I think Tom and ray said the 65 MLH was the most efficient spead for a car to travel? Would this be for all makes & models? I have an Impala without a tachometer to watch the RPM’s. On the highway, is auto cruise efficiency that much different than driving w/o cruise?
A lengthy discussion on the relationship between mileage speed has already been undertaken here: http://community.cartalk.com/posts/list/140809.page
The answer to that one is no where close to that simple. Follow the link above.
55 to 60 mph in overdrive is where you should be for that scenario.
However, the way people drive today, you’ll likely get run over during a fit of road rage.
Depends what you drive and how you drive it and you will have to experiment. If you have a big van with a 4.6 engine, it might not make much difference if you stay near the speed limit.
too bad your car doesn’t have a tachometer.
i have found that while speed is a significant factor in mileage, and that generally it’s always best to be in as high a gear as possible, keeping the RPMs moderate to low is the single BIGGEST factor.
i actually get BETTER mileage in low speed (35 - ~52 mph) suburban/urban driving than i do on trips. largely because in the former, i can always keep my rpms at 2300 and below. even as low as 1500 rpm.
unless i stay at 55 mph, my rpms on trips are always HIGHER, for LONGER periods. hence, the engine is ALWAYS turning faster, burning more gas.
Though not directly related to the question:
Back during the gas shortage of the mid-70s, there was a big competition to see who could get the most mileage out of a gallon of gas. Big oil and car companies participated. The vehicles they built were very small “go-cart” type vehicles.
- The winner got around 1600 miles per gallon.
- The average speed they all traveled at was about 12-14 miles/hour.
- The competitors that built farings for the front of their vehicles got about 200 more miles per gallon than those who didn’t.
You need to be in the highest gear and operate the engine at high enough RPMs that the engine does not lug. For an Impala with a 4-spd automatic, 55 MPH should be fine. I would, and do, drive 65 or 70 MPH if that is the speed limit. I even drive 55 if that is the legal limit. I get about 28 MPG in rush hour traffic in a similar car.