OK, wanted to give you an idea of the fluctuation in MPGs I get with my Fit. I just returned from a trip to Cali which was the exact same drive as a couple months prior. Here is the deal.

75-80mph/70-80 degrees = 42 mpg.

75-80 mph/80-107 degrees = 34 mpg.

It seems heat kills MPG big time in the Fit!

 

Which direction was the wind blowing?

There is no way anyone can get over 40MPG+ at 75MPH+ without the aid of the wind.

 

It was downhill all the way from CO or NV. Driving from Hendersonville NC to Bat Cave I've gotten over 100 mpg. The tests we have run only automatics on level ground really get 40 mpg at 70 mph

 

Both times I was traveling from Vegas to Mission Viejo, CA. Have no idea which way the wind was blowing.

 

I could see this happening, Vegas to LA is downhill through a lot of the flat portions. There's a huge hit above 75 but I can still manage ~40 mpg at 75 on the highway with an MT, I would think it'd be a little higher with AT.

I have noticed too, that MPG suffers signifigantly with extreme dry heat.

 

I got terrible MPG from Vegas to Baker, CA.
I got 42+ MPG the other way 'round.

It's all about hills and wind, imo.

 

The roads are pretty flat here in the Midwest. I forgot about about gravity.