Three Laws of Fuel Economy

[Ernie Rogers]

I updated the Rogers' Law to make it more sensible (and more accurate-- Lug Nut take note) and added some new ones. Of course, laws aren't to be fully accepted as such until they have been checked, revised and amended. We will have to wait and see about that part. Number 2 is the only law that's original with me. The others have been known for some time, with some small variations.


Three Laws of Car Fuel Economy
Ernest Rogers May, 2008


1. In highway driving, for each 5 mph that you slow down, your mileage will increase by 10%.


2. For any trip with a present average speed of (mph) and fuel consumption of (mpg), if you speed up to save time, the extra fuel you will use can be estimated by--

Extra gallons = (mph /mpg) (minutes saved /35)

In words, if you divide your normal speed by your usual mpg, then multiply by minutes you want to save (by speeding up) and divide by 35, that's the amount of extra fuel you can expect to use. It is a handy rule to see the fuel cost for speeding to save time.


3. Very efficient drivers use pedals less and can get 30% better mileage than inefficient drivers.

[fumesucker]

Originally Posted by Ernie Rogers

I updated the Rogers' Law to make it more sensible (and more accurate-- Lug Nut take note) and added some new ones. Of course, laws aren't to be fully accepted as such until they have been checked, revised and amended. We will have to wait and see about that part. Number 2 is the only law that's original with me. The others have been known for some time, with some small variations.


Three Laws of Car Fuel Economy
Ernest Rogers May, 2008


1. In highway driving, for each 5 mph that you slow down, your mileage will increase by 10%.

That depends highly on the car.. My daughter has an Expedition with the SOHC 4.6 V8, a towing package and the mpg readout. There is zero difference in mileage between 65 mph and 55 mph (22 mpg) and at a steady 35 mph it gets about 17 mpg..

2. For any trip with a present average speed of (mph) and fuel consumption of (mpg), if you speed up to save time, the extra fuel you will use can be estimated by?

Extra gallons = (mph /mpg) (minutes saved /35)

In words, if you divide your normal speed by your usual mpg, then multiply by minutes you want to save (by speeding up) and divide by 35, that's the amount of extra fuel you can expect to use. It is a handy rule to see the fuel cost for speeding to save time.

Again, it depends highly on the car in question.

3. Very efficient drivers use pedals less and can get 30% better mileage than inefficient drivers.

As we have discussed here multiple times already, otto cycle engines are more efficient at higher loads than they are at low loads, this means using a lot of throttle at low rpms.

I can beat my daughter's mileage in her Expedition by considerably more than 30% by paying strict attention to the mpg readout and driving accordingly.

11.5 mpg versus 17 mpg

[mdl76]

[quote=fumesucker;101389]That depends highly on the car.. My daughter has an Expedition with the SOHC 4.6 V8, a towing package and the mpg readout. There is zero difference in mileage between 65 mph and 55 mph (22 mpg) and at a steady 35 mph it gets about 17 mpg..



Your daughters car may defy the laws of aerodynamics...
Amazing

[Biffmeistro]

Not really. If it's already a horrid car for aero, and it has excessive horsepower.

The gearing might put the car into a more efficient rev range at 65 than at 55. The ease that it makes the necessary power at the higher speed could very easily account for what logic would tell us would be a loss of MPG.

[fumesucker]

Originally Posted by mdl76

Your daughters car may defy the laws of aerodynamics...
Amazing

It has to do with gearing and the BSFC map..

Higher speeds put the engine in a more efficient portion of the BSFC map and below 44 mph the truck drops out of overdrive lockup and efficiency plummets.

[kamesama980]

^^agreed. my truck gains mpg till about 28-29 at 75. 4 banger brick of an S-10. course that was with the mech. fan and full grill block. with the e-fan and no grill block, it's down 2 mpg on the highway but up 1 mpg in town and mixed (where I do 80% of my driving) tho admittedly the efan conversion was right around the time of the change in weather.

[fumesucker]

Lake Wobegon

Where all the cars are geared for better gas mileage at a higher speed.

And where all the children are above average.

http://en.wikipedia.org/wiki/Lake_Wobegon_effect