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 mph 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 mph 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 mph, 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. :-)

 

You're underestimating the nerdiness of your fellow board members! While I don't have a hybrid, I do have a lean-burn Civic HX, which was about as fuel efficient as it got back in 97 without buying a Geo Metro. Using an OBD2 ScanGauge and a few different long, empty stretches of road, I did similar test with my own car, then graphed them. I also put in estimates for the Prius for comparison purposes. These are "average" figures, and both vehicles can produce even better numbers in ideal 80 degree weather.

http://www.autocrossing.com/misc/junk/mpg-mph2.gif

 

LOL. I try not to go overboard, but I suppose it is already too late since I am here on this message board!

My observations are closer to 70mpg on a perfectly flat road at 65mph. I never get that over long distances because there are ups and downs in the road, traffic, etc. Also you burn some getting up to that speed in the first place. I can get higher going down hills, but you burn a lot climbing hills. I usually climb hills at 40 mpg. That 40mpg up the hills really lowers your average.

In pure highway driving, I see about 55mpg. For instance, on my many trips to LA I sometimes fill up around Carlsbad (Northern SD county), and when I reach LA I have about 55mpg. Of course, that never lasts long if the cars is used while I am there! But at least auto-stop keeps it from burning gas while idling. Idling is a killer in LA.

 

Highway mpg is all about aerodynamics. Friction increases in proportion to the *square* of the velocity.

This is why it *kills* me to go over 60 mph :-))

As far as EPA goes: highway driving is modeled at 48 mph (really), but the average mpg number you are quoting has been decreased 15% to better mimic realworld driving.

 

i wish my commute was flat i would land such greater mpg

the low mpg you got for 72 mph makes sense, since mpg peaks at around 55 mph and drops sharply after that. correct me if i'm wrong

 

Nerdiness? I have a chapter in that book too.

My commute is 46 miles each way and it takes about an hour to complete, about half is 55MPH multiple lane freeway and about half is 45 limit highway.

I suprised that simple math shows my average trip speed only 47MPH which is a major contributor to my numbers.
It the slower speed only adds about 10 minutes each way.
Sometimes breaking ranks with the masses can save you alot of money, especially with today's gas prices!

 

I at times have seen the fcd go to 80 mpg while going 65 - 70 mph. That is going on a flat freeway ( not going downhill..lol) It usually doesn't last for too long maybe a 1/2 to a mile at a time. My normal speed is 55 - 60 mph. I think not just temp. but the road surface has alot to do with that. It's mainly on this one section of freeway, but on rainy times I've seen it barely hit 50mpg on the same section. Kevin

 

i think this might be the lean-burn kicking in - saw this on a couple posts before about a temporary spike of mileage on flat roads at constant speed

 

You fellows seem to be watching your MPG very closely. Buy a pint of ACETONE in the paint department and add just 1 oz. for each 5 gal., then tell us what you get. No less than 1/2 oz., no more than 1 1/2 oz. per 5 gal.

 

The minor differences in speed that result in major differences in MPG are correct. Another post said this, but I'll put it in colloquial terms; whenever speed doubles, wind resistance quadruples. It's the single biggest factor in fuel efficiency.

On a road trip last week from Texas to Kansas, I drove 460 miles @ 70 mph and got 45 mpg. On the return trip with a nasty headwind, I got 40 mpg. Huge difference.

BTW, speed limits in the rural midwest actually are 70 mph. West Texas even has a stretch of I-20 with an 80 mph limit.

On short trips -- 30-40 miles -- I've found that keeping the cruise control on 65 (vs. 70) yields roughly 1 mpg better economy, with an unnoticeable increase in time-to-destination. That's a good trade.

Also, the lower speed results in lower stress. I'm not on the lookout for cops with radar guns, and I'm not constantly speeding up to pass slower vehicles, since I'm one of them.

Maybe I'm turning into my grandmother, but I sure enjoy driving more, especially because I know that I'm getting to the same destinations I used to visit, but I'm giving OPEC a lot less money to do it.