Reported mileage way off (too low)

There is so much variation in the fuel bladder that you can't make any sort of calculation based on one tank. You'll need to total the mileage over many tanks to get a reasonable calculated value.

Tom

 

I am starting to do a three tank average on my spread sheet. I may need to go to four. My normal observation is one of a short lived high that come close to the calculated average, followed by falling MPG for the second third of tank or so. After that the MFD average very slowly rises to a final that tends, but is not always, a bit below the calculated average for the tank. In very cold weather the MFD reads a bit high.

 

I'm very new at this since I haven't even filled up once but I know there are some discrepancies with the MFD mileage....I only have about 220 miles on the car but have noticed that the MFD is displaying 212. I got the car and reset the MFD when it had 3 miles on the odometer so there are 5 miles unaccounted for. I'm going to have to see over time but I think there are just some optimistic errors with what the car reads overall. I would recommend what I am planning on doing and just refill when I am told to do so and then do an overall calculation to see what mpg I'm actually getting. As far as the low gallons of fuel needed to refill the car, I have read that refilling on inclines/declines do affect the tank display. Good luck to you!

 

It IS unusual for the bladder to be unable to accept that much of a difference---in normal-to-warm weather. I have heard of almost that much shrinkage in cold weather. Thus I'd say the automatic shutoff on that pump is suspect. The only other possibility was an air pocket in the filler neck, but I doubt it. Assume you tried to keep pumping a few times, with it repeatedly clicking off.

 

I just had a similar experience. 3016 miles/53.49 gal=56.38 mpg. Way to high I think, the consumption monitor gave me an overall average of 52 to 53 for the entire trip. That would indicate a 4 gal difference between the first and last fill up. To much I think, I might believe 2 gal but not four. My experience with the car after 15K miles leads me to believe my milage was closer to 52 than 56 but I can't explain the discrepancy. I would love to think I got 56 mpg for the trip but I don't think I did.

 

I have to say that sadly I have manipulated the bladder? (It is OK for those of you that have done the same to come foward) I have waited for a cold day to fill up knowing that it would make my "calculated" MPG higher than on a warm day. Of course in the end I know that it averages out, but I can not resist the total high of the MPG that is over average. Can you forgive me, my brother and sisters?

 

I don't hdrygas you were my hero you know ... OK you're forgiven!

b