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Avg. fuel number reads drastically different when I pick up my car from repairs

I took my vehicle in for repairs after some front end damage from hitting a flying piece of plywood on the highway. The average fuel economy was posted at 18.4 before repairs. The day I pick up my vehicle from the shop it now reads 13.4 average. Thoughts?

The battery was disconnected while being repaired and the average was reset to zero. Moving the car in the parking lot boosted the reading to 13.4.

Drive it normally, the average will come back up.

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Whenever I pick up my car after servicing, the mpg readout inevitably reads ~2 mpg less than when I brought it in. When you take idling in the shop and also moving the car around on the shop’s premises into account, and then throw-in their post-service test drive that is undoubtedly not done with an eye toward good gas mileage, that inevitably leads to a drop in the average mpg reading.

The next time that I fill the tank and re-set the system, my mpg readout always returns to my usual/normal numbers, and–more than likely–the OP will have the same experience.

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Measure your fuel economy “manually”.

Note you car’s mileage the next time you fill up with gas. At your next fill up, note the number of gallons to fill up the tank, and the mileage. Calculate the difference in mileage, and divide by the number of gallons of gas. Do that several times over a few weeks, and that will give you an actual MPG figure.

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I’d hit the reset button and start over on the average.

Shops like to reset things like this for some reason and shut everything off or they disconnected the battery. In either case, looking at the average over a long period of time is really not very meaningful. Going downhill you might get 50 mpg and stop at a stop light down to zero so on the average you might be at 30 so how is that useful? I usually just reset the thing on the highway to get an idea of highway mileage and pretty much ignore city and overall average. You have to understand what average means-time and miles traveled.

On both my Mustang and F-150. There are two separate trip odometer readouts , each one gives you time in hours/minutes/seconds since you last reset it, the distance covered since you last reset it, and the average MPG since the last reset. I reset one when I took delivery of each vehicle and have never touched it since . The other one I reset at each fill up. My thinking is that if I ever have a suspicious drop in fuel economy, I can compare it to the historical average at will, and then figure out what’s causing it (recent driving trends, etc.).

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That’s the A and B trip meter on my car. On a trip I usually use the A for the total trip out and back and the B for each way. Like I said, I see no reason to accumulate a lifetime mpg and hours of driving figure and I do a lot of top offs and various driving patterns which makes tank comparisons not valuable information. But to each his own.

Old habits die hard with me. My first cars had neither trip odometers nor mpg readouts. I had a little freebie from my insurance agent that clipped on the sun visor. It had five dials that I set to the odometer reading when I bought gasoline. The next time I bought gas, I would mentally subtract the reading on the 25¢ gizmo from the insurance company from the odometer reading. I would round off the number of gallons it took to fill the gas tank and do a little adjustment of the miles traveled and mentally do the division. This was all the accuracy I needed to compute the mpg.