Is lower RPM always more efficient?

[kamesama980]

I'm just hypothesizing how and why.

I doubt any one of those factors would help alone and assume it's the combination. No I don't have real-time data, it's an '87 so real-time readouts are a little more involved. Just trends: 3 or 4 tanks of hard use highway that are 1 mpg better than slower steadier highway and a few tanks from hauling heavy loads like that. Which brings me to driving styles like you mentioned: driving hard and fast at 70-80 gets better mpg (tank) than steady 60, 65, 70, etc. loaded to the tits at the same speed doesn't seem to bother it much.

Kind of like a friend of mine whos car, by all rights, shouldn't be running for the 180k miles, missing/broken wires, vacuum leaks, lack of tuning, and random parts thrown at it. Yet somehow, with an automatic trans to boot, it dyno'd better hp and torque at the wheels than the original advertised @flywheel power specs.

Driving style is pretty similar empty or loaded (except the mentioned hard/fast driving with friends late at night in the city when everyone thinks the freeway's part of the autobahn). the biggest difference is that when loaded down like the latest heavy trip the throttle can be open over 50% just maintaining a steady 65 mph. Accordingly it'll vary from 0-100% depending on hills and traffic wheras empty it'll be 25% to cruise (guess) and vary 0-75% with WOT not really happening. Reaction to hills about the same either way... steady throttle unless speed gets really low.

My mention of the bearings is that it's got tapered rollers in front, recently repacked with amsoil grease and readjusted by yours truly which made a noticeable difference coasting and a solid axle in back which means straight rollers. straight rollers are VERY efficient and won't increase friction nearly as much with weight as any other style bearing. Tires are kept at about 35 psi (sidewall max), I think I upped it to 40 for the latest heavy load.

BUT my truck aside, all other things being equal lower rpm will yield better mpg until you get them so low the engine can't make enough power to cruise. (Like F-bodies and other cars using the T-56 6-speed... you can't even use 6th till you're going at least 70 mph. but once you get there it's doing 1700 rpm @ 80 mph :-p)

[1993CivicVX]

Originally Posted by theholycow

On another forum, a driver with a Saab turbo swears that his fuel economy is terrible if he stays in a high gear and opens the throttle more, which apparently puts him in boost, and that his fuel economy is much better if he downshifts to run at higher RPM and stay out of turbo.

I'd guess that a modern turbo car with the computer-controlled turbo is more efficient if you do it like him, downshift enough to stay out of boost..

Does this logic translate to lean burn? Substitute going out of lean burn with turbo.... I've been wondering how much better fuel economy the VX gets in lean burn at a given rpm compared to WOT at same RPM.