What is the most fuel-efficient highway speed?

Opinions?

The slowest speed that will allow the use of top gear. Driving this slowly will no doubt be unsafe. Follow the traffic; it's not worth initiating a pile-up from all the people swerving to miss you ...

I'm almost always driving faster than traffic, so that's not an issue. I'm just curious. Technically, one could drive at any speed (above idle) in top gear, but I doubt 20 mph in 5th gear would result in stellar mileage, nor would it be good for the motor.

My guess is that it's somewhere in the neighborhood of 50 mph.

torque peak and yeah 45-50 is about the ideal - its slightly under the torque peak but still within the normal engine controls.

but like GoFaster said . . .

Two things to consider here. Do you have a stick or auto? After that look @ the torque curve for your particular setup with engine speed and gear ratio in final. The speed that you reach in final ratio at the top of the torque curve for the rpm's is the most fuel effecient place. Do not mistake horse power to torque. Twisting power is torque and horse power is the movement of the torque. When your engine is at the place where torque is highest, the engine is at optimal out-put for the work that it can accomplish. For the pontiac 455 that I built, and the transmission setup that I had, 110mph was the peak place for fuel mileage. It actually got better mileage at that speed than when I drove 65-70. It turns out to be some math to figure all that out, but once you get the concepts down it is easy to see.

So for a 5-speed, what speed are we talking about?

Let me make sure I understand this correctly: peak gas mileage will always be obtained while operating the car in top gear at the RPM that corresponds to the engine's peak torque output? In other words, mileage is INDEPENDENT of velocity?

45-50 . . . I've got the mileage competition numbers somewhere . . . based on time if you are really good you should be over 60+ mph if you watch your accelerations.

So much as to do with HOW you get to cruising speed, unless you are spending 14 hours at a stretch without stopping.

My car can run smoothly at 35 mph, and I would expect that it would get in the 70 mpg range. My last hiway trip brought 56.7 @ 60mph. I got 62mpg with my '80 rabbit driving mostly 50 mph, but it wasn't worth it.

That is great mileage for a Rabbit. My mother, in the early '80's, drove a diesel rabbit for several months which was obsessively maintained and tuned by Bosch diesel engineers as a test bed for their injection technology. The glove compartment was crammed with data recorders. It ran smoother than any other non-TDI I've ever heard and only got 55mpg!

As far as the original post, I think once you get over 50mph the air resistance REALLY starts to climb. Fluid dynamics, just like most powerboats would fall apart at 80mph if you gave them enough engine to do it without serious hull reinforcement. The water starts to act like a whole different material at those speeds. I'd guess 45-50 to be most efficient, but I'll stick to 75-80 and STILL getting double the mileage of most everyone else on the road!

I remember reading about that here somewhere where a forum member said that it was in top gear at around 2200 rpm's... I think that that is @ or near 56-58 mph.

I'm not exact on that data but hell you're asking a great Trivial Pursuit question for some wacko trivia fanatic like me!

Personnally, with my 01M 4-speed automatic, I've had the best fuel economy when I had a feather-foot doing the ol' double NICKLE give or take a mile per hour. I observed 46.2 MPG that way. I will tell you that I probably won't replicate that anytime soon... why? Too damn boring, especially since there's so much else to do while driving...

like watching the chick ahead of you FARDING.

Shawn

Mark,

When Jonathan Bartlett and I did the fuel economy run in the Tour del Sol last May using his 1996 Passat 5-sp with over a quarter of a million miles on it, we spent 5 hours doing a 30 mile interstate loop, using instantaneous mpg from a trip computer to determine that 47mph WITH the wind (pretty breezy that day) and 40mph AGAINST the wind netted 90+mpg. Of course, we had Tour del Sol signs, our car number on all sides, flashing lights, and a big fat sign in the rear window indicating 'Maximum MPG Test' to justify doing these kinds of insanely slow speeds (along with about 20 others) amid 85mph traffic flow - it's NOT recommended!~

As with everything else, YMMV.

I've gotten 61mpg on a couple of occasions in my NB 5-sp doing 4th-5th and 45-55mph all day long (with the A/C on).

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