RPMs: what do they really mean?
filed in 1985 supra
Question: I had a 1985 Toyota Celica, an automatic, which would coast at 60 MPH at 2100 RPM. I promptly traded it for a 1986-1/2 Toyota Supra, a 5-speed, which coasts at 60 MPH at 2900 RPM.
I’m looking at new cars, all 5-speeds (Maxima SE, Celica GT-S, Accord EX, Integra GS), and would prefer to buy a car this time with an RPM ratio to speed which is closer to that of my former Celica: it seems as if the car is working less for the same speed when it’s at lower RPMs.
Does anyone know if and why this is an unnecessary concern for me?
Answer: A couple of factors come to mind. First is wear and tear. If most of your driving is done at 60 MPH or so, then it stands to reason that engine components like valves, rings and bearings, whose life expectency is more accurately measured by engine revolutions than by vehicle mileage, won’t last as many miles running at a higher RPM.
Second is fuel economy. This one is less straightforward. The efficiency in energy conversion is probably slightly better at the lower RPM, but the only way to tell, short of a horrible mathematical model, would be to just measure the two. Don’t be misled by a graph of fuel consumption against RPM; most of these that I’ve seen are measured with the engine at full load, which is of no value here.
A third consideration is engine noise, which one would expect to increase with RPM.
Of course, the innate differences between car makes could well offset, or even overshadow, any of the above differences. In the end, the final analysis is probably subjective. (I know what you mean, though; I don’t care for my engine twiddling at 3000 RPM when I’m cruising.)
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