Gas Mileage on the Interstate

[Yuriman]

It appears the chart ignores losses before the flywheel, such as thermodynamic losses in the engine. For example, for a modern gas engine with a compression ratio of 11.6, the max theoretical efficiency would be 37.5%, so 62.5% of the energy in every gallon of gas goes out the tailpipe or into the radiator.

In realty the actually efficiency is probably quite a bit lower than the theoretical max.

So while aerodynamic losses are large, more than 2/3 of the energy is lost to waste heat no matter what speed you are going.

You're right, of course. I'd like to add that some of the losses before the flywheel can be changed by things like gearing, too; for example bringing engine load up closer to 100% at a lower RPM generally increases effiency, both by increasing cylinder pressure and by reducing pumping losses, as do driving at higher altitude and increasing intake air temperature, so now that I think about it, that chart isn't all that useful.

Here's a fun one I picked up over at Ecomodder when I was doing research on gen1 Insights before buying one:

Boy do those parasitic losses grow.

 

[Zorba]

I don't follow your logic. Real world interstate driving in LA means I rarely get above 65😉

I've been averaging about 30-31 in my Fozzy.

By real world interstate driving, I meant when you are driving between states, not within a city, hence the name 😉.

I think my Fozzy has something wrong with it, but the dealers won't do anything unless it throws a code. The last one told me they could don't anything unless I could provide a fuel log, so I did provide my fuel log, and then they still wouldn't do anything 😡 Or everyone else lies about their fuel mileage at steady state 75 mph.

 

[Zorba]

You're right, of course. I'd like to add that some of the losses before the flywheel can be changed by things like gearing, too; for example bringing engine load up closer to 100% at a lower RPM generally increases effiency, both by increasing cylinder pressure and by reducing pumping losses, as do driving at higher altitude and increasing intake air temperature, so now that I think about it, that chart isn't all that useful.

Here's a fun one I picked up over at Ecomodder when I was doing research on gen1 Insights before buying one:

Boy do those parasitic losses grow.

Something I forgot to mention, though, is as your drag increases per your original chart, the amount of thrust need to overcome it increases. Meaning your engine has to deliver more power, so losses from engine inefficiency would be roughly proportional to the drag chart.

This is why electric cars can get an 100mpg equivalent. Since electric motors over 125HP typically have an efficiency of >92.4%. Of course there are losses converting the chemical energy in the battery to AC power for the motor, if we say those are only 70% efficient, you get a total efficiency of ~ 65%, where in the real world gasoline engines are probably down around 20-25%. So right there you should get roughly 3x better range on an equivalent amount of energy.

 

[tsupersonic]

'13 BMW 335xi: 36-38 MPG on highway @ 70-80 MPH. It's rated for 20 City/30 Highway. I could probably get slightly higher if I engaged the Eco-Pro mode.

Not a big fan of cruise control - while it is very convenient, it is very disengaging driving experience. Not to mention, I don't need to maintain a set speed going uphill.

Just did a trip yesterday 360 miles - (rental) '13 or '14 Jetta SE: 39 MPG highway. Still have more than a 1/3 of tank left

 

[Raduque]

Hrhrhrhrhrhr

I just drove 430 miles on around 29 gallons of gas.

75-80 the whole way.

I love my truck.