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
Maybe you are hauling something heavy? Cars do not need extra fuel when they get up to constant speed. Basically, they travel on inertia + engine adds minimal power, unless you are hauling something heavy (so there are more losses due to friction)
I don’t see anyone mentioning slip ratio: since the tire is elasticated compressed in front of the tire contact patch and stretched behind it, the wheel center will rotate more rpm than the distance traveled divided by the circumference of the wheel (as calculated by the loaded rolling radius). How much the wheel center rotates compared to how much it would theoretically when freely rolling is called the slip ratio and is dependent on the tire construction, vertical load and how much torque is transferred through the tire. When driving a constant speed, the slip ratio isn’t very high as the only torque transferred is that of rolling and wind resistance but when you accelerate, the slip ratio may be so high that the amount that the wheel center rotates faster than the road speed is apparent on the engine rpm.
The maximum torque is transferred at about 12-22% slip om dry asfalt, if your engine is so strong that your acceleration is traction limited so if you are looking at the rpm while still accelerating and compare to when you are cruising, then the difference may be noticeable.
Simple explanation might be a hill?
The air conditioning compressor increases RPM because it’s an additional load on the engine
If it is indeed a standard manual transmission, the engine is directly connected with the wheels and will always turn the same RPM at a given speed. The two things that would potentially change that is wheel/tire diameter and clutch wear.
If the wheel/tire diameter increases the RPM required to reach the same speed decreases.
If the clutch is worn the engine is no longer directly connected and the RPM could be arbitrary. Only a portion of the power is being transferred to the wheels while the rest is being converted to heat and clutch bits.
It is possible that this is a “manual” automatic transmission. You may be able to select gears, with the difference in RPM explained by the torque converted being locked or not.
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