Diesel cars/trucks run about 2,000 RPM or so, and the pulley set on these are usually about 3:1 in order to get about 6,000 RPM from the alternator. A gasoline car will run up to an average of about 3,000 RPM, so the gasoline pulley set usually has a ratio of about 2:1 to get the 6,000 RPM. I have read most vehicle alts are built to survive 10,000 RPM.
A cars starter battery has many very thin plates sitting in the electrolyte in order to provide a lot of plate surface area in a small package. After drawing a lot of amps for 3 seconds to start an engine, the battery is hardly drained at all, and the alternator only needs to run for a few minutes to charge the battery back up.
While the car is running, it is drawing most of the current it needs to run from the alt, so the alt is sized according to the max running load that is expected.
If you drive at night with the headlights on, and the A/C on, the radio, etc. you will need a fat alt that puts out a lot of Amps. A riding lawnmower will have the cheapest, smallest alt they can find. It is common for modern car alts to put out about 100 Amps, my truck alt puts out 130 Amps. -Ron
http://www.1stconnect.com/anozira/SiteTops/energy/Alternator/alternator.htm
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