Maximum speed

[burnhal]

Here's a question I have occasionally pondered over for many years.
What factors limit the top speed of a car.

Take my diesel focus as an example. At 70mph in top gear the engine is churning along at 2250 rpm. Now the engine redlines at 4500 and will quite happily go to 5000 for short bursts. So if we take 4500 as the top rpm- thats double the revs at 70mph so if the engine could get to 4500 rpm in top it should do 140 mph.
Obviously it wont- I think top whack is something short of 110mph.
So given a straight road of whatever length is neccesary, would the engine eventually drag itself up to 4500 in top?
So what stops it actually doing that?

[markysngc]

Several things as you have already figured the maximum engine revs and gearing, though these are rarely the limiting factor, it is not that often you will find a car that can redline in top gear.

The main limiting factors are aerodynamic drag (i.e. wind resistance) and you also have rolling friction (the tyres on the road) though this is fairly minor.

The fact is that your engine would get the car up to a certain speed and then the engine simply would not have enough power to over come the wind resistance which increases as speed increases.

[Toureman]

No it would never reach that 140mph speed due to wind resistance.

It would probably reach a higher top speed in your next to top gear.

[bigboots]

Also you will find the power curve will fall away at the top end of the rev range, giving less torque and less power my diesel will rev to 5000 but in anything other than first or secnd gear it is gutless, accelerating through the box changing up at 4000 rpm gives the best result
I also top out at 110 in top gear at about 4200 rpm but it takes a while to get there after 90 mph

[markysngc]

and of course you know you top out at 110 because you have checked this on private land, just as I did when I found my car tops out at about 125 ;)

LOL

[burnhal]

Thanks to one and all!! And- yes- we all use that private road for testing!!

[dumbo1001000]

That's all sound theory but why does a honda v-tec go to so high in the rev range and shouldn't all these factors affect it after all the previous type-r is as aerodynamic as a sideboard.And My old gtv loved 8000rpm?