Calculating your actual gas mileage

One of the more common questions people ask when doing an engine swap or upgrading your turbo is “What kind of gas mileage do you get?”

Well, this answer is going to depending on a few things:

  1. How is the engine tuned? If you’re running pig rich then you’re going to get terrible gas mileage.
  2. How do you drive? If you’re constantly mashing the gas pedal and generating a lot of boost then you’re going to get terrible gas mileage.
  3. Are the fundamentals of your car set up properly (i.e. are your tires inflated to the correct psi?).

All of these things will contribute to your gas mileage. My recommendation is to do a controlled test that incorporates your typical driving habits so you can get a real, true answer as to how much gas mileage you’re getting.

This test is so ridiculously easy!

  1. On your next fill-up at the pump, reset your trip counter to zero. This test assumes that your trip counter is in working condition.
  2. Put a full tank of gas in the car.
  3. Drive your car as you normally would until you’re ready to fill up again.
  4. At your next fill-up, write down (or take a picture) of your trip counter and the number of gallons of gas you put into the car.
  5. Divide the trip counter by the gallons (use decimal points for accuracy).

The result is your actual miles per gallon.

Trip Counter
Gas Pump

Using my example above, I would take 230.1 / 13.699 = 16.796 miles per gallon.

My driving habits included a lot of rapid accelerating, some downtown driving, some highway driving. My AFR is typically 15 at cruising speeds to help. This low mpg is likely due to my own driving habits. When the weather is nice I drive the 240sx for fun, not for gas mileage.

Do this test yourself and see what kind of mileage you’re actually getting. You might be surprised.

*** EDIT ***

I did a more thorough test without hammering down and here are the results for casual driving: 17.9 mpg City and 22.1 mpg Highway. It’s not bad as long as you aren’t getting into the boost a lot.



4 thoughts on “Calculating your actual gas mileage”

  1. nice tip! my s14 is consuming a lot so ill have to do this math 😀
    Dude a have to ask you some questions about which clutch do you recommend me to go with.

    1. I like the SPEC Stage 3 or Stage 3+ clutch with matching flywheel depending on your torque requirements. I did not like the SPEC Mini Twin clutch – it was too stiff for street driving.

      1. Nice, so right now are you using a spec stage 3 or 3+?

        I was doing some research and come up with this numbers:

        Spec Stage 3 for $286.71 + shipping
        Spec Stage 3+ for $456 + shipping
        Clutchmasters FX400 6 puck for $445 free shipping
        ClutchMasters FX500 4 puck for $306 free shipping
        Competition Clutch Stage 4 6 puck for $364 free shipping

        Which one would you go for a fun street driving and sometimes drifting that could hold 500whp without wear a lot? Like you i dont want something like the twin disk to wear it out driving normally in traffic.

        Thanks again rad

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