Dear Wanderer,

Great answer. I had no clue that pushing the clutch pedal when going downhill would turn on the gas. Thanks.

 

Air resistance goes way up when you go faster. It's proportional to the square of the speed. And Fits aren't very aerodynamic.

 

From my month with my GE I would have to agree with your assumption

 

I have found that about 40mph is a good spot for max fuel economy. Not high enough for large air resistance (which increases with the square of speed), but fast enough to be in top gear at low RPM. I have an AT.

But it's pretty uncommon to find a 40mph-speed-limit road where you can go for miles and miles without slowing down and speeding up. Don't bother trying to go below the speed limit, at least any part of the US I've been in; you'll just cause road rage and accidents and middle fingers. I promise you that is not worth any fuel economy savings. I once took a long trip going 62mph on a 65mph interstate. I got 2mpg better (Honda CRV) and the entire time I felt like I was going to die as geriatric Buicks approached from behind and had to swerve into the fast lane to avoid me.

And that's probably why 45mph looks like an optimum to me - I can occasionally find a constant-speed 45mph road and it's great. 45-mph roads with lots of stops are terrible. I'll probably never find a 35mph road that's constant speed, so likely I'll stick with me 40-45mph recommendation.

 

By the way, extending that graph that was posted (and my own argument) to the limit --

For maximum fuel economy .. stay home.

 

Today I was on a 1 mile road in town, with a 45 mph limit. No traffic, so I set the cruise to 35 mph (1650 rpm+/-) M/T Fit. For a 1/4 mile or so, I was getting mid 50 mpg. Then I turned the a/c off and it went up to mid 60's !!!
I haven't reset the scan gauge to track this mile trip yet. Just watching the SG instant mpg.

I've noticed the a/c uses a lot of fuel. If I'm getting 35 mpg, and turn the a/c on it drop 5 mpg or so. I leave it on. It's FL and the Summer!
I have the a/c on 99.5% of the time. Just play with it sometimes. I still have averaged 32+ mpg, combined, with no highway cruising.
Great mileage for a car with so much storage.