Fuel calculations are off

[jembo]

I've noticed that my 'miles to empty' and 'average mpg' readings in the message center don't agree with each other, and in fact they're pretty far off. For example, I filled up the car to full yesterday, so that's 18 gallons. Today, after driving a total of 20 miles after the fillup, with it telling me that I average 16.5 mpg, it says I have 348 miles to empty. Now, I know that I currently have at most 17 gallons left in the tank. If I have 348 to empty and average 16.5, I would have to have 21 gallons left. So, it's calculations are off by a good 20%. Is this a common thing or is mine having a problem somewhere? Thanks

 

[Calabrio]

My Mark VIII message center has been giving me bad information for a long time now. It constantly gets the average and instant mpg wrong, which then puts all of the numbers at odds with each other. It always says I've used fewer gallons (usually 1 or 1.5 less) than I have, and that means my mpg numbers are artificially inflated.

I've yet to find a solution or explanation for this.

 

[owlman]

Mine overestimates my mpg pretty consistently by 1mpg.

 

[GWL]

The miles to empty can change with different driving conditions. On the road after a few fill ups mine will estimate more distance than it will after a few days of around town driving. The only time the message center gives "instant" current mileage is right after you reset the system. From then on it is an average since the last reset. Tire size can also make the calculations off, even with stock tires. My LSE came with tires that were 1.7% off from tires that were were on the 16" wheels. The 18" tires that I have now mounted are slightly smaller in circumference and get smaller as they wear out. That is one of the advantages to the SCT flasher that allows fine tuning the tire size.

 

[97stscaddy]

I notice that with mine after I go on a long trip. Ive driven over 400 miles on a tank when it said I had 250 miles to empty at the beginning of the trip. After the trip when I fill up again it will say I have ~375 miles to empty, or some number close to the last reading on the trip odometer.

 

[camthman]

mine messed up RIGHT after I put my FORD GT fuel pump in.

 

[joegr]

I believe that the "miles to empty" is based on a running average of the last 500 miles. The average MPG display is the average since the last time you reset it. (I think that in reality it is more like the running average of the last 1500 miles, if you never reset it.)