mileage calculation

skeeter123 · Post by **skeeter123** » 28 Jul 2010, 08:49

Does anyone know the particulars on how the computer calculates the mileage? I've gotten so used to using it, I haven't done the fill-it-up and calculate it yourself method in a long while. Last time I did, I think the manual calculation was more than the car-computer. Questions:
What is the averaging algorithm? IE how long/far does it take to go from inner-city stop-go measurement to an accurate highway-long distance measurement?
What if you sit at idle for 10 mins, or an hour? Does computer reduce calculated mpg? It knows your speed is zero, does it count that in? (my manual claculation certainly does).
By the way, the computer-mpg on my na-S60 has gone down from 28.5 to 27.4, in concert with my daily comute becoming much shorter. But I recently took a 140-mile trip and expected the mpg to go up to 29 or 30 highway. Didn't happen. I'm about to replace the spark plugs (it's time) and am wondering if that would get me back 1 or 2 mpg....

Post by **priapism** » 28 Jul 2010, 13:34

doesn't it just continually divide distance by fuel used?

Post by **IVIUSTANG** » 28 Jul 2010, 16:38

Yes, the computer does account for idling and driving variation; however, it usually makes big adjustments slowly. For instance if I idle for half an hour it won't start lowering the range until after several minutes. When I enter the highway it can take a good 10 minutes+ for my range to go up a good 30 or 40%. No algorithm, but the estimate is not always real time, but is buffered from current fuel economy data over an average time I would say.

- Jesse

Post by **priapism** » 29 Jul 2010, 07:48

Cars use very little fuel at idle, that could be the reason for the smaller change when idling.

Post by **matthew1** » 29 Jul 2010, 10:44

I think it continuously divides miles/fuel used, nothing fancy.

On my camping trip a few weeks ago we came off the highway and idled for 5-10 minutes and it went from 30 to 28 mpg.