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2nd Gen Non-Engine/Transmission Overhead Trip Computer: Fact or Fiction?

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Engine/Transmission (1994 - 1998) Vacuum pump can not keep up

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I'd like to find out what others think of the trip computer in the overhead console. Do you think it's accurate? If not, how far off are the readings for average fuel economy?



While driving down the road at a steady speed mine will shoot up to 28 mpg for a few seconds, then down to 15 mpg. It seems to settle down after 20 minutes or so but still has what I feel are wide "mood swings".
 
The computer can give you a fairly good idea of what's going on, but after you add power enhancements like injectors or a fueling box, it will be way off. I don't think it is normal for the computer to swing drastically like you indicated.
 
fact or fiction?

You are looking at the instantaneous mileage. Depending on throttle setting and grade, you may see numbers from 7 to 99 mpg. If you watched and remembered all of these readings, and averaged them, you would get the same number shown on the average MPG display. It looks crazy, but is actually pretty accurate. I typically get about . 4 mpg better than the calculated average. This is only good for unmodified trucks though. It is based on the computers fueling rate request. A box stretches the pulse to add addtional fuel and the computer doesn't know about that, so the computer will make some serious errors in calculation.

A good example of calculating fuel economy is an exteme case. If you had one truck getting 1 mpg and another getting 100 mpg, most people would think your average would be about 50 mpg. This is not true. If you drove each one 100 miles you would use a total of 101 gallons for the 200 miles driven. 200/101= 1. 99 mpg.

This explains why you frequently see 50+ mpg displayed, but only get around 20 when you fill up!
 
These wide swings are on level ground with steady pressure on the peddle. I'm still on my first tank of gas :D and haven't done the old style mpg computation (fill up & take odometer reading, fill up again & figure distance driven and divide by gallons used = mpg).
 
The more often you reset the MPG to zero, the more you'll see wide swings in the readings. ;)



After a couple of tankfuls, without reseting the zero, you'll see less drastic swings in the readings.
 
My average MPG on the display is usually within . 5 MPG. My truck is stock. The instanious display doesn't help much, but is interesting to see 4 MPG going up the pass and 99 going down.
 
Correction

slo-ryde you said you are on your first tank of gas:confused: These trucks run on diesel for a fuel. Don't forget and put gas in it.

The old pen and pencil is the accurate way to figure mpg. My overhead is off a couple of tenths +/-.
 
Yes mine does the same thing. I think if you got out and checked the road there would be slight inclines and declines. No road is perfectly level. The cruse is electronic and tries to maintain exactly the same RPM's. I love to watch it every morning as I drive to work at 5 AM. Helps to keep me awake. Fill up and reset your average. Then watch it for about 100 miles. It will swing all over the place until you get enough miles so that the instantaneous does not affect the average as fast. Fun to see how high and how low you can make it go.
 
My experience with the trip computer is limited as my vehicle is brand new. The two times that I have hand calculated my fuel mileage, the computer has been exact. For example I hand calculated once that my mileage was 21. 29. The fuel computer showed my mpg as 21. 3. A couple of days ago I again hand calculated my mileage and came up with 18. 81. My computer showed 18. 9. Like I said my experience is limited due to the vehicles newness. My vehicle is stock and according to some other members, bombing really throws the computer off.



My instant mpg will shoot up to 99mpg when I am coasting in gear. I suspect that it stops at 99 because two didgits are as high as the computer will go. I can't say for sure, but I expect that the engine uses none, zero, zilch fuel while coasting in gear. I have a Volkswagen Diesel (TDI) and have verified that it uses no fuel whatsoever when coasting in gear. A friend of mine hooked up his laptop computer with some special software to my VW as I was driving. It showed various fuel parameters and showed that the car was using no fuel while coasting in gear. I didn't believe him when he initially told me this but seeing is believing. Like I said, I SUSPECT that the Dodge Cummins is the same way, however, I may be wrong on this.



I find it hard to keep the fuel computer instant mpg the same even using the cruise control. The slightest incline (or decline) will affect the readout as well wind speed and direction. For now though, I am extremely happy with the computer. However, time will tell.







CAUTION: The preceeding post is only MY opinion and a collection of rambling thoughts. Following, emulating, extrapolating, or thinking about any part of my post could cause world annihilation and extiction of spotted owls and/or other endangered species. Reading of my posts have also been known to make babies born naked. EXTREME CAUTION URGED!
 
Re: Correction

Originally posted by lschultz

slo-ryde you said you are on your first tank of gas:confused: These trucks run on diesel for a fuel. Don't forget and put gas in it.

The old pen and pencil is the accurate way to figure mpg. My overhead is off a couple of tenths +/-.



I'll have to be careful with my choice of words. In the future I'll try to refer to it as fuel. :cool:
 
The Compass is accurate so is the thermometer. Everything else is close but not dead on.



If you think that's bad put a set of 33" tall tires on then se what happens to the speedo meter and the odo. :eek:



Holy MPH Batman!:p



Darrell
 
Interesting. My 99 only has trip average for fuel mpg. What year did they start the instant MPG?



For a tank, my trip indicator is usually +/- . 5 MPG.



-Vic
 
I think they're all milage average.

Try reseting the MPG display to "0" while driving on the highway.

It'll show some amazing fuel milage, and will show quick changes as you get on and off the throttle.

Remember it's an avarage, the less miles accumulated since last reset, the quicker the display will show changes.
 
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I wouldn't have ordered the trip computer on my new truck, but I bought a truck off the lot and it had it. I LIKE IT!!! It seems pretty accurate except that I ALWAYS get 1-3 MPG BETTER than the computer shows! I am convinced that watching it and learning how to drive to get the best economy has really improved my overall economy!



Steve
 
I am convinced that