Quote:
Originally Posted by Stealthee
Horsepower doesn't affect the top speed in given gears unless a car cannot reach the gear limited speed. For instance in 91-93 3000GT VR4's the gear limited top speed is ~208 mph. The car is horsepower limited and cannot reach that speed, but that doesn't affect the theoretical top speed of the given gear as it is a constant.
The formula I have seen for calculating top speed in a given gear is
(RPM x tire diameter in inches)/(final gear ratio x 336)=speed in gear
You figure final gear ratio by figuring overall reduction ration x gear reduction ratio.
So if your final drive is 3.73 and your gear ratio for 5th gear is .70 you multiply 3.73 x .70 to get 2.611
Then lets assume a 6000 redline with 245/45/17 tires you'd have
(6000 x 25.7)/(2.611 x 336) = 175 mph top speed
Again, just because a gear can do 175 mph, doesn't mean the car has the power to reach those speeds. The main reason is for low RPMs while highway cruising.
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Thats what I meant to ask, the calculation gives you 175 mph but the car might have a real top speed of 140 mph. Using power =Fv equation I can come up with a pretty accurate top speed for whatever car I have tried but I just don't know how to calculate the top speed in each gear that the car can do. I will give you an example:
lets say a car has 75 HP(56kW) of power and revs to a max of 6500 RPM with wheel radius of 0.4572 m. in this case
Car A can do 6500 RPM
Gear ratios for Car A are:
1st = 3.55
2nd = 1.96
3rd = 1.3
4th = 0.89
5th = 0.71
Final Drive = 4.18
Calculation example for 4th gear at 6500 RPM:
Prop Shaft speed = 6500/0.89 = 7303.4
Wheel speed = 7303.4 / 4.18 = 1747.2
Vehicle Speed = 1747.2 * PI * 0.4572 = 3172.12 m/min
Vehicle speed (MPH) = 118.27 MPH
(In real life the car can do around 95-100 mph at 6500 RPM in fourth gear)
Calculating maximum vehicle speed from power:
Power = Force*Max Velocity
Frontal area (A) = 2.06 m^2
Drag Coefficient (Cd) = 0.32
Air density (p) = 1.25
Max Velocity = Power/0.5 * A * p * Cd * max velocity^2
Max velocity ^3 = Power / 0.412
Max velocity = (56000 / 0.412)^1/3
Max velocity = 51.4 m/s
Max velocity allowed (MPH) = 115 mph
From real life I know the max velocity calculated for Car A is very accurate because in real life this car would do 113 mph.
Reversing the equations I get the maximum RPM in fifth gear to be 5039 RPM at 115 mph.
So you see the fourth gear calculation tells me the car would do 118 mph which isn't correct as it is past the maximum allowable speed even before shifting to fifth.
Now how do I find out the correct speed in fourth at 6500 RPM as I know the car will definitely hit the rev limiter (6500 RPM) in fourth gear?
I know you have said horsepower doesn't affect top speed in given gears but how do I calculate the correct speed the car can do in each gear given its characteristics? For gears upto third gear I get pretty accurate numbers but then the overdrive gears (4th and 5th) give me ridiculously high speeds.