Manual V. automatic gas mileage


#1

I have a four cylinder five speed manual Ford Fusion. At 70 mph in fifth, the engine is revving at 3k rpm’s.



I’m currently driving a loaner car, which is the same model except it’s a five speed automatic. At 70 mph this car is at only 2400 rpm’s.



Everything I’ve read indicates the manual version gets one mpg better highway fuel economy. Given the higher rpm’s the manual is operating at, how is this the case?


#2

There are many factors that go into what kind of mileage a car will get. RPMs are not the most important one. In addition the most efficient RPM will vary with the total weight of the car, the road conditions, the type of transmission etc. Lower RPMs are not always more efficient.

The biggest difference is usually the driver and the driving style.

There are many different kinds of automatic transmissions and some are more efficient than others.

The numbers reported in the owner’s manual have been measured for one specific set of conditions and your mileage will vary.

Consider this. IF lower RPM was THE factor, the auto makers would all just change the gearing to reduce the RPM at all speeds.


#3

The EPA highway gas mileage is not the car’s 70 mph fuel consumption but the total of a simulated high speed trip that includes acceleration to cruising speed and a few speed variations.
The automatic may very well use less fuel at a steady state 70 mph.

Did you have the loaner long enough to check its gas mileage?

The math involved with figuring average gas mileage is the same as that for figuring average speed. The fuel used during the acceleration phase of even a rather long trip may not be offset by the lower cruise fuel consumption.

Example, if you get 5 mpg for the first half mile accelerating to cruise speed, what mpg do you have to get for the second half mile to average 10 mpg? The answer is infinity, the only way you can average 10 mpg at that point is to switch the engine off and coast the rest of the way.


#4

With all things being equal you’ll get much better then 1mpg cruising at 70 with the manual then the auto if there’s a 600rpm difference.

But how much of your trip are you actually cruising along at 70mph?


#5

To answer your question would take in depth knowledge in how the Federal testing is conducted to determine mpg ratings. I do know that the old test simulated a highway speed of approx 50 mph and the new test is higher, but how high?

At 70 mph if you just drove at that speed all day the auto would beat your 5 speed manual for mpg based on the lower rpm. So why did Ford use a different final drive ratio? I suspect if the final drive of the manual had the car at 2,400 rpm you’d find the car needed to shift from 5th down to 4th fairly often on hilly stretches of road. Since the auto does this for the driver it is OK. In the manual the driver has to make the change and Ford must believe this would not be acceptable to many drivers. 3,000 rpm is in a better torque range for your motor and it can cope with hills better since it is operating in its “sweet spot” for power.


#6

The EPA mileage is measured on a stationary simulator, and cannot accurately reflect driving conditions. It is only useful for comparing one car with another to levey emission taxes.

Also, as mentioned, some automatics are more efficient than others. It’s quite possible for a sedate driver to get better gas mileage with an automatic.


#7

There’s more to it than just the type of transmission. In this case, the final gear ratio for the automatic is numerically lower than the manual. This means that fewer RPMs are required to turn the drive wheels once. Since the outer diameter of the wheels are the same for either transmission, the engine doesn’t work as hard for the automatic; the gas mileage is higher. If you check a lot of EPA milages for cars with both transmissions, you will see that several automatics have higher highway mileage than an otherwise identical manual transmission car.


#8

Usually your manual transmission is more efficient than an automatic because of the clutch versus the torque converter. The clutch has a little more grip on the engine and loses less power, but they are making automatics better and better. Also rpm is not a good way to judge economy, if the engine is not under a load it takes less fuel.


#9

The automatic transmission needs constant cooling, but the manual doesn’t.