Weight and engine RPMs

[ghiachimp]

At the risk of asking a dumb question, I’m curious to know if the weight of vehicle affects the rpms of the engine.

By that I mean if you took the transmission and engine out of one car and put it into another that weighed less, would the rpms be lower in the lighter car when both cars are measured travelling at the same speed?

[busdaddy]

RPM depends on transmission gear ratios and tire size, weight would have nothing to do with it unless the clutch was slipping, or it was an automatic of some sort and horribly overloaded.

But running the same RPM in a heavier vehicle might create cooling problems at some speeds, if the engine has to work harder but the fan is still moving at a slower speed damage might occur. Much like the problems those "freeway flyer" transmissions cause.
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[ghiachimp]

Thanks for your input. As I said, probably a dumb question but the more I thought it over the less sure I became....

So just to be clear: if 2 cars with identical tire size, transmissions and engines (but one car was lighter in weight than the other) were both travelling at 60mph their tachometers would both be reading the same RPMs?

The reason behind my question is this is :

I'm seeing 2600rpm on my tacho at 60mph . I have a 1641cc engine, stock 165/80/15 tires, and an AM code 1300 transmission (assuming its therefore 4.375 ratio). I expected to see around 3000rpm.

The biggest difference between my car and a standard Beetle is it has a 356 coupe fibreglass body, hence it weighs less and that's why I was curious if weight could have an affect on the rpms I'm seeing.

Of course, there are other variables: it's entirely possible my tacho is inaccurate, maybe the transmission has been rebuilt to a non-standard ratio. They're the 2 unknowns that come to mind right now.

[vwracerdave]

Weight has nothing to do with engine RPM. If your seeing a difference between two cars it's in the gearing and tire size, or yes possibly not accurate tach.
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[kpf]

Could also be an inaccurate speedometer.
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[KTPhil]

[ghiachimp]

Here in the UK we tend to call Speedometers "Speedos" and tachometers anything from "Tachs" to "Tachos" to "Rev Counters".

But yes it is a third thing for me to check - perhaps my Speedo is reading false as well. It is nearly 70 years old !

[viiking]

As others have said the revs should be the same.

But the other thing to consider about load on the engine is the frontal area and the drag coefficient on the 356 body I would ass(ume) would be slipperier in the air than the bug.
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