Why do cars get such better mileage at speeds under 65 mph?

Machine and Scr, thanks.

You can’t optimize a car that way - air resistance goes up as square of speed, and you can’t change that fact.

You could design a car so the engine is most efficient at 65 mph. But that won’t make up for the fact that you need 70% more energy per mile to maintain 65mph than 50 mph.

When I was in graduate school (late 1980s), I had a 1981 Plymouth Reliant. 4 cylinder, manual transmission, no A/C, very few options, and none of the safety equipment a similar car would have today. The thing got over 40 mpg on the highway (I also drove strictly at 55-57mph, after having gotten 2 speeding tickets in a month :D). I would imagine that a similar car, today, would be lucky to reach 30-32 mpg on the highway.

Best mileage is in a bubble right about or below 2000 RPM, where friction losses are lowest. At least in modern, overpowered cars.
Best mileage in cars that aren’t overpowered might be at peak torque, which would be 4000 RPM or so in modern 4-cylinders.
Few cars turn 4000 RPM at 55-65.

For these purposes, small cars making over 50 HP and large cars making over 80 HP are overpowered if you are planning on avoiding very hilly terrain.

So it basically costs me an additional 1.5 gallons of gas to drive to Vegas at 80mph than it does to drive there at 55mph? $5 vs an additional 30 minutes on the road? Totally worth it.

One of the bigger ironies being that it needs a lot of that extra safety equipment on it because all the other cars are carrying that extra weight too.

Obesity in humans has nothing on the metaphoric obesity of cars in the last few decades…

If it take you 2.8 gallons of gas for you to drive to Vegas at 80 mph, then yes, you save “only” 1.5 gallons by driving at 55 mph instead.

If you get the SFE option the Fiesta it is rated at 40MPG highway. The EPA numbers are apparently pretty conservative, since most Fiesta owners report beating them.


From what I read, the Fiesta Econetic makes the regular Fiesta look like a fuel hog (over 60MPG) , but it isn’t available in the U.S.

Well, I was talking about mileage I actually got. Are you suggesting that, government rating being apparently nearly the same, I could get 50 mpg out of a 2011 Fiesta driving the same way I did the Festiva?

Possibly. In my manual 2011 Fiesta, I average pretty close to the 37 mpg EPA figure for my interstate commute. My average driving habits include modest speeding (70-75 mph) and fairly lively acceleration. If I stick to the 65 mph speed limit and drive somewhat conservatively, I get around 43 mpg. There aren’t many 55 mph highways around me, so I’ve never measured my efficiency at that speed. Still, I’d guess you’d see something around 50 mpg, since there’s a significant reduction in air resistance.

Also I think the engine will be running a little more efficiently – it’s around 3250 rpm at 65 mph, vs 2750 at 55 mph. This brake-specific fuel consumption chart for a similar (but not identical) Ford four-cylinder engine suggests optimum efficiency is between 2000 and 2500 rpm.

The Fiesta Econetic (which to note, is a diesel) is rated at 63 mpg using the European fuel economy test; that test will give the same vehicles a higher rating than the US one, as it involves lower top speeds, gentler acceleration, doesn’t use the AC, and so on. For example, it rates the regular gas Fiesta at 48 mpg highway, compared to the 38 mpg the Fiesta gets under the US test. I am guessing, given the ~20% difference between the Euro and American figures for the regular Fiesta, the Econetic would probably get ~50 mpg highway on the US test.

Still very good, but the extra cost doesn’t make sense in the US. Checking Ford’s UK website, the Econetic is £1850 more expensive than the gas engine of similar power output (though the diesel has nice torque advantage). That is $3030 at current exchange rates. According to AAA, the average price of gas in the US is $3.58; diesel $3.88. So if the regular Fiesta gets 38 mpg, and the Econetic 50 mpg, the regular Fiesta will cost 9.4 cents a mile, while the Econetic will cost 7.7 cents per mile. So to make up the $3030 difference in price, you would have to drive nearly 180,000 miles. Then you can start saving money.

The Econetic makes sense in Europe since it doesn’t have to pay road tax (since its CO2 output is just under the legal limit to not pay those taxes), the fuel prices in general are higher, and in many European countries, diesel is cheaper than gasoline, due to not being taxed as much.

Also, the US (especially California and the other states that follow CARB) has much stricter laws regarding NOX (nitrogen oxides) emissions, that diesels are bad at, than Europe does. That is not impossible to get around, but requires even more expense for fancy emissions control equipment (that reduce mileage) and/or Urea injection. Which adds a bit of extra cost.

By performance I was talking about powertrain design. Of course you can’t optimize coefficient of drag for a particular speed, you can only try to get it as low as practical.

I have to admit that the Econetic may not make much sense in the U.S. I was doing the math and a Honda Insight looked better if you do a lot of city driving.

I would be happy if they had a car that used a flywheel for regenerative braking. They are potentially cheaper and more reliable. I’m concerned about the cost when you have to replace the battery pack on a conventional hybrid.


From what I have read on various user forums, the batteries in Toyota & Ford hybrids usually have little trouble lasting 200-300 thousand miles, basically as long as the rest of the parts. Honda’s hybrids, on the other hand have had under-spec batteries that often run into issue long before then. Though the new Honda hybrids have switched to lithium batteries, so they may or may not be fine.