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Un­der­stand the lingo of your GPS

What is best for tow­ing: a man­ual or au­to­matic gear­box? And is it true that you should never tow in fifth gear? There are a few un­writ­ten rules around a camp­fire: you never, for ex­am­ple, tell a man what rugby team he should sup­port or what car he ought to buy. And if you like your friends (and your front teeth), you will also never start a de­bate re­gard­ing au­to­matic ver­sus man­ual trans­mis­sion – nor one about tow­ing in fifth gear.

THE TORQUE CURVE

Tow­ing is not merely a mat­ter of push­ing the ac­cel­er­a­tor to its limit in or­der to just get go­ing. Even be­fore you jump into the car, there is some nec­es­sary home­work to do: con­sider the torque curve of your tow­ing ve­hi­cle. If you fail to find the curve in the owner’s man­ual of the ve­hi­cle, then pay a quick visit to the in­ter­net and search for it there. Should you re­fer to torque, you need to know at what rev­o­lu­tions (r.p.m) the en­gine of your car func­tions op­ti­mally. For this pur­pose, you get a generic torque curve that re­sem­bles an arch.

Al­ways try to tow in such a way that the per­for­mance of the en­gine falls in the top 20% of this curve. It is also in this per­cent­age sec­tion that the en­gine runs cooler and more eco­nom­i­cally. In the re­main­ing 80% if this curve, the en­gine labours in some way or an­other, whether it is dur­ing ac­cel­er­a­tion or be­cause the revs are too high or too low. (By the way, when not tow­ing, you should look at the up­per 30% of the curve).

WHAT ABOUT FIFTH?

When you look at a five-speed gear­box, the fourth gear is of­ten the one with a 1:1 gear ra­tio (or clos­est to it). In other words, the gear­box might just as well not have been there at all, since the rpms of the gear­box and the driver shaft are the same.

For some un­known rea­son, there is a myth that pur­ports it is not a good idea for the gear­box to tow in fifth. How­ever, there is no log­i­cal rea­son what­so­ever for think­ing along th­ese lines. The fifth gear can­not sim­ply break. A gear breaks when it has to work with­out oil and there­fore over­heats.

The fifth-gear myth prob­a­bly orig­i­nated in the sev­en­ties and early eight­ies when reg­u­lar fam­ily cars were fit­ted with only four gears. At the time, fourth gear was also the 1:1 gear and was the one that you se­lected for tow­ing. Sub­se­quently, a fifth gear was added – the gear was known as the over­drive. (From first to third, the en­gine revs were more than that of the drive shaft. In fourth, it was the same, but in fifth, the drive shaft ro­tated faster than the en­gine – hence the name. Fifth gear was not in­tended to be a pow­er­ful gear but rather one to let your ve­hi­cle drive more eco­nom­i­cally.

Mod­ern-day en­gines how­ever, are so far ad­vanced and so pow­er­ful that your ve­hi­cle can eas­ily tow in fifth. In some of the sixgear boxes, fifth can be the 1:1 gear, but it is no longer a given that a gear­box has a 1:1 gear. It may be that the ra­tio of any gear is slightly higher or lower.

AUTO BOXES

In prin­ci­ple, an au­to­matic and a man­ual gear­box work ex­actly the same. Ex­cept for you hav­ing to step on a clutch with a man­ual trans­mis­sion (an au­to­matic gear­box has a mech­a­nism in­side it that does ex­actly the same job), the gears are iden­ti­cal. Some man­u­fac­tur­ers ac­tu­ally rec­om­mend us­ing an au­to­matic gear­box when you want to tow any­thing heavy. An au­to­matic gear­box also helps in­ex­pe­ri­enced tow­ers to con­cen­trate on their driv­ing since they don’t have to worry about chang­ing gears.

GO­ING DOWN­HILL

Go­ing down­hill is fine for coast­ing, but re­main in gear. Peo­ple err by se­lect­ing lower gears when go­ing down a steep hill. Wear and tear on the gear­box is then the same as when you’re ac­cel­er­at­ing. Rather use the brakes to con­trol your speed. That’s what they’re there for, af­ter all. Keep the en­gine revs within the 20% zone when go­ing down­hill and, should the go­ing get steeper, re­duce speed by ap­ply­ing more brakes and gear­ing down sytem­at­i­cally. Do not take the car out of gear when go­ing down­hill. A ve­hi­cle weigh­ing 2 tons ac­cel­er­ates faster that a rac­ing car over a dis­tance of 100 m and at a slope of 30 º .

If there’s an up­hill stretch af­ter the down­hill, you’ll want to se­lect the lower gear at the right time to climb the hill. Shift down at the bot­tom of the hill so that you reach the max­i­mum revs in the 20% zone of the torque curve (for in­stance 2 280 r.p.m in the pre­vi­ous ex­am­ple). As you lose speed while as­cend­ing the hill, keep gear­ing down ac­cord­ing to the 20% prin­ci­ple.

CHOOSE THE SAME TYRES

The size of the tyres on your ve­hi­cle has an im­me­di­ate ef­fect on the gear ra­tio of the gear­box rel­a­tive to the speed at which you are driv­ing.

When re­plac­ing tyres, it is im­por­tant to keep the cir­cum­fer­ence of the new tyres within 10% of the orig­i­nal mea­sure­ments. (It is best to stick to the orig­i­nal di­men­sions as the man­u­fac­turer in­tended.)

If, for ex­am­ple, you change from a 17” rim to an 18” one and the 17” tyre had a 75 mm pro­file, you will now have to fit a 60 mm pro­file tyre on the 18” rim. Some man­u­fac­tur­ers’ war­ranty be­comes void if you fit tyres ex­ceed their spec­i­fi­ca­tions and even some in­sur­ance com­pa­nies will refuse to hon­our your claim if any­thing goes wrong be­cause of this.

STAY COOL

Re­mem­ber, not all cars are de­signed for tow­ing. When you tow, there’s an ad­di­tional bur­den on the gear­box, which can cause it to op­er­ate at higher tem­per­a­tures. It is good for the oil and the gear­box if the oil re­mains cool. If you sus­pect that your gear­box is over­heat­ing, you can al­ways con­sider fit­ting an oil cooler.

It’s also im­por­tant to re­mem­ber that oil ages. If you of­ten tow a car­a­van, you should to re­place the gear­box oil ev­ery 5 000 km.

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