Quick drop in MPGs
 

I have a 2013 RAM Tradesman diesel 4x4. It's 15 months old and since new I have always gotten 22-23 MPGs around town and more on hgwy. Two weeks ago the dealer repaired a recall on the fuel line heater and also replaced the 2 fuel line filters. Since then my MPGs don't go above 20 around town. I drive to work and home the same as always so there shouldn't be that much difference before and after the changes. Has anyone else seen the same situation? Thanks.

Randy6821

I have not experienced this, but I have not had those changes done to my truck.

The 22-23 MPGs you are talking about, are they as calculated by the Truck info center (computer), or hand calculated by you?

If MPGs are Truck info center (computer) calculated, then my guess is that your MPGs might actually be exactly the same as before. Depending upon the way the truck computer calculates MPGs (I have no idea), if the computer uses any values affected by the changes made by the dealer, and those values are different, regardless if those changes actually affect MPG (I doubt it, but possibly) then the MPG calculated by the computer will be different.

The only accurate way to calculate MPG is to manually/physically determine how many gallons of gas/diesel were used for a known distance (miles). The computer is really just to be used for a quick and dirty comparison, given that everything else remains the same (no work done on your truck), imo.

Using any electronic device to measure MPG adds a degree of uncertainty (error).

Since I'm retired, I have some time on my hands, so I got to thinking why you might have seen this change after the dealer worked on your truck.

There are probably others on this forum that know for a fact why it changed. I don't, but my guess is that the computer uses fuel temperature and/or fuel flow to estimate the amount of fuel used to travel the distance as measured by the odometer.

The computer might use the ideal gas law (PV=nRT) http://en.wikipedia.org/wiki/Ideal_gas_law, or some form of it, to estimate the amount of fuel used.

Since Temperature of the liquid fuel [might/probably] affects the Temperature of the gaseous fuel, and the fuel filters [might/probably] affects the rate of flow (amount) of fuel, and [if/probably] the computer uses either of these values, the MPG as reported by the computer changes.

Also, gaseous diesel or gaseous gasoline is not an ideal gas.

In both the before and after case, the computer is only estimating the amount of fuel used (and the distance traveled, actually).

The MPG before and after the change was/is probably inaccurate.

Bob_boater,

Thanks. The MPGs have come back up some to 20-21. For an experiment I'm going to estimate by the gallons used the next time I fill up and compare to the truck's computer. I'll post the difference.

Did they also do a flash update to PCM? that would change your MPG information display to be more accurate.

Don't know if they did a flash update. I'll have to research that. Thanks.

I know how large a US gallon is and I've never heard of the kind of mileage you are getting around town so I would hazard a guess that the original readings were way off, you wouldn't even make those with an Imperial gallon and forget liters as it will cost way more money to fill up.[coffee]

Busboy,

You may be right but I only know what the average MPGs were on the dashboard screen for the last 15 months. I'm now averaging 20-21.8 around town so it's closer.

Wow, that beats the tar out of my 12 mpg and I thought I was doing good. lol

MPGs
 

Staarma,

A friend of mine said that his friend only got 13-14 from his 2013 diesel but he had a gear ratio of 3:75 and mine was 3:43. Not sure of the correct numbers because I'm at work but that seems to make sense that his was geared for heavier towing ability. I'm not a gear head so I don't know. I'm going to fill up this week and calculate the MPGs when I have to refill to see if the true MPGs are close to the computer read out. I'll let you all know.

Yeah, I have the 3.42 rear gear. My overhead and hand calculated pump MPG's are within a tenth. Hopefully yours are too.

After after a couple of months figuring the MPGs manually, I've gotten anywhere from 18 to 21 MPGs, in town and highway, all depending on how fast I drive. I also found out I have a 3:73 rear gear ratio. The computer on board always showed 1 to 1.3 MPGs higher between fill ups. So, I'm still pretty satisfied. Thanks for all the replies.

Those are very good numbers.. Do you have a crew cab? short box or long box, single or dual rear wheels and what type of transmission, 68RFE or the Aisin?

lo mpg
 

I have a crew cab. 6' 4" box and single rear wheels. I just looked at the window sticker and saw where I had a 3:42 (I misquoted before) rear and six speed trans but it doesn't say what kind of transmission.

low mpg
 

I forgot to say it is also 4x4