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
Road conditions, the mood of the engine, and the load in the car can affect the RPM. It can also be affected by hills or the wind because you are putting more/ less pressure on the engine to produce power due to the load.
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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
If you’re in the same gear the speed of the engine will *always* be directly linked to the speed of the car – so either you weren’t in 6th both times, or your speedo is wildly inaccurate, or your clutch is slipping (or you’re riding it).
A 400rpm difference suggests you weren’t in 6th both times.
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.
Looks like I need to get some things checked… Thank you everyone!
Headwind and tailwind would explain this, also if one run was flat and the other had a slight uphill.
Is your car an automatic? Because a 400 rpm drop is similar to what the lockup in the converter will do. It may be failing to lock up.
Your original assumption would be correct. Assuming the drivetrain is rigidly coupled and not slipping, achieving the same speed in the same gear will result in the same engine speed. It’s a fixed relationship between the engine and wheel.
Either there’s something that’s variable (the clutch slipping for example) or your measurements are incorrect. It definitely wouldn’t change by 400 RPM. The tachometer or speedometer could be faulty too.
I may be wrong, but what came to mind is the incline. RPMs provide torque on the axel, propelling the car forward. It’s not directly converted to MPH, which you notice when you press the accelerator and the RPMs zoom up but the car’s speed gradually climbs up. So, with RPMs providing force, the changeable part here since everything else is the same, is the amount of energy required to stay at 80 MPH. If you’re going uphill, you need more energy to continue at the same speed than if you’re going downhill. Your two RPM measurements weren’t far off, so it could have been a small incline difference.
TLDR: Uphill, level or downhill?
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