Every internal combustion engine has an ideal rpm rate, where it will produce the most mechanical energy relative to the fuel consumed. What this means for a car is that based on a whole host of factors (wind resistance, tire inflation, gear ratios, etc), there will tend to be some speed that will end out being ideal. But since some of those factors change based on conditions outside the car, you can't always say X mph is the best speed. It's ultimately based on how hard the engine is working (and thus, how fast it's spinning). If you have a car with a tachometer, you're better off looking at that than the speedometer since the efficiency correlation is to rpm and not really mph.
I've found on my car that if I keep my rpms just under 2k, I get pretty decent gas mileage (relatively speaking). Since that ends out usually being around 75-80 mph, that works out just fine. On long drives, I'll usually just cruse at that speed changing only when traffic conditions warrant it. If I'm in more of a hurry, I can always drive faster. I find I tend to do that more when heading home from somewhere than when heading to somewhere. So heading back home from my brother's house in LA usually involves barreling down the 5 at 85-90 mph late at night. As to gas? I suppose if I drove a car that got good gas mileage in the first place, it would be more of a concern. Kinda made that decision when I bought the car.
Edited, Oct 9th 2012 1:49pm by gbaji
King Nobby wrote:
More words please