I just saw another thread and it was indicated that EPA numbers for fuel economy are too high for "all cars".

I beg to differ.

For memorial day I had to transport some stuff from San Francisco Bay Area to Las Vegas. I rented a Chevy Silverado, long bed, 4.8l V8 engine. 285 hp.

With the gas prices I looked on the web how that car was rated, and EPA says:

City (mpg) 15 - 17
Highway (mpg) 19 - 21


I drove 115 miles, filled up and it needed 5 gallons. Yes, that's 23 mpg. I had gone 60 on average because I wasn't on the interstate yet.

Then I drove 420 miles (nice to have such a big tank), speeds between 70 and 90, never over 100 mph. Through the mountains.

When I filled up in Vegas, I calculated 22.6 mpg!

City driving in Vegas, well, congested freeway driving, to be accurate. 21 mpg then.


I'm just writing this as yet another example how it is possible to do better than EPA's numbers, and I didn't even have optimal conditions (cruise control @60 would be optimal).

On a side note, it's fun to smoke a V6 Mustang in such a truck. I'll never want to own one though.

-Peter

 

The EPA numbers are obtained by running standard tests that are supposed to be representative of city or highway driving. The tests describe vehicle speed versus time traces that must be followed for both city and highway. Fuel economy is then calculated. The only guaranteed way you are going to match the EPA fuel economy numbers is if your driving style and routes happen to give you vehicle speed versus time profiles that are very similar to those used in the tests.

For example, the cruising portions of the highway tests specify vehicle speed around 60-65 mph. Therefore, manufacturers definitely calibrate the powertrain to achieve maximum efficiency at whatever engine speeds correspond to 60-65 mph in top gear. So, if you normally cruise at 80 mph or even 70 mph on the highway, don't expect to achieve the EPA number for highway fuel economy.

The same thing goes for city driving. The test specifies a bunch of stops and starts, with cruising speeds anywhere from 10 mph to 45 mph. If your typical driving habits aren't in-line with the driving style laid out in the test, you're not going to get the EPA number for city driving. Keep in mind that when this test is run in a MT vehicle, the vehicle is almost always shifted at the best-fuel points. If you're not shifting at the best-fuel points, which you're probably not because you'd have to take a lot of experimental data to determine these points, then you won't achieve the EPA number for city driving.

However, in a lot of ways, the EPA tests specify vehicle speed profiles that are a bit extreme, i.e. you normally wouldn't drive the vehicle like that all day. So it is very possible in some cases that you will achieve better fuel economy than the EPA estimates. There are so many factors that affect your fuel economy that it's very difficult to determine exactly why you fall short of EPA numbers or exceed EPA numbers.

In the case of the RX-8, I'm baffled, because it seems almost all vehicles are falling well short of the EPA numbers. This is strange, because the EPA test is run with production U.S.-spec vehicles, and it would be way too risky (from a legal, financial, and reputation standpoint) for Mazda to try to alter the hardware or software after the fact. There's something missing from the puzzle.

 

best points to shift for fuel economy unfortunately are like at 2500rpm, thats why if you've ever driven a car with a shift indicator light it always seems way too low...and freaking annoying too

 

My uncles 1990 Isuzu Rodeo had a shift indicator light...almost as soon as you shifted into 2nd/3rd that damn thing came on. Much more annoying than useful.

 

The EPA sucks.