You have mentioned that fitting bigger tyres affects the gearing of a car. If the gearbox ratios stay the same, what do tyres have to do with it?
In wondering how to best answer your question - for readers who may not be mechanical engineers and might not have enjoyed physics classes at school – I asked 20 diverse motorists whether their cars were fitted with a “speed selector”. All (!) of them said “No”. I advised them that all their cars certainly did have speed selectors, and asked them to guess what this component might be and do? The consensus answer was “something to do with the accelerator pedal” or part of a “cruise control system”. Sorry, wrong again.
Speed Selector was the first (and initially the only) name given to what English speakers now call the…“gear lever”, or the “gear shift” in whatever language it is that Americans speak; but then they also have hoods and trunks instead of bonnets and boots (all equally guilty of confusing cars with clothing).
Car tyres.
The French, bless them, have remained steadfast. Their name for gearbox is “Boite de Vitesses” (the box of speeds) and even the Italian name includes the word “Velocita” (aka velocity or speed). Ironically, the real origins and meaning of gearing can best be found on bicycles! Bikes with five gears are, rightly, universally referred to as “5-speeds”.
Balancing power and pace
For a driver to get the right balance of power and pace in any and all circumstances, he must be able to select the speed… of the engine! It is that which enables and governs first movement from a standstill, accelerating to higher “road-speed”, and maintaining a desired road-speed whether the vehicle is going uphill, downhill or along the flat. Because it is the speed of the engine that delivers the right amount of power to do all those things. And it is engine-speed limits (max revs) which constrain the car’s potential.
If an engine is to deliver power (to do work) it must also be fed, and the driver serves the necessary meal size with a spoon called the “accelerator pedal,
To make all this more understandable to non-techies, the principles are not unlike the “revs” of your legs when you walk. Up a very steep hill you will take lots of steps (high revs) but they will be very short steps (first gear), the distance you cover in one minute will be less so your road speed will be low and your energy consumption (fuel) will be high. Walking, or even running, along the flat, each stride will be longer, and even if your leg speed is lower your road speed will be higher, and your energy use will be less.
Bicycle riders will have the best “feel” for these principles, because, without an engine, they have to do the work themselves. Gears make riding up hills easier, and riding down them faster. The riders scoff energy bars and water as they go along.
If an engine is to deliver power (to do work) it must also be fed, and the driver serves the necessary meal size with a spoon called the “accelerator pedal”.
Gear shift lever.
Next, although the power of car engines is rated by a specific number of brake horsepower, that is the most power each can deliver; it is not what they produce as soon as you turn them on. Most car engines “idle” at between 700 and 1,000 revs (which delivers very little power) cruise easily at 2,000 to 3,000 when power demand is moderate, reach and maintain peak power between 4,000 and 5,000 revs, and start to pant at anything near 6,000. Above that they are likely to have a heart attack.
Staying in the power band
To keep power plentiful but not stressful or unduly hungry, around 3,000 revs is a broad median. And drivers (or automatic control systems) will use their “speed selectors” to keep their engines at between 2,000 and 4,000 at almost all times.
Most gearboxes have five gears, which means there are four gaps/steps between gears. Gear ratios are selected to make the transitions smooth, so the driver never needs to go outside the median bounds before or after changing up or changing down.
Car manufacturers choose the ratios of each gear (combined with the final drive differential gearing) and the gaps between them very carefully, taking account of each particular engine’s power-band characteristics, the weight of the car, its likely usage patterns and passenger/cargo loads, and so on…and the circumference of its tyres! That “size” determines the “distance” element in road-speed, “work” and “power” equations.
At a given rev level, each rotation of the engine delivers a specific amount of power. So if you need more power at the wheels you have to rotate the engine more often for each wheel rotation. The original gearing is designed in that knowledge and with essential regard to the original tyre circumference. If you change that circumference, you change that equation.
So if you fit a different tyre size that changes its circumference (tyre width is not a direct factor) by, say, 10 per cent, you will interfere with the original gearing balance of ratios and gaps. To retain the original balance of revs to wheel rotations, you would have to change the gearbox and final drive ratios by 10 per cent too.
