Vehicle MPG gauge

[RB18C]

Is anyone else’s MPG gauge always off? I track my MPG on fuelly and my trucks gauge is anywhere from 1-2 mpg off. Think they do this on purpose to make ppl think they are getting better gas mileage? Haha

 

[brucie]

Mine is between .3 and .5 mpg too optimistic. Accurate enough for me

 

[RB18C]

I wonder why mines always so far off. I’d be happy with .5.

 

[brucie]

I've only hand calculated 5 or 6 tanks.

 

[Silverbeard]

I’ve owned several Dodge/Ram trucks over the years and the inaccuracy ranged from about .5 mpg over to almost 2 mpg over. Current Ram is a 2019 3500 with a Cummins. I only 4000 miles on it so far, so things may change, but it’s about .75 mpg over. It’s been my experience that if driving conditions change significantly between fill ups, such as highway driving then heavy towing, that the estimated mileage will have a bigger error.

 

[litzdog911]

Is anyone else’s MPG gauge always off? I track my MPG on fuelly and my trucks gauge is anywhere from 1-2 mpg off. Think they do this on purpose to make ppl think they are getting better gas mileage? Haha

Is your speedometer accurate? Changed wheels/tires?

 

[Deleted member 80]

This is my third Dodge/Ram pickup, and the Lie-O-Meter has always read optimistic. My '05 CTD was the worst, by far, sometimes being 5-6 mpg off when towing. The '15 gasser was generally 1.5-2 mpg off, and the '19 is generally running 1-1.5 mpg off. My wife's old '06 Charger also read way higher than actual.

 

[RB18C]

Is your speedometer accurate? Changed wheels/tires?

I do have different tires. But they are the stock size.

 

[Gondul]

Those things are notorious for being 'off'... same story on my wife's Subaru (Forester and Ascent)... it reads a higher MPG than what we are actually getting... I generally ignore it and also track via Fuelly

 

[Wileykid]

My '19 3500 SRW lie-o-meter is running 2 mpg off on the high side.

 

[jsalbre]

I’m finding (after 5 tanks) that my meter is a little under .5 mpg off from Fuelly. I call that close enough for my needs. Probably the most accurate I’ve ever had in a vehicle.

They’re never going to be perfect, and even a couple PSI change in tire pressure will affect tire circumference and thus odometer readings enough to impact the calculation.

 

[Distillusion]

There's really not much excuse for them being off more than a tiny amount. The ECU knows exactly how much gas it's feeding the engine through the injectors at any given time. And it's a trivial calculation to compare it to distance traveled at the same time.

 

[Marshfly]

There's really not much excuse for them being off more than a tiny amount. The ECU knows exactly how much gas it's feeding the engine through the injectors at any given time. And it's a trivial calculation to compare it to distance traveled at the same time.

Agreed. And the tech exists to periodically calibrate the speedometer with the GPS.

My Yamaha outboards are accurate on fuel consumption down to about 1-2% when I reset the gauge on fillup. I bet it’s dead on and that just variability from when the hose stops. These trucks can be just as accurate.


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[jsalbre]

Unfortunately it’s not that simple. There are things on cars that are very different from boats (aside from the wheels!). If you really want to know more reasons why, read this Car and Driver article.

 

[Distillusion]

Unfortunately it’s not that simple. There are things on cars that are very different from boats (aside from the wheels!). If you really want to know more reasons why, read this Car and Driver article.

That article is based on older concepts of measuring the gas in the tank, and all those factors are valid in those circumstances, with recirculation error and other issues. But now, ECUs monitor fuel pump pressure and control how long each injector is actually open. Recirculation doesn't effect that. That data can give you a very precise measurement on the volume of gasoline actually released into the cylinders. And miles traveled is a function of turns of the wheels, which are also monitored.

Yes, fuel density is variable by temperature. But that's where pump measurements and hand calculations have the most error. Gasoline coming out of a below-ground tank is at a near constant temp. But you put it in the truck, and then it warms up or cools off in your truck, depending on the weather. If it warms up 30 degrees after coming out of the pump, you could end up with 3% more volume as the density decreases. So you add 20 gallons, you could end up with 20.6 gallons actual volume later. That's going to throw those hand calculations off by 3%. You think you only added 20 gallons. Your computer measures 20.6 going through the injectors. Which is correct?

And of course, that means that the gas you have is now less energy-dense, which will reduce your calculated mileage somewhat. Exactly how much depends on the refinery and what it did in processing the gas. If it's 30 degrees colder, you lose .6 gallons, and your computer measures 3% less going into the engine.

 

[jsalbre]

Yeah, I didn’t realize the age of the article so there’s a good chance it’s at least somewhat out of date, but I also don’t have any specific data on what the Ram’s inputs for the MPG calculation are.

I’m sure there’s some reason that current vehicles don’t calculate MPG well, and I doubt it’s anything to do with laziness or cheapness on the manufacturers part. The ECUs in these things collect pretty much every possible piece of data so it would just be a matter of writing up the proper formula if there aren’t any external complications that can’t be accounted for. Maybe someone knows a Ram engineer we could poke at for an answer?

 

[Distillusion]

I'm of the opinion that since gas volume changes with temperature, miles per "gallon" is an inaccurate calculation at best. Put it in your truck, now you don't know the actual volume. Fill your truck on a *really* hot day, you might gain 5%, and on a really cold day lose the same. That's going to screw up the truck's run, and it's part of the reason that numbers vary by season. So I'm good with anything that seems within a few percent of "close."

One oddity I've noticed is that I always got at least 16 average on my 5.7L. I would reset one of the computer's two trip odometers every couple of thousand miles, and for any road trips. It was never less. The other odometer I left alone. After 30k miles, the "average" lifetime on the second odometer showed at 13.9 mpg. Clearly some odd error in calculations once you start racking up more miles on the trip calculator. I haven't put enough miles on the 6.4L to get such a big difference, yet.

 

[jerryw1000]

Lots of things go into it to include: inaccurate fuel gauges....inaccurate gas pumps....inaccurate odometer.....temperature....evaporation....tire size. It gives me a general idea of how it's doing and keeps me from running out of gas.....I'm happy.

 

[RB18C]

So since I compare with the Fuelly app. And the app takes nothing other then miles and gallons into consideration, would the truck then be more accurate then the app?

 

[Deleted member 80]

Nobody despises GM more than I do, but, despite all the variables, their Lie-O-Meters are generally much, much closer to hand calc than any Chrysler product I have owned.