DIS vs manual MPG calculations

**gcon45** · 23-03-2012,05:48 PM

Hi guys.

I reset all my trip settings at my last diesel fill to see how accurate DIS was.

Done the usual calculation (litres filled divided by 4.546 to get the gallons and divid the mileage by the gallons) and found that I was averaging 43.5MPG.

DIS over the same trip however calculated my average of 47.9MPG.

What do you think is at fault?

The odometer or the DIS?

I would have thought they work together?

My tyre size is 225/45/18. This isn't worlds apart from the rolling radius of the original 235/45/17 tyres so I don't think this would adversly affect the odometer readings.

Any thoughts?

**RMH1** · 23-03-2012,05:58 PM

This is normal dis is always optimistic and reads above actual figures.

**darich** · 23-03-2012,06:53 PM

DIS is erring high.
I think every car with an on board computer does the same.....but by varying amounts.

My DIS is displaying an average of 30.7mpg....but using the odometer and brim to brim method on a smartphone app, my average is 27.83 (over 7300miles and 1year)

**gcon45** · 23-03-2012,07:44 PM

Aw...

That's what I thought.

I'm averaging a whopping 57MPG on the longer runs according to DIS.

That's probably around 50 in real world terms in that case.

Still, not bad.