The original gearing might typically give about 20 engine rotations for one wheel rotation in first gear and about five in fifth gear, with the other gears and the spaces graduated in between. For practical reasons, the gaps between first and second and between fourth and fifth are usually smaller than the others. If the gearbox and final drive cogs are left untouched, a tyre circumference about 10 per cent bigger will change those figures to about about 18 and 4; a 10 per cent smaller circumference would change them to about 22 and six. Each gear will be “longer” or “shorter” in effect than it used to be. It may be initially confusing that what we call “lower” gears have “higher” ratios (and vice versa for what we call “higher” gears; so think of gears as “longer” or “shorter” to avoid that semantic booby trap).
In effect, if tyre circumferences are changed, every gear will be slightly longer or shorter than the original. Longer gears are less powerful for acceleration and steep climbs, but allow slightly lower revs at given cruising speeds and, at max revs, a slighter higher top speed. Shorter gears give brighter acceleration and more climbing zest, but higher (fussier) cruising revs and a lower top speed. The speedometer and odometer won’t tell you that – they read the gearbox – so you will have to mentally adjust the dashboard readings by the relevant percentage (or use a GPS to get the truth) in calculating actual speed, distance and things like fuel consumption. Careful with speed limits. When the speedo says 100, might actually be doing about 110.
Overall, there are pros and cons, not rights and wrongs. Do what best suits your particular needs and preferences – bigger, same or smaller tyres. But make any change moderate. The consequences of a tyre-size change are direct, noticeable and inescapable, and that hopefully answers your question on what tyre sizes have to do with gearing.
Picture: Picture of gear lever, picture or large tyre and small tyre, picture of gearbox, picture of a rev counter/speedo dial. These are the main elements of the message.
Correct points for jacking and towing
How do I find the jacking points, towing points and greasing points on the underside of my car? Joff.
Jacking points are parts of the floor that are strong enough (or specifically reinforced) to support the weight of the car on a small trip of a jack without bending or collapsing. They are commonly placed at the extreme front and back of the main chassis rails (very strong steel beams), and amidships where the chassis rail meets reinforcing cross beams near the door sills.
Cars can also be jacked up on their beam axle (amply strong enough, but curved so be careful about slipping) or on strong suspension elements like wishbones (beware slippage here, too, because wishbones will change angle as the car lifts.
Position the jack so its upper pad will be at right angles to the wishbone surface when lifting is at full height.
Towing points will be at least one strong hook or steel loop under the front bumper (usually to one side) and similarly under the back bumper. Except in emergencies or odd circumstances, and always with special care, do not tow by attaching to any other part of the car.
Grease points
Many grease points that used to have nipples to make them regularly serviceable are now replaced with “sealed-for-life” joints. Serviceable joints that still have nipples are most commonly the universal joints at either end of the propshaft(s) and on two on each front wheel – one at the ball joint on the steering track rod on each side, and the other on the lower swivel joint on the front stub axle. 4WDs may also have a nipple on their front wheel tractor joints.
Some cars have a nippled grease point to lubricate the hand-brake cable, usually positioned between the back wheels.
It should be stressed that greasing is not about topping up. It is about completely replacing old grease with new. So the fresh grease should be pumped in to the nipple (by hand pump or pressure pump) until all the old grease has been visibly forced out of the escapement, and the new grease can be seen emerging (plentifully) behind it. So do the greasing in a place suitable for some mess…probably not on your living room carpet. And wipe each nipple and vent when you’ve finished.
My car sometimes fails to start, what’s wrong?
My car battery is okay, but I sometimes get a loud click and no action when I try to start the engine. Where should I look for the problem? Serena
The starter motor. To confirm that, switch on your headlights and try to start the car. If the lights go dim that means there is an overload, and a jammed starter is almost certainly the problem. Also, a battery that is “okay” for lighter loads is not the same as a battery that is “strong”. Check that, and its connections.
For an on-the-spot remedy, locate the motor by following the live cable from the battery to a cylindrical component about the size of a litre can attached to the side of the engine near the back.
At the front end of that gadget, there is probably a protruding shaft with a square end. It might be concealed by a dust cover which you will need to unscrew and remove.
Fit a spanner over the squared end and turn it clockwise; this will often free the jam to get you going again…to a garage that will be able to remove the starter motor and check and lubricate it as a more permanent solution.
Otherwise a push-start (if your car has a manual gearbox) is necessary in the first instance to get you going. For an automatic, or if the battery is simply too weak, jumper-start cables from the battery of another car (with its engine running at mid revs so it gives a stronger kick to your system) might do the trick.
