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
RPM is how many revolutions the engine is making which, at a constant gearing (ie the same gear) should translate to the same road speed regardless of load and weather etc.
Speedometers themselves are generally not precise and, depending on the type and age of vehicle, may have some variable error such as being affected by tyres being under or over inflated, as it’s often actually a measure of the RPM of the wheels (rather than the engine).
Tachometers may also be variable in their accuracy depending on how they’re driven.
What could be changing includes
– your clutch could be slipping, meaning it’s due for repair replacement real soon now
– loss of grip of the tyres on the road but this seems unlikely at constant speed, more likely under acceleration or braking (eg aqua planing)
– variability in the speedometer/tachometer esp if they’re an older mechanical types
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