Cars: How Do I Verify City MPG?

Factual Questions
1
      • For cars two measures of fuel economy are typically given, highway and city. I can figure out a way to test my vehicle’s highway MPG, but what are the standards for testing city MPG? How fast or slow does one drive, and how many stops-and-goes are there per mile? - MC
2

::shrug:: I dunno. It’s your city driving, so just figure it on your work route and be done with it. Perhaps not how they do it, but screw them.

–Tim

3

I’m not sure that it’s all that important to verify the consumption rate…those numbers’ primary purpose is to give you a basis of comparison between cars, for the most part. I’m not sure there’s any real testing protocol that goes into the development of those numbers. Then again, I could be wrong.

4

I’m sure the same method you use for hwy MPG would be close enough. Are you concerned about YOUR consumtion or testing your car against industry standards?

5
      • I wanna know because I want to test my own truck (if I can), to see if it is up to its original figure.
  • For highway MPG it’s easy; you just drive until you run out of gas, pour a known quantity of gas into the tank and again drive until that runs out, noting the odometer before and after. I would assume the 55 MPH speed limit is the intended cruising speed (as it was at the time of my vehicle’s manufacture). This method isn’t exactly perfect, but averaging the distance traveled with 5 or 10 gallons is likely pretty close.
  • The problem with finding city consumption is as I said, how fst do I drive, how many stops and starts should I be averaging per mile. - MC
6

At one time, and this may still be the case for all I know, the MPG numbers were generated as a side result of emissions testing. To do this, they put the drive wheels of the cars on rollers so they could have the exhaust hooked up to sensors. They run the engines at different speeds for city vs. highway regimes, but I forget the details.

There’s an obvious problem with this method of testing, of course, it doesn’t reflect actual driving conditions. There’s no allowance for air resistance, for example.

So when people got their cars and figured their mileage, they found that they usually got less miles per gallon than the official numbers. Of course, this generated numerous complaints. And what did the officials do about them? Did they modify the testing procedure to run the car on an actual city street? Don’t be silly. That would require too much paperwork, to say nothing of real work. Worse, it would cost money. Instead they came up with one of the all-time famous disclaimers “your mileage may vary”.

7

Similar in the city. You top off your tank, write down the mileage (or zero your trip odometer), and drive normally. As dtilque already said, you won’t be able to simulate the factory test, unless you drive on a set of rollers and don’t actually go anywhere, so just drive like you normally drive in the city. Obey the speed limits, stop for lights and stop signs, etc. When you’re near the bottom of the tank (NOT out of gas, just low–you want to use as much of the gas as possible so that you can get a better average) then go fill your tank to the top again. Write down the mileage and how many gallons you put in.

The gallons to fill the tank is the amount of gas you burned. The difference in odometer readings is how far you drove. Divide. Miles by Gallons.