Flumoxxed or Bamboozled ? : On board MPG calc.

hand calculate mpg.

there is a reason they are often referred to as 'lie-o-meter'

Hey, I keep my receipts and write in the odometer reading too. I put the info in a spreadsheet to see how I did on the trip. (I'm a Mechanical Engineer and I can't help it).

Doug,

Excellent, thank you. I have never recalibrated, reset, or
manipulated the original settings in any way.... need to
hit the manual. Sounds like operator error to me. Next time
I'm in for service will add " check the mileage calculator "
to the list. I kind of wish it was just simple like my refill
calculation... I will check the mile markers (good idea).

Funny, I still save refill receipts and write in odometer
settings...need to get hip with the new technology !

Charles

The instantaneous reading is calculated over a very, very short period of time/distance. The average mpg is calculated from the last time you zeroed the reading until you reach a mileage limit (that tends to vary among the manufacturers). Your owner's manual may or may not tell you what that limiting distance is. For example, if you never reset the gauge you may always be seeing the average for the last 200 miles (or whatever the limit is). Unless you've had your computer re-programed the change in tire size will not have been factored in. Check highway mileage against mile markers to see the impact.

James ,

I see... "calculates it over a period of time" .. that is
different then what I expected. Need to pay better attention
to that info.. I did change the OEM tires ... any effect on
readings ?

Doug,.... mine falls into the "optimistic" category.

Charles

The vehicle computer samples the pulses of the fuel injectors and calculates the fuel delivered based on the amount of time the fuel injectors are open and delivering fuel. Strange thing about that - if your vehicle is able to show the instantaneous mpg, if your fuel pump is going bad and not delivering fuel, the instantaneous mpg reading will go really low because the engine computer is calling for fuel and the injectors are being held open. By the way, all vehicles seem to be optimistic in determining their mpg.

It analyzes your driving, computes it and calculates it over a period of time then thinks that is your mpg. That is the short answer.