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Gas Mileage Displays In Cars: Accurate Or Optimistic?

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Gas mileage trip computer

Gas mileage trip computer

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How do you measure your gas mileage?

Do you take mileage readings after each tank, refill to the brim, calculate how much you've used...and repeat at each fill-up? Or do you press the trip computer button a few times to bring up your tank average?

If you're doing the latter, there's always the risk the on-board computer isn't quite as accurate as you'd hope.

Last year, both Edmunds and The Detroit News reported that fuel readouts were often around 5 percent optimistic, with some cars as much as 15-20 percent out.

MORE: Want To Track Your Real-World Gas Mileage? Here's How

While it doesn't affect the amount of fuel you're actually using from tank to tank, it might give you a false sense of how well your vehicle is performing when it comes to fuel economy.

Gas gauge

Gas gauge

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You'd then brag to your friends about how frugal it is. They'd go out and buy one too, and before you know it everyone is driving around in cars that lie about their fuel efficiency.

But is it really that bad? Wards Auto suggests not--and says that, contrary to previous research, trip computers are probably the most accurate way of determining your real MPG, more so than the traditional tank-to-tank method.

Wards offers several reasons for this line of thinking.

One is that if most vehicles were over-reading, then most vehicles would happily beat their EPA-rated economy figures when driving in the real world. In their experience, and in our own experience, that isn't the case--most vehicles are pretty close to the EPA numbers.

The next is that the same software used to calculate instant and average MPG also tells you how much fuel you have left in the tank.

At what point does the fuel computer suddenly start telling the truth about your mileage--before, or after the range indicator has dropped to zero miles and you're out of fuel?

MORE: Gas Mileage During Dealership Test Drives: Mostly Meaningless

Lastly, even the experts think trip computers are accurate. Modern cars control fuel flow and exhaust emissions very precisely--data also used to calculate the car's economy at any given time.

If those sensors were so inaccurate, your car probably wouldn't be very efficient in the first place, as it wouldn't be working correctly.

In the end, despite the odd inaccuracy here and there, the trip computer could be the best way of determining your real-world fuel efficiency.

And even when it isn't, it should at least be consistent with itself--so can still be used as a tool for improving your driving habits, by beating your previous economy figures.

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