Trip/mpg displays

I have a 2001 F350 diesel Lariat 4x4 7.3 liter. It has a display that tells you the average mpg of the vehicle. I live in Colorado on the eastern plains. My friend that is building a house up in the mountains borrowed my truck for a couple of months and the mpg display claimed he was getting between 22-25 mpg and that included pulling trailers. I have my truck back now and went to my friends house also out here on the flat plains of Colorado. In a 60 mile round trip journey the display finished with a 30 mpg average. I seriously doubt this is accurate. I know I need to do the math my self by recording the miles and gallons over a long period of time but have not done so yet. What opinions do you have on the accuracy of my display? Thanks. Roggenite

For comparison:

Your display is probably pretty far off. Towing a trailer in mountains, your friend would be lucky to get 15 mpg.

Sounds a little like the display on the 2013 Corolla I rented for the weekend that at some points displayed 99mpg but I think that one is more related to throttle usage since just cruising down the highway at steady throttle displayed such high figures. A few MPG off is not that unusual but this doesn’t sound right to me

I seriously doubt this is accurate

It’s NOT accurate. These are notoriously inaccurate. I find the gas gauge more accurate. A simple thing like a headwind or tailwind will effect the reading.

I think mine in my cars are fairly accurate. There is a difference between current and average though. So if you look at the current mpg display it’ll range from zero at a stop light to 99 going downhill but over a couple hours the average will still be about 28. Bottom line is though, what difference does the display make? The mileage will be the same regardless of the display. Kind of like knowing your daily bank account balance-what difference does it make?

I never trust the average mpg indicator …it doesn’t tell me any pertinent information.
What I use the most , and both trucks are set primarily on, is the display for ‘‘Miles to empty’’.
This information is much more relative to my day to day activities.

But here’s an oddity on my 08 Expedition ;
around town it’s petty dang close and I can count on it, full well knowing that it is showing ‘’ if this keeps up this way’’ averages.
YET, When I head out on I-40 for the 140 trip to Albuquerque ( 280 round trip plus Abq in town ) it never seems to recalculate its avereage untill I get home and down to the last 1/4 tank.
– Here’s what I mean ;
As I cruise down the motorway I watch the miles to empty subtract one at a time AND look out for the mile post markers.
It will subtract just one mile from the miles to empty total even though I’ve just drive 1 3/4 miles…over and over again, not just for the first few recalculating miles but for the whole trip as long as I’m cruising at about 85 or so.
– and yet, after I get back to town it’s accurate again as the needle approaches empty in the next few days.

That readout does sound questionable.
However, those who say that all of these devices are unreliable are wrong.
I can tell you that I have double-checked my mpg readout multiple times with a calculator, and
I found it to be consistently “off” by only .4 mpg, which I think is an acceptable error rate for an instrument that is not laboratory-grade.

I’m sure the readout is questionable but it could also depend upon the function being used. If that truck is anything like my Lincolns it provides a running average MPG and an instantaneous MPG feature. The latter doesn’t mean much as it figures mileage every couple of seconds. Depending upon terrain and the speed at which coasting is done it’s possible to hit 99 MPG. In the real world it doesn’t mean much as that only happens for a nano-second and is quickly sunk by other factors.

If the 30 shown is the instantaneous feature then it’s possible the truck got that. It’s just not long lived.

I’ve religiously used the feature on my Lincolns for about 15 years and have found it to be amazingly accurate; all backed up by the gallons used per miles driven method.
The readouts have always been accurate within .1 MPG. About once a year they seem to go stupid for a few hours or even a day or so and provide some off the wall readings but other than that they’re not an issue at all.

Are you sure it didn’t accidentally get reset from mpg to kilometers per gallon?


Nice try, but . . .

In europe, fuel economy is calculated liters used per 100km or L/100km

I dunno but when I would switch to metric it didn’t seem like it converted to liters but I have to confess I never checked the mileage in metric. It was just fun to do it once in a while to relieve the boredom.

It’s a lot more intuitive to estimate average fuel economy when the display reads liters/100km instead of miles per gallon. If you get zero miles per gallon for one mile and 100 miles per gallon for the second mile, it does not average to 50 mpg (the easy intuitive answer), You actually average zero mpg because you burned an infinite amount of gas for the first mile. On the other hand, if you got zero liters/100km for the first mile and 10 liters/100km for the second mile, your average is indeed 5liter/100km.
Miles per hour averages the same way and a lot of people can’t believe their average speed was so slow when they covered nearly the entire distance at 70+ mph.

