How gearing can make the difference in a car
Gear ratios are crucial for any car, whether it has 1 gear or 10. So here are some things you should consider when buying a car:
The gear ratio of the highest gear on a car is very important. It determines what RPM the car will cruise at, which in turn determines fuel efficiency on the highway and the noise level. In general, a taller gear ratio means better fuel mileage and less noise, although you’ll have to shift down if you need to overtake.
The ideal cruising speed is usually around 1600-2000 RPM for diesels and 2000-2500 RPM for petrols.
For example, the Toyota GT86 has 6 rather short gears, meaning 70mph in 6th gives over 3000 RPM. This explains its rather poor CO2 figure of 181g/km - this is a lot considering it is a 2-litre, 200 horsepower car with a drag coefficient of just 0.27. It also means quite a bit of engine noise (especially with an aftermarket exhaust) - this may or may not be a good thing to you. Sure, these problems are partly solved with the automatic transmission, which has taller gear ratios, but automatic GT86? I thought not.
In general, cars with more power have lower cruising RPMs since 70mph is barely ticking over for them. For example, my parents’ Golf Mk6 1.6 TDI has a 5-speed manual and cruises around 2300 RPM. On the other hand, their SEAT Ateca 2.0 TDI (which has considerably more power) has a 7-speed double-clutch and cruises at a much lower RPM of about 1700. This is why some V6 cars only use very slightly more fuel than their 4-cylinder counterparts.
Aggressiveness refers to how short the gears are. The more aggressive a car’s gearing, the lower the speed that redline is reached in each gear. This improves acceleration and makes the car feel more sporty, at the expense of top speed and fuel economy (this can be gotten around by adding more gears). However, high-performance cars often have less aggressive gearing because acceleration in the early gears is traction limited, and shifting many times can be tedious. The Dodge Challenger Hellcat for example has its 1st gear hitting the limiter at 63mph in order to avoid shifting in the 0-60mph sprint, leading to a quicker time.
The ultimate example of non-aggressive gearing is the Koenigsegg Regera. It reaches 250mph in 1st gear. The reason why a transmission is not used is because it would be a waste. Until 186mph (and possibly higher), acceleration is limited by traction, so having extra wheel torque from shorter gears would be useless.
Close vs Wide ratios
The gap between the gears is also a big factor. The closer the gears are together, the higher up the power curve the engine can stay, meaning better acceleration. Closer gears also means better fuel economy since the engine can be kept in its optimal RPM zone for fuel consumption. However, having closer ratios either makes the highest gear too short or makes the 1st gear too long, both of which can be undesireable (see earlier). The solution is either more gears or wider ratios.
The type of transmission is usually matched to the car. Close ratios means more shifting, so close-ratio transmissions are fitted to sportier cars. Wide-ratio transmissions tend to be fitted to cars with wider powerbands such as diesels. Formula One cars have very narrow powerbands, meaning they use very close ratios. The difference between 7th and 8th gear is only about 1.12 times, compared to about 1.25 on passenger cars.
However, closer ratios won’t always mean more performance. My bicycle’s rear cassette has 8 extremely close ratios (1st gear is 30 teeth, 8th gear is 11) which means it is easy to stay in the optimal rev zone to make good use of the power you put in. However, when accelerating hard (as I always do xD), I have to fire through the gears extremely quickly - the first five shifts usually take place within 2-3 seconds. This means more time is spent shifting than putting down power - I’m pretty sure I would be quicker with a 5-speed wider-ratio cassette. This is less of a problem in cars with dual-clutch transmissions, but it can be a nuisance with manuals unless the driver is very good at shifting.
Final drive swap
If you are not happy with your car’s gearing, the easiest way to fix it is a final-drive swap. This means getting a new differential with a different internal gear ratio. A lower number means longer gears and a higher number means shorter gears. This won’t allow you to control how close or wide the gear ratios are, but it will allow you to change all your gears by the same amount. If you do this, you will have to adjust your speedometer and odometer to take into account the new ratio.
This kind of swap can be relatively simple in a front-engined rear-wheel-drive car, but in some front and all-wheel drive cars it cannot be done. Many cars use a transaxle which has a differential built into the transmission, meaning the final drive ratio cannot easily be changed. Also, some cars (I think the Audi R8 does this) actually change the final drive ratio for different gears (think of it as two transmissions in series). Unfortunately, more and more cars are opting for integrated units, making it impossible to work on them unless you take them to a licensed garage and pay extorsionate amounts of money.
I hope you enjoyed this blogpost and learned something new :D.