Re: Most fuel-effecient speed.

Depends on air density, flow of traffic & countless other items.

USUALLY it's targeted for 55 MPH / 90KPH

Plug in a tachometer & Vacuum gauge Low rev's high vacuum best results "usually".

Let us know how it works out.

I've hit 40 MPG in my Olds 98 @ 70 MPH over 900 Miles in the summer driving across the Prairies and 55 MPG in an 1984 Diesel Rabbit @ 50 MPH driving from Windsor to Niagara Falls.

At what RPM is the engine rated for maximum torque?? Usually the ENGINE is most efficient at that speed. Running at that speed in top gear is GENERALLY the most efficient

I drove it from Kitchener to Kingston at 2AM and got better than 45MPG (It's a long time ago - I think it was actually almost 49)

It worked out to be running at the max torque RPM in overdrive - with the tires at maximum full-load pressure. 2 adults with minimum luggage.

The gearing on the '77 was changed, and the mileage decreased, and was achieved at a lower speed. (but it DID have more spunk with the same HP.)

No. Cars may be tuned so that they run as effeciently as possible at 55 MPH, but are most efficent lower speeds like around 40 MPH (varies from car to car).

Aerodynamic resistance is the limiting factor.

That mat be true for power, but not for fuel efficency.

Engines are more efficent at lower RPMs than higher ones with maximum efficency at around 1000-2000 MPH.

Jeff