ORIGINAL: bl1nkage

but despite that fact... what is a good gear ratio to get without sacrificing alot of top end? because i really dont want to take the top end off because i might want to race this car one day... i mean if i go all the way to 4.10 how much top end are we talking?

ORIGINAL: CrazyAl


Like everyone else has said, you can think of gears like levers. The effect the gear ratio change has on your acceleration and your top speed is exactly the same, and the math behind it is simple.

Let's say you have a Manual GT with stock 3.55 rear end gears and you switch to 4.10s. 4.10 divided by 3.55 = 1.155. That means that you'll get 15.5 % more torque transferred to the rear axle at all times. In other words, you'll accelerate 15.5 percent faster.

BUT, this works in another way--your theoretical top speed will be REDUCED by 15.5% as well. I say theoretical, becasue this assumes a "perfect world" with no drag (air resistance).

Let's assume for a second that there is no such thing as friction/air resistance/drag. If you had your Manual trans GT pegged out at redline (6000 RPM) in 5th gear (0.68:1), with stock rear-end gears, your axle would be turning 6000 / 0.68 / 3.55 = 2486 RPM. With stock 27" diameter tires, you'd be going 199.6 miles per hour. If you then changed to 4.10s your top speed would drop by 15.5%, or about 169 mph.

Now, in the real world things are a bit different. Air resistance is the #1 thing that reduces our car's top speed (over a long distance--in the 1/4 mile you rarely get fast enough for drag to be a problem). Air resistance (drag) goes up exponentially as you go faster. In other words, to go twice as fast, you need FOUR times the HP. If you were to get on a long, straight, road and floor it in your car, you'd never actually make it to redline in 5th becasue the air resistance will be holding you back.

So, a "stock" Mustang's top speed is the point at which the maximum force provided by the engine (modified by the gears) matches the aerodynamic drag, and this is much slower than 200 MPH. If you change your gears to 4.10 then you are decreasing the maximum RPM that the engine could be spinning the rear axle at. BUT, on the other hand, you are gaining that 15.5% more torque (force) at the wheels. So, it's problem that is not easily solved except by an engineer who would have access to the engine torque curves and the aerodynamic figures for the car. The top speed might actually improve a bit (becasue the engine can better overcome the drag), or it might go down. But likely it will be fairly similar.

Now keep in mind this is your absoloute top speed over a LONG distance. In the 1/4 mile you will probably never go fast enough to reach the aerodynamic limit of your car, and in that case the 4.10 gears will INCREASE your speed through the traps.

4.10 gears are generally regarded as the best choice for Manual trans cars. (3.73 for Autos) Practically speaking, you won't see a change in your top speed. And let's face it, how often would you really reach that point anyway? Better acceleration you will feel EVERY time you pull away from a stop or power through a curve. Maximum top speed you'll only encounter every once in a while, if ever.