheels spin. To gain one thing is to lose another and vice versa!

For example, you need 200NM of torque to maintain 90 km/h, and in fourth gear you get 200NM of torque at 20% engine load. Change up to fifth gear, and all of a sudden you produce only 150NM of torque for the same engine load of 20%. You begin to slowly lose speed, so you need to depress the throttle pedal slightly more to get 200NM of torque out of the gearbox to maintain the 90km/h, increasing engine load to 40%.

Next time you’re about to go for bigger tyres, consider whether the benefits outweigh the potential effects on economy. More importantly, remember light throttle cruising in a high gear is fine towing a load. But as soon as you start to apply more throttle due to headwinds and hills, think about changing back a gear earlier to keep the throttle load light and let the gearbox do the work instead of the engine. Recently, a 1HZ on our dyno produced 278NM in third gear and 146NM in fourth. You can see gearing dramatically affects pulling power. Next time your vehicle is on a dyno, get the operator to do a third and fourth gear test and show you the results.

Safe Driving

Andrew