I was driving on a perfectly flat interstate in central california the other day, 70 degrees, no wind, and no traffic. So I decided to ditch my "lifetime mpg" trip meter, in favor of using the trip meter to do some observations of the MPG of my HCH at various speeds.

In all cases I observed the mpg until it settled into a value and stayed there for a while; usually this took about 10 miles to stabilize. As I stated above, the external conditions were pretty much ideal.

My observations:

72 miles per hour: 45.0 MPG
66 miles per hour: 52.9 MPG

Interestingly, if you add a little traffic to the 66 miles per hour speed, such that you periodically slow down a bit, then accelerate back to speed, the MPG goes up to around 54.5.

I was a little bit surprised that the 72 miles per hour value would be as low as it was; I thought it would be a little bit closer to the rated value of 48 MPG. On the other hand, my overall experience with the car has been that I get about 52 MPG when traveling at 65 miles per hour, which is about 4 MPG over the EPA rating of 48; so the value I observed in my test was consistent with that.

I'm not sure if any of you are as nerdy and analytical as me, and have done this type of measurement, but I found it somewhat enlightening. And now that I have freed up that trip meter, I will probably waste even more time measuring MPG from place to place. :-)