Determining your car's speed for optimal fuel usage

In planning to take the 20 hour, 1250 mile round trip to Cincinnati in a few weeks, I'd love to know how to maximize fuel economy. The estimated gas cost of $90 - $120 (and burning 35 to 50 gallons of gas in 3 days) slightly haunts me.

I found some interesting links on what hurts MPG and being more efficient. But I've heard that every car has a single optimal RPM at which they get the maximum fuel efficiancy. Of course, your gear dictates speed which alters wind resistance which alters efficiency, but let's assume 5th gear; cruising gear.

Is there a way to calculate or otherwise determine that optimal RPM and/or optimal speed? Can I determine any other conditions needed to help (ie: drive slower up a hill to avoid overworking the engine?)?

Hm, that fueleconomy.gov site is pretty cool, especially the find a car feature. Haha, "cars that don't need gasoline" is interesting. No hydrogen powered cars on there. Someday, hopefully.

Well, aerodynamic losses increase proportionally to velocity squared. So slower is better.

Friction losses within the drivetrain (ignoring gearing for a moment) are proportional to velocity - but since distance travelled in a given time is also proportional to velocity, that's a wash. Your losses are proportional to distance travelled.

So that leaves the engine and gearbox. The latter is unlikely to have noticeable efficiency differences between gears. The engine is a beast unto itself. Each engine has a different optimal rpm. In general, the lower the rpm that the engine can cruise at without bogging, the better.

But velocity is really not even half the equation.

Acceleration is the major factor. Cruising at 90mph (theoretically of course ) is going to be more efficient than continually fluctuating between 40 and 60 mph because of crappy traffic,

Last year on the way to Cinci my car got its (then) best ever gas mileage when 'cruising' with Rob. Traffic was light and for most of the tank stayed out of the left lane. We didn't exactly drive slow. It hit the 31mpg quoted

Of course, that record was soon forgotten on the way back when my poor car spluttered its way home. (Still beat the city mileage though)

That record stood until recently when I set cruise control at 55mph for about 300miles which managed 35mpg.

The peak eff point changes with load too. If you're going up an incline, it may be better to drop a gear and use high rpms.

Why not get an mpg meter? My Jaguar came with one that could read mpg on the fly as well as average. On long trips I would put it in instant mode and fiddle with the cruise control until it peaked.