michail71

I have a US region 2013 C250 that has been reporting a lower MPG than what I thought it had been getting.

On a road trip I kept track of the trip computer and the actual fuel used when I refilled. The total MPG reading I got for that tank of gas was 30.5 by the trip computer.

When I refilled the car and calculated the MPG it came out by a factor of 1.2 times higher. Which was 36.5 MPG. This MPG also matched up better with the car's range figures and what I was seeing on the real time consumption gauge.

This got me to speculating the cumulative MPG figures displayed in the trip computer are in British gallons. One British gallon roughly equals 1.2 US gallons. This would result in a number that is 83.3% lower.

knowbenz

Those readings are hardly ever accurate and I would just use it to give you a general idea.

michail71

But if the trip summary is so inaccurate how is it the range and real time readings all seemed to be spot on? The trip computer even reports down to the tenths place. Which indicates an inherent level of accuracy.

Everything added up for me except they summary screen. If I convert from British to US gallons that then falls in line. But I guess the standard in England would be the liter now?

My previous car allowed for setting units. But it was also accurate compared to the old fashioned pump measurements. One of the choices was for British gallons, which would show numbers around a few miles less per gallon.

I'll run a few more tanks and compare actual to the summary. If it keeps coming out to 83% less than actual this could be what's going on.

knowbenz

Did you ever reset the counter?

michail71

Yes, I did for the trip. It was also a 500 mile non-stop round trip, so the "from start" was for the whole trip and I reset the other one at the half way point. The reason I did the reset was I had unloaded a passenger and luggage and started back around in lighter traffic.

I'm going to gather some more data before I jump to any more conclusions and see if the pattern persists.