4 speed vs 5 speed

[niky]

It depends.

Depends on how long the gears are, whether they're both manual or both automatic or whatever.

Basically, the more gears you have, the longer you can stay at a certain rpm while accelerating.

For example, in a 4-gear car (this one's a Lancer GT)

Speeds at 1000-2000 rpm per gear
1st gear: 10-20 km/h
2nd gear: 19-38 km/h
3rd gear: 29-58 km/h
4th gear: 40-80 km/h

Whereas, in a 5-gear car (Lancer ES, MT, same engine)

Speeds at 1000-2000 rpm per gear
1st gear: 9-18 km/h
2nd gear: 16-32 km/h
3rd gear: 23-46 km/h
4th gear: 30-61 km/h
5th gear: 41-82 km/h

A four gear box gives you long gearing, which makes for good highway economy, but since each gear has to cover a wider range of speeds, either you accelerate slower or use more power to get up to speed.

A five gear box gives shorter spacing between the ratios, so you can accelerate more quickly at the same revs... which gives you better fuel economy in the city.

This is as long as we're talking manual versus manual or automatic versus automatic. If it's manual versus automatic, your standard 4-speed automatic box will always give worse economy in the city, as, beside the longer gearing, it's also heavier and takes more power from the engine to operate than the manual.

The opposite is true of the CVT. Some CVTs are almost as light as manuals, and because they have no fixed ratios, you can stay at exactly the same rpm the entire time you're accelerating, which is the most fuel efficient method of using a gasoline or diesel engine.

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BTW, a six speed transmission is even better, but once you start running that many speeds on an econo-box, you get some ridiculously short gears...