The overhead computer always reads 2+ mpg better mileage than I'm actually getting after I do the math when I fuel up.
I put the SRT-10 wheels and tires on the truck when I bought it new and after realizing the speedometer/odometer was innacurate I had the dealership recalibrate them. My question is this. Do i need the have the fuel mileage calculations recalibrated also? It's more than 10% off which seems far too much to me.

Jason Carden

 

What the overhead computer saying you are getting like around 25mpg. I think 25mpg is very unlikely to happen even with the MDS engines. I think 16-22mpg is what you will see with the MDS hemi's. And I think the computer will reset itself at awhile thats why one day you may see 16mpg and 2days later see 19mpg.

 

No the computer will state something like 16.5 - 17.5 mpg. When I do the real math at the pump, I usually get a number around 14.5 - 15.5. I have the 5.7L Hemi and it's not the MDS model. At this point I've put about 2500 miles on the truck and the computer's fuel mileage has never been close and is almost always 2 mpg or so off.

 

Mine is usally withing 1MPG of what my pen and paper says. I wonder why the diff between all of our trucks computers?

 

Am I wasting my time leaving the truck at the dealership to try and get it fixed? I can't trust the display at all. Moreover, since the mpg is wrong, I can't rely on the DTE on that display either. I might have 50 miles left to empty or I could be sitting on the side of the road dead while my truck says I can make it another 50 miles.

 

The most common reasons that the overhead trip computer reads incorrectly are that the fuel pressure at the injector rail is high or low, or that one or more fuel injectors are leaking some fuel in the off position of the pintle.

The overhead trip computer makes an estimate of the gallons of fuel used by keeping track of the number of milliseconds that the fuel injectors are turned 'on'. This "educated guess" is counting on a steady spray that can be thrown off if anything is out of spec. It depends on the fuel pressure being within the high and low limits. Since Dodge puts a pretty "loose" limit of plus or minus 5 psi on fuel pressures, that limits how accurate the overhead trip computer can be.

Ask the dealership to measure your fuel pressure, not just at idle but out on the road at typical highway speed. If you want to do it yourself the gauge kits generally cost about $40 at the auto parts stores.

It also would be worth re-setting the odometer part of the overhead trip computer and checking it against about 50 to 100 miles of highway mileposts to be certain that part is accurately calibrated too.

 

Not to inconvieniant you, but you could also figure out "well, I get about X MPG, a quarter tank lasts me Y miles, so, I have about Z miles left to go." My Dakota gets about 18MPG highway, my tank is a 22 gallon tank, so, I get about 100 miles per quarter tank. I can look at my gas gauge, see "Oh, its pretty nearly as at empty, I can get another 30 miles."
Besides, if you are really that worried about running out of gas, get off the road and buy more gas!! When I'm traveling on the highway, I never go below 1/4 tank.
As far as the overhead computer being off, HankL brings up a few good points. It's probably just something is not giving off "perfect" readings.

 

Mine's been off about a consistant 1.5mpg since new. No big deal. I run it close to 0 on the DTE and don't sweat it. It still has 3 gallons or so left when it reads 0 miles DTE. I know cause when I fill it up, it never gets to 34 gallons. Usually around 29.5-30.5 gallons per fillup when I go to 0 miles DTE. Most times I don't le it fall below 1/4 tank. Doesn't hurt the wallet as much when I fill up then

 

My 2005 is only about 0.5mpg off from the actual, and I fill the tank to the brim every time. Its the only way to know that you have not put in less or more fuel that the last fill up. the first 6 month I had the truck, the computer would add a max of about 1 mpg, now its almost bang on.

 

the make 5.7 06 hemi's without MDS?... thought that only the 6.1 was MDS free this year.