Car Hire Conundrum

I drove home in a hire car tonight. Very nice Vauxhall with a diesel engine and a rev counter. I was driving along at 60 miles an hour in top gear and the rev counter showed 2000 rpm. I was wondering if I had enough information to work out the speed I would be travelling at if the rev counter showed 3000.

Comments

[Paul]

Grrr, Vauxhall I'm more of a Ford guy myself :)
Why not just put your foot down and try it?

[Bob]

90MPH?

[JDog - Joel Parkey]

Nope, you'd need to know atleast wheel radius! Provided thats what the RPM references!

[JDog - Joel Parkey]

Actually, my above method is to be accurate, if you play with the above numbers, I would have to agree with Bob!

[Edd]

Assuming no air resistance, 90mph, otherwise you need ot account for drag at ~v^2.

[Reedy]

2000rpm being 60mph, will not be 90mph at 3000rpm

When the speed gets up a greater increase in power is needed to get greater speed


You've got other factors to take into account:

Car weight, air resistance, weather/road conditions, engine size and therefore power, bhp, torque, whether it has a turbo.. etc


And rpm is that of the engine, not of the wheels. For example, when you are stationary and the engine idling, the rpm is usually around 900rpm, but are the wheels moving? i think not

Power is also a curve, not lineear




I seem to remember Jeremy clarkson mentioning to get a few more mph on a car that did nearly 200mph, or was it faster acceleration, they'd need to quadruple the turbos or something, so instead they increased the aerodynamicity of the car

[peter]

Assuming a mechanical transmission (gears, no torque converters), the same gear, (top) then the speed MUST be 60 x 3000 / 2000 = 90MPH. Wheel radius is irrelevant unless it varies with speed. Speed / power relationship is non-linear, but with the above criteria engine speed / road speed most definitely IS linear.

[peter]

And the speed is zero when the engine is idling at 900rpm is because the thing is in NEUTRAL. i.e. no gear at all.

[Reedy]

Yeah, i was pointing out that the RPM was that of the engine, not the tyres

[Rob]

Yep. I was overthinking the problem I reckon. It should be a straight proportional calculation, given that if the car was in gear and stopped the RPM and the speed would both be 0. This means that according to the rev-counter (which had a red line at around 5,000 rpm as I remember) the top speed of the car would be around 150 mph.....