Disappointing MPG results

[md13ffhguy]

The 6 mpg decrease per 10 mph speed increase doesn't make sense. That is a linear progression. Air drag increases with the square of the velocity (speed). It is a lot worse than that rule would indicate.

BINGO! Too simplistic. I get the generalization, but to pin specific numbers to it is misleading. You can't generalize like that and apply it to drivers in Maine, Florida, Nevada, Michigan, mountains, cities, etc... So many variables in play with these cars, which btw, are awesome. ;)

[Cobra348]

BINGO! Too simplistic. I get the generalization, but to pin specific numbers to it is misleading. You can't generalize like that and apply it to drivers in Maine, Florida, Nevada, Michigan, mountains, cities, etc... So many variables in play with these cars, which btw, are awesome. ;)

<chuckle> Prime example - computer average MPG when I started my run to FL and back a month ago was about 42.+. Hit Georgia and it was 43.8 and cllimbing. Dropped like a rock on the flatlands in GA and FL then climbed back up when I got into hills and stuff. Speeds were 65+ most of the time, with darn little city or <45 driving. My estimate would be 96/4 split highway/city.

 

I always base my MPG on the calculated actual from gas receipts. And the car is super generous anyway (2 to as much as 4+ MPG). I have a compact spare which by all the gurus should drop my MPG because of the weight (30-35 pounds) but it doesn't even seem to bother the car.

Edited July 2, 2018 by Cobra348

[lolder]

Yes, the wind resistance goes up as the square and my numbers are approximations. Sun light air conditioning load goes down slightly with increasing speed as the car exterior is cooled by the airflow. The ICE is also operating at higher power in a lower specific fuel consumption area. Between 50 and 80 mph you can bank on that 6 mpg/10mph loss. I've watched this for nine years. Headwinds and tailwinds are -/+ 3 mpg/10 mph wind speed. When you speed against headwinds in cold weather you'll run out of gas before the car warms up.

[ptjones]

For the 2010-12 FFHs, from a reference point of 46 mpg at 50 mph and 60º F, mpg decreases 6 mpg per 10 mph speed increase. For every 10º F. temperature increase up to about 95º, the mpg increases 2 mpg. The 2013+ FFHs starting figure is about 50 mpg instead of 46, I think.

I tested for this 5.5 yrs. ago. This shows the difference between MPG vs OT and with Grill Covers on and off. It's about 2 mpg for each 10*F change in OT.

Paul

[Waldo]

The 6 mpg decrease per 10 mph speed increase doesn't make sense. That is a linear progression. Air drag increases with the square of the velocity (speed). It is a lot worse than that rule would indicate.

 

No it's not a linear progression, because MPG drop is not a linear progression. The difference in actual fuel consumed dropping from 36mpg to 30mpg is much more than the difference between 46mpg and 40mpg. So that somewhat balances the exponential increase in drag.

[ptjones]

BTW my test was going on the FWY for 7 mi. and back at 70mph to make everything the same, just temperature changing.

 

Paul

Edited July 8, 2018 by ptjones