wow you are obsessive! and to think i just stop for gas when the light comes on, fill it with premium, select "no" when asked for a receipt and drive off.. i never know how many gallons i put in and i never look at what it cost to fill the tank or how man miles i got on the last fillup.
its just one less thing i need to worry about, or concern myself with.

 

This is because car manufacturers intentionally calibrate speedometers and odometers on the "safe" side. This means that the speedometer should read a little high, and the odometer should read a little low. The 38 MPH average is just a simple calculation of miles driven since the car was started (or reset) divided by the time since the car was started (or reset).

 

I don't bother checking mileage at the pump anymore since every pump is calibrated differently. The ECU might not be perfect, but at least is will give you consistent results.

 

Yep, I have noticed this too and the cars calculation it about 2-3 mpg more than mine.

 

I think the answer to this could very well lie in what the computer treats coasting at... if it caps it at 99, the computer weighted average of your drive will be less, assuming that coasting actually returns more than 99...

 

When you are truly coasting, the ECU is not providing any fuel, so instantaneous MPG goes to infinity.

 

right, but it depends on how the computer that calculates your tank MPG treats it. if it says anything above 99 is 99, then the average will be low. if it's smart enough to hold off calculating until the next time you use gas and then divide the coast distance by that gas use, then it should be accurate.

 

actually i think it maxes out at 60 mpg my acura TSX maxes out at 100 mpg. i know because if you reset it while going downhill and the car in neutral engine ideling my TSX returned an average ecomoney of 99 MPG the IS shows 60 MPG (maybe 70 i forgot) but in any case i dont think this is a factor. also i can tell you that runnig in neutral down hill is considerably more fuel efficient than simply coasting in 6th gear for 2 reasons. 1. when you finish the hill your car will roll for a while longer without the engine still slowing it down, and 2 without the additional revs on the motor each of which still has to delever the same duty cycle as when ideling but it has considerably more injector squirts because the engine turns more revolutions than it would when ideling over the same distance (miles)
its probably not much but i have played with fuel economey because i was bored for many miles in the TSX and can attain easily another 3-4 miles per gallon by pushing in the clutch verses coasting while in gear whenever gravity can move the car for free.

 

My experiences are similar to Bichon's, actual calculated mpg is .5-1 mpg higher than TANK AVG display. . .

 

Just because the guage is capped at 99, 90, or 60 MPG doesn't mean the computer is considering it to be that value. How would you expect the ECU to display an infinite instantanious MPG while rolling down hill in gear?

Coasting in neatral down hill would use more gas since the car would still be using gas while the engine idles. If the car is in gear (engine braking), the ECU is NOT delivering any fuel.

 

Just filled my tank on the way home. Displayed tank average 21.7. Calculated mileage: 346.2 miles / 15.1 ga = 22.92. Very typical. I wonder if there is a calibration setting one can do with a factory scan tool?