My trip computer is usually 1 to 2 MPG lower than calculated. When I get home, I'll post the difference for a few tanks.

 

I recently had my first fill-up. The calculated MPG was 1.7 better than the dash.

 

Well it really seems that the results are very consistent. I know I take gas at the same station at the same time and never top off. Its more about convenience than intent.

 

*Summons the thread necromancer... I waited a while on this just to get more tanks, and because the pumps at work are old-school dials rather than flashy digital jobs so I wanted to be sure I averaged out as much as possible. With 11 tanks now in, my hand-calc is 1.7mpg better than the FCD (50.1 vs 51.8). This begs three questions:

1. Any idea on how to tell whether or not I've got that supposed firmware update that brings the two closer together?
2. Those getting your cars now, are you seeing the same discrepancy (meaning the firmware change doesn't help anyway)
3. Is it appropriate to occasionally apply a "correction tank" to the database in order to bring things back to sync?

 

because one has to factor in when gasoline vapors are being brought up because of the sun. Park it in shady areas always and I bet the calibration will be almost dead on.

 

I'm not sure if this makes any sense. First of all, I believe all of these cars have vapor recovery systems and vapor loss is extremely minimal.

Second, if gasoline vapors are being lost, the calculated number would be artificially low. For the HCHII, it is the display that reads low.

Imagine an extreme scenario. Throughout the course of driving a tank, 5 out of 10 usable gallons of gas are lost in the form of vapors. Only 5 gallons were actually used by the ICE. Let's assume the car was driven 250 gallons. In reality, 5 gallons were used to achieve this and the real FE would be 50mpg (close to what the dash says). However, upon filling the tank, you put in 10 gallons and calculate your FE as 25mpg. Because of vapor loss, the calculated FE would be artificially low.

 

Same here. Now with near 17,000 miles on the dash MPG in comparingdash to actual is about 1 to 2 MPG off.

Last read 50.2 MPG actual was 51.38

alster

 

I believe the variance is based on how empty the tank is. If we onlky used lets say--3 gallons and then gassed up, I bet this discrepancy would disappear.

When you drive the tank to the last two dots or less, the discrepancy is always present.

This is likely due to how the fuel consumption is translated into MPGs. As the miles travelled per tank increase, it becomes increasingly difficult to bump up ( or down) the average MPG based an actual driving-- however-- the start up effect appears to be disproportionate on the trip odometer average MPG reading.

Anyway, my last difference was 0.9 as of today when I filled up after 512 miles. On any full tank its been between 0.8 - 1.4 calculated greater than meter reading.

 

rysa, that's the whole reason I waited until I had more tanks to judge from. Personally, I usually gas up when I have three or four blocks left, just because having pumps at work make it so convienient. The abnormality, though, exists on all tanks, and since it's so common, it's a fair call there's something not right in the detection components or math.

 

What I am suggesting is that if we plotted a graph of gallons used at time of filling up vs calculated/dash MPG difference, that as more gallons are used at time of fill up the greater discrepancy would be seen. Its unlikely math is consistently wrong across most users-- its more likely an issue of how detected fuel use is translated into dash reading MPG and how the miles travelled affects that translation.