Quote:
Originally Posted by mvrick208
Hello,
Can you tell me if there is anyway of calculating the top speed in each gear of a vehicle given its horsepower and gear ratios?
Since Power = Force*Velocity
Velocity = Power/Force
Force = Drag Force + Rolling Resistance
--> Velocity = Power / (Drag Force + Rolling Resistance)
This does give me the top speed of the vehicle but it doesnt tell me what the top speed in each gear would be?
This is how I found the top speed in each gear before:
Prop Shaft speed = Engine RPM / Selected Gear Ratio
Wheel Speed = Prop shaft speed / Final Drive Ratio
Vehicle Speed = Wheel Speed * PI * Wheel Diameter
The problem with the above is that for overdrive gears it gives extremely inaccurate and high speeds.
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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.