How do you know when to change gears?

Answer 1

If your car has a tachometer, that is the dial with a needle that moves to the right as you increase engine RPM, it should have different colored zones goingto red on the right side of the guage. Never "rev" your engine, that is depress the gas pedal, to the point that the needle (or digital display, depending on the car) goes into the red area on the tachometer. That is the origin of the old term "red-lining", meaning going beyond the mechanical capability of the engine. It will harm the engine if you over rev, either on purpose, while the car is in neutral, or by down-shifting a mechanical transmission at to high a speed. The red-line limit of an engine depends on it's design. Older cars not built for racing, say a 2000 Mercury Pacer, will have a red-line at about 3000-3500 RPM. IT can go up to 5500 RPM on cars designed for street racing.

Answer 2

I have several answers for you, Please chose the most appropriate for you:

Greyhound Bus with 2 stroke Detroit power:Each gear is reved until the engine governor stops the engine accelerating any more, now its time to shift.

Down shift when the maximum road speed of the next lower gear is reached.

Keep your 2 stroke Detroit power reving, it likes it.

Heavy Truck or Greyhound type bus with 4 stroke power:These engines are provided by Detroit, Caterpillar, Cummins, Mercedes, White/Volvo.

These engines have a very specific power and operating RPM band.

The heavy duty engines usually want to operate between 1150 RPM and 1700 RPM

In this case for fuel economy you want to keep your RPM as low as possible. A properly set up engine it will accelerate better and climb better in the proper RPM range.

My Diesel Pickup truckIs the same as above except the RPM range is 1500 RPM to 2700 RPM My Hot Hatch winter car:I have a simple system:

As I pull away gently the engine feels willing and eager, as the speed increases the engine start to feel less eager to go, its time to shift.

Faster acceleration feels the same until at full throttle you rev the engine to "Red Line" (Red marking on the tachometer that indicates maximum safe RPM)

Down shifting is done when the engine starts to LUG or Labour as the engine slows down and is running to slow for the load on it.

My Race Car:Is fitted with a shift warning system and a rev limiter.

The yellow light comes on means be ready to shift, red means shift now, the engine starts to cut out means you should have shifted 200 RPM ago.