Drivers that are averaging 55 to 60 mph are covering most of the distance at 70 to 80 mph.
Drivers that are averaging 20 mpg are driving a lot of distance at 30, 40, and even 100+ mpg. It’s those few miles where you get one or two mpg that kills the average.

Db, if the odo is accidentally reset to kilometers, as modern odometers can easily be, and the owner is using its reading and dividing by gallons used to get his mpg, he’ll be calculating kilometers per gallon.

We ain’t in Europe.

I don’t trust readouts period. I might if they are ever proven to be accurate but I’m not holding my breath. They are probably not even in the ballpark if truth be told. When my oil life indicator shows less than 50%…I’m changing my oil and oil filter and resetting the whole darn system to 100%.

Ya know in my old 89 Riviera, I went for 6-8 years driving over 100 miles a day without a working gas guage. Instead of looking at how much gas was left in the tank, I just used the computer to tell me how much was used. It was amazing how close it was when I filled up. Never did figure out how the car computed average mpg and gallons used and miles to empty (or something like that-memory is fading), but it did, all the while the low fuel light on and reading 1/8th of a tank due to the not working float in the tank. So it was obviously looking at injector pulse width, speed, and who knows what to figure it out.

“I don’t trust readouts period. I might if they are ever proven to be accurate but I’m not holding my breath. They are probably not even in the ballpark if truth be told”

…but I already stated that mine reads a consistent .4 mpg optimistically, and ok4450 stated that his readout is only off by .1 mpg. Don’t you believe us?

Yes…but there is a caveat here. My TPMS readouts are dead on the money for each tire but my fuel remaining readout is a bust. I just got back from a trip and it said I had 120 miles remaining but I was running on fumes when I got to the gas station. My vehicle holds 16 gallons and I put over 15 gallons in it. Besides, I drive a lot of new vehicles, mainly Ford Fusions, and their fuel remaining readouts are all over the place. We recently had one driver who ran out of gas with 108 miles remaining on the readout. I drove one that got down to zero but since I had a 1/4 tank left I just drove on. I wanted to see if it went into negative gallons but it just remained at zero for about 80 miles. My point is…most of the readouts that I see are not even close so I just don’t rely on any of them. Not even the TPMS. I still check my tire pressure on a regular basis.

“I just don’t rely on any of them. Not even the TPMS. I still check my tire pressure on a regular basis.”

That is very wise, as the TPMS is supposed to be used only to warn you of sudden catastrophic pressure loss while you are driving. It was never intended to substitute for manually checking tire pressure, even though many people now think that they never have to bother to check their tire pressure anymore because of the presence of a TPMS.

In reality, those naive/misinformed people could be driving around with a pressure loss of several lbs in all of their tires before the TPMS warning light actually lights up.

This subject was being debated about half a dozen years ago when I was about to go on a road trip so I kept a close running tab on actual fuel used as compared to the dashboard message center. Granted, some units may be unreliable and even reliable ones may wander off now and then but the units on my Lincolns have been amazingly accurate over the last 15 years.
I won’t post the cities and computations I kept from that trip but will show the miles, actual, and display readings of the various stages from the little log I kept.

393 miles - 26.026 MPG - 26.1 display.

226 miles - 27.098 MPG - 27.1 display

269 miles - 27.008 MPG - 27.0 display

353 miles - 25.841 MPG - 25.9 display

That has been pretty much the norm on my units and even things such as range, miles to empty, and so on are near dead on. It stuns me that it can be as close as it is because I would look at a +/- 2 MPG as being pretty accurate.

Thanks @VDCdriver . I believe that high end vehicles like Lincoln’s probably have better electronics than your everyday vehicles @ok4450 . That may be the reason that your ride is so accurate. I would expect that if I bought a car or truck with a big sticker price. I tend to stay towards the middle of the market when it comes to motor vehicles.

I bought an add-on vehicle computer back in the early 80’s that was surprisingly accurate when it came to speed, quantity of fuel and mpg. It had the added benefit of being a cruise control unit. It worked well until the magnets fell off of the drive shaft. The company was already out of business by the time I tried to order another set of magnets. Speed and the cruise control stopped working but the fuel quantity and mpg worked until I sold the car.