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I have no idea about cars so the question might be a bit silly but I can't manage to find the answer anywhere on the web.

Basically, as far as I have undestood through my own experience; (Please ignore the numbers, it's just an example) I can be using 4th gear to go at 80 km/h while reving at 2000 rpm and using 6.0 litres every 100 km but on the other hand I could be using 5th gear and I could be going at the same speed, 80 km/h while reving much less at 1400 rpm and using less petrol, something like 5.5 litres every 100 km.

This afirmation must be so wrong because it makes no sense to me that there aren't many cars with more than 6 gears. It all depends on the torque? Does it depend on the horsepower? I have no clue.

I have a 1.7 Opel Corsa from 1997 which using 5th gear (max I have) while reving at 3000rpm I manage to go arround 120km/h, but of course with 4th reving at 3500rpm I manage to go nearly at the same speed.

So, why aren't there more gears?

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    Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Sep 3 '18 at 17:50
  • Comes down to the power output of the engine being equal to the rolling resistance + aerodynamic drag. Once that is equal you don't go any faster, then think about useable rev range and matching engine speed to road speed and the good answer by @Juhist – Solar Mike Sep 3 '18 at 18:20
  • There are 8 speed automatics, also Honda filed for a patent on a 13 speed automatic last year. – Moab Sep 3 '18 at 19:10
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    @Moab correct if I am wrong, but are companies adding more gears to help the cars get marginally better fuel economy? An 8 gear auto just sounds excessive, and (in theory) will be prone to breaking sooner than a 5 gear transmission, right? – Sam Sep 3 '18 at 20:11
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    @Sam I agree with you on both statements – Moab Sep 3 '18 at 20:21
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They do.

Many new cars are equipped with a continuously variable transmission (CVT). Such a transmission has an infinite number of gears, essentially.

But let's consider manual transmissions only.

Basically, you could do two things with the extra gears: have more closely spaced gears or have a higher top gears. Both of these have small gains. A reasonable 6-speed gearbox can quite well approximate the optimal RPM for the conditions, and making the top gear higher would only help with fuel efficiency downhills. It's not always that lower RPM would mean higher efficiency. Not only that, but also a human driver is poor in estimating the optimal gear to use, so by having let's say 10 gears, you will miss the benefit due to the human operator.

Here's the basic engine operating line of Toyota Prius that has essentially an electric CVT: http://techno-fandom.org/~hobbit/cars/SAE-bsfc.gif -- this line is optimized for fuel efficiency. The reason this infinitely variable CVT works is that it's computer controlled and thus you will get the gains from it.

As you can see, at some point of time when you start demanding more power, you will need higher RPMs. These conditions occur when driving at high speed due to high air resistance.

Now, if you had a 7-speed manual transmission, where would you put the 7th gear? 5 gears is easy, and 6 gears requires hiding the reverse gear somewhere. 7 gears, not so intuitive to use.

I'm glad that car designers haven't gone to the route of bicycles where you have something like 30 very closely spaced gears.

  • Fuel efficiency downhill is irrelevant, if the ECU is any good. My (cheap, European, 8 year old) small car has a 5-speed manual box, and the "instantaneous fuel consumption" display goes off the scale (more than 99.9 miles per gallon) coasting downhill with my foot off the gas. – alephzero Sep 3 '18 at 19:59
  • And with "conventional" automatic transmissions (not CVT), Car and Driver magazine frequently reports on 10 speed automatics. – fred_dot_u Sep 3 '18 at 19:59
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    The truck designers did though : lots of gears for a small working rev range, based on your last bit about bicycles. – Solar Mike Sep 3 '18 at 20:00
  • The reason bicycles have many gears is really just a marketing ploy. 21 gears sounds a lot more fancy than 3 gears, even though most people will only use a small range of their gears (unless you are a competitive cyclist). Adding another 3 gear ratios is relatively cheap if the bike already has a front and rear derailleur. – Sam Sep 3 '18 at 20:05
  • "6 gears requires hiding the reverse gear somewhere" I used to drive a manual transmission car with effectlvely 6 gears. The 4-speed box was followed by an "overdrive" sun-and-planet gearbox with a separate, electrically operated clutch. A small electrical switch on top of the gearstick selected between the conventional 4-speed selector working with "1 2 3 5" or "1 2 4 6". The overdrive clutch was designed to operate at any engine power so changing between 3-4 or 5-6 was just a flick of the switch. The only "gotcha" was that changing from 4 to 3 at more than 80mph over-revved the engine! – alephzero Sep 3 '18 at 20:06

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