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post #1 of 5 (permalink) Old 06-16-04, 09:02 AM Thread Starter
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What determines roadspeed

Im trying to figure out something that has been bugging me for a while. If you increase your power does that mean you will be travelling at a faster speed?

Basically, im asking if for example, your car does 100km/h at 3000rpm at 3rd gear. Now lets say you increase engine power, does that mean that at 3000rpm you will be travelling at 120km/h or wateva since you are making MORE power at the same rpm?

Or is road speed governed solely by the gear+ diff ratios?


This is probably pretty simple but I cant think of a definite answer.

Thanks for the help.


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post #2 of 5 (permalink) Old 06-16-04, 01:57 PM
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Re: What determines roadspeed

Even if you double the power output of the engine, 100kph/3000rpm still the same, just get there a lot quicker

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post #3 of 5 (permalink) Old 06-16-04, 01:57 PM
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Re: What determines roadspeed

Quote:
Originally Posted by T_Terror
Im trying to figure out something that has been bugging me for a while. If you increase your power does that mean you will be travelling at a faster speed?

Basically, im asking if for example, your car does 100km/h at 3000rpm at 3rd gear. Now lets say you increase engine power, does that mean that at 3000rpm you will be travelling at 120km/h or wateva since you are making MORE power at the same rpm?

Or is road speed governed solely by the gear+ diff ratios?


This is probably pretty simple but I cant think of a definite answer.

Thanks for the help.

If you increase the power you are making more RPM.

What determines the speed is:

Engine RPM * Gearbox Ratio * Diff Ratio

Example:
3000RPM * 0.8 * 3.11 = 746.4 (im not sure if thats correct)

0.8 is for top gear from memory.
Your diff ratio will depend on what car you have.


So if it is correct that means in 1 minute your wheels spin 746 times!
So calculate the circumfrance of you tyre and you can determine how far you can go in 1 minute

IE:
1.5m x 746 = 1119 meters/min
1119 meters x 60minutes = 67140meters (or 67.14km)

So that gives you 67km/h
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post #4 of 5 (permalink) Old 06-16-04, 08:11 PM
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Re: What determines roadspeed

Quote:
Originally Posted by T_Terror
Im trying to figure out something that has been bugging me for a while. If you increase your power does that mean you will be travelling at a faster speed?

Basically, im asking if for example, your car does 100km/h at 3000rpm at 3rd gear. Now lets say you increase engine power, does that mean that at 3000rpm you will be travelling at 120km/h or wateva since you are making MORE power at the same rpm?

Or is road speed governed solely by the gear+ diff ratios?

This is probably pretty simple but I cant think of a definite answer.
Thanks for the help.
Your second asumption is correct..
Road speed is determined by what gear you're in and the diff ratio.
(larger or smaller diameter tyres would also be an influence as the circumference would change)

As normell says.. More power just means you get there quicker.. and the car would be capable of a higher top speed.

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post #5 of 5 (permalink) Old 06-17-04, 12:59 AM Thread Starter
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Re: What determines roadspeed

Ok, I see now. Thanks for the response.

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