I have a manual transmission and was under the impression that a certain RPM on a particular gear will always generate the same MPH. The other day I noticed I sit at 2,800 RPM in 6th gear @ 80 MPH, and other times it’ll be at 3,200 RPM – Could some explain?
In: Engineering
A few possibilities.
Your observations are wrong and you were in a different gear.
Or…
You have some kind of malfunction/slip in your drivetrain.
Or…
Your tire inflation is WAY off and dramatically changing the wheel diameter. This, I think can be discounted since you’re talking about a 14% difference. EDIT: Note, this would change your true speed-over-ground but not what your spedo and tach register. Thanks, u/hydraulis
Because your original impression is correct. In a manual, with the clutch disengaged you have a direct correlation from RPM to tire rotation to speed. Going up a hill and not changing your speed will not change your RPM. But you’ll have to give it more throttle, lowering your vacuum (or raising boost), putting more air and fuel into the pistons, and increasing your combustion pressures.
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