Gas Mileage Fluctuation

[myronf]

I drove 100 mi yesterday to see my son. Speeds were between 60 and 75 mph. Gas mileage 22.9 mpg.

Driving home today same speeds, same temp - gas mileage 26.9 mpg.

Why??

I also noticed that the mileage is much better at 60 mph than 75. why?

Engine rpms are at 2500 rpm at 70 mph. Is this correct? sounds a little high to me.

By the way I have had this big difference in mileage b4. Same route different mileage.

Myron

[gibbons]

Not having seen the route you drive, I will just toss out some things that change economy.  Are the start and end points the same elevation?  Reading the instantaneous display, I get 26 going to work and 22 coming home.  There is about 300' of elevation change in 23 miles.  Sounds silly, I know.  Was there any wind either day? That makes a substantial difference.  Next time you see a semi-truck coming up behind you, note your mileage.  After he passes, drop in close behind and "draft" him (be careful doing this).  You can see your mileage go way up!  

The reason that you get less mileage at 75 than 60 mph is that the wind resistance increases to the square of the velocity.  That's an engineering Fluid Mechanics tid bit.  That's not the only thing going on a higher speeds, but you get the picture.

[myronf]

Elevation is the same.  Yes there was a headwind on the poor mileage trip.  Maybe 10-15 mph.  The difference in mileage seems excessive for a medium breeze.

In regards the air resistance going up with the square of the velocity, I understand fully, but seems to me that there is not this kind of difference with any of the cars that I have ever owned.