How do you figure out car's optimal speed...

There are many great suggestions here. I've been trying to stick closer to the posted speed limits and that is helping a lot. On flat ground, at 55 MPH, my tachometer reads 1400, at 65 MPH it reads 1700. I'm certainly staying beneath the 2000 mark on my tach, more often.

I am also a firm believer in Cruise Control, too. My foot isn't as exact on the gas pedal as my Cruise Control can manage.

GeoThorn
 
I can only say that Gollnick must not live in an urban area such as Washington, DC, and its environs. If I drove in the manner that he suggests, I might as well stand still as so many people would be cutting in front of me that I would never get any where. If you leave the suggested gap of one car length for every 10 MPH on any road, sure as Hell some impatient person will be cutting you off, especially on the Capitol Beltway, I-495. On that road, with its 55 MPH speed limit, you are an actual traffic hazard if you are driving at less than 65 MPH in the far right lane as that is the lane where all of the impatient people seem to want to gun past everyone else. Nobody around here has ever heard of lane discipline.
 
I think Hugh is correct, it depends upon the surrounding traffic.
If you live where I live stop and go is not the norm, it is the constant, but you can lessen the effect of this type of traffic on a lower m.p.g. by using some common sense that Chuck pointed out, in a high traffic environment it is more of a balancing act of flow go I go; flow stop me stop.

Abrupt change can wreck M.P.G. I have to admit that an open road is much more fun and the key there for me is a steady pace (use the cruise control if you have one) at about 2100 rpm in overdrive. I have probably made around 120 trips during the last 6 months taking care of my mom while she is recovering and it is kind of fun to see if I can make the interstate portion of the trip without touching the brakes or varying speed.

When traffic is flowing on the Interstates in this town, going with the flow of traffic means 65 to 70 mph at a minimum. If you are driving at the posted Interstate speed of 55, you will be putting yourself at greater risk of an accident by the significant increase in the numbers of vehicles that will be passing you and riding your bumper.
 
We took a trip to Ottawa in our Dodge Caravan this summer and averaged about 25 MPG on the Interstates. We used the cruise control and stayed with the traffic, running about 60-65 MPH. Where we got killed on the mileage was going through mountains, of course, because a 3 liter V-6 pulling a 4000+ pound vehicle up a steep grade is not going to get great mileage no matter how you slice it. When I drive my Focus out to visit my sister in Columbus, Ohio, I get great mileage except from Frederick, Maryland, to west of the Ohio River and for much the same reasons, it is all up hill or down dale. Going over the Eastern Continental Divide is almost always a 3rd gear moment in whatever car I am driving.

That Focus has a button controlled overdrive on the auto transmission that makes the car quite difficult to drive legally in areas with a 25 MPH speed limit. I have to take it out of overdrive because the car just will not run at less than 30 or so in overdrive.
 
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