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I own a 2003 Civic Si (manual) Coupe (not HB)... just wondering does anyone know at what revolution do you switch gears at for maximum fuel efficiency?? I was talking to a friend and he said that he went to a honda forum site and found out that for his '93 prelude he needs to shift gears at around 3000 rpms for better fuel efficiency/gas mileage.

 
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im guessing you are canadian because our 'si' coupes are known as an 'ex' in the states. when you say 'si' on here people generally think you are talking about a hatchback. from now on just refer to your car as an 03 ex coupe just to make sure you get the proper help for your model. i have an 02 si in canada, but i call it an 02 ex on here.

anyway... a good shifting rpm target is around 3000 for fuel efficiency although rpm does not have as much to do with it as most people think. the best way to get very high fuel efficiency is not so much to shift at a certain rpm, but moreso in how hard you push the gas pedal down. gas mileage is obviously affected the most by the amount of fuel pushed into the system, so when you start driving from a stop at a light, the best way is to just take your time to get up to speed. it takes longer yes, but you also burn probably only 1/2 of the fuel you normally would when you press it down generously, or even 1/3 of the fuel compared to when you floor it.

same thing goes for cruising at a certain speed. our cars are pretty efficient around 110km/h but youd be surprised at how much extra fuel you burn cruising at 125-130. its not much faster, but your foot has to be pushed down way further to keep at that speed so you burn way more. its all about how heavy you like your foot to be. personally ive never really cared. we have one of the most fuel efficient cars out there as it is, and even when we burn a lot, its still nothing compared to most other cars.

 
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2800 rpms you will save more fuel if you get up to speed normally instead of very slowly. if you accelerate slowly, your rpms will stay higher longer and that will use more fuel. this is esp true in a hybrid car.