October 01, 1987

Straight Torque - Gearing and Tyres

OK trendsetters. So we've looked at Power and Torque, and we've spun it up on the dyno, and we've had a bit of a rave about what it all means. But hang on, all that P & T (let's just call it energy) is still up in the driveline somewhere. It doesn't do any good till it gets out onto the road, and that happens through the wheels.

You may have heard people rambling on about gear ratios. Very briefly, when the energy comes out of the motor it has to pass through several gear ratios: the gearbox, the transfer box, the diff centres and the tyres. (yes, that's right, the tyres!) Have a look at these figures for the 1300 Sierra. (Other models, just check the back of your workshop manual somewhere)
  • Lo range 2.268
  • Hi range 1.409
  • Diff Centres 3.909
  • 1st 3.650 
  • 2nd 1.946 
  • 3rd 1.422 
  • 4th 1.000 
  • 5th 0.793 
Now it's very simple. If you want to know the final gear ratio for an give gear combination, you simply multiply the three appropriate figures together.

For example, take a look at your main gear for 4WDing which is low range first gear (usually just called low-low). It would be 3.650 (first gearbox gear) X 2.268 (low transfer gear) X 3.909 (diff centre) = 32.827. Don't take my word for it… check it yourself.

The same goes for any other combination of gears … gearbox X transfer X diff = gear ratio. OK, so you might be thinking this is a really useless piece of information, and you might be right. Until you start comparing it with a few other 4WDs and asking a few question. We'll stick with low-low for a moment. Look at this…
  • Audi Quattro 14.000
  • Subaru Sportswagon 21.996
  • Suzuki Sierra 32.827
  • Mitsubishi Pajero SWB 35.440
  • Landcruiser Sahara 39.023
  • Toyota Hilux 43.583
  • Landrover 110 Diesel 47.837
  • Mercedes Benz 300GD 48.634
Notice anything? The really capable 4WDs all have one thing in common - a really low low-range gear ratio. They are all around the 40 to 50 range. By the way, gear ratios work back to front, the bigger the number, the lower the gear. Conversely, vehicles that are not generally regarded as "serious" 4WDs like the Audi Quattro and the Subaru, have quite high gearing (lower numbers).

That should immediately tell you something. Having a decent low range gearing is a very important characteristic if you intend to use your vehicle for "serious" offroad driving. So where does the Sierra fit in then?

At 32.8:1, it is the worst geared of the whole bunch if you don't count the Audi and the Subie, which are really just passenger car derivatives and not real 4WDs. Now, generally speaking, most Suzuki owners can get away with the gearing as it is because of their applications for the vehicle. Let's face it, the number of people you will find that use their Suzukis to go to Wirraba Ridge or Heartbreak Hill are in a definite minority. And because the vehicles are so light, and the motors are really quite efficient for their size, even the diehards can get away with the standard gearing in most situations.

But here is a problem to think about. The Sierra comes standard off the showroom floor wearing LR78/15 tyres. When Joe Public picks up his new Suzy and starts to think about modifications, he generally wants to start with different tyres (you know, rooly big ones!), and so he heads down to the local tyre dealer (who spends most of his time working with passenger sedans, so often doesn't think about this stuff), and gets a set of 10Rs fitted. They look great, but when the guy drives it out on the road, he suddenly wonders why it doesn't go fast anymore. It gets along ok on a flat road, but then dies in the arse at the first sign of a hill.

What this unsuspecting chap has actually done is to raise all his gear ratios. The original tyres were about 28 inches tall; his new ones are about 31 inches tall … that's about a 10% increase in all of his gear ratios! The poor little motor was never designed to pull that around… it simply doesn't develop the amount of torque needed to keep the wheels turning. Even if it did, when you went off road, you would have to do everything at far too great a speed just so the motor wouldn't stall. It would be like driving everywhere in high range.

The final result of all this is that it is very unwise to go to a much bigger tyre (in diameter) unless you are prepared to do some serious modifications to the rest of the drivetrain to bring the final gear ratio back to where you started from (or lower!) If you're not prepared to do these mods, and you still want your car to perform well, then don't use any tyre bigger than a 215/75-15, or about 28.5 inches in diameter.

On the other hand, if you really don't care how much of a dog it is to drive, and you don't mind using second gear to climb hills, and the farthest offroad you get is your front lawn, but you don't care because at least it looks mean, then go for it. Try Unimog wheels.

Chris Betcher
October 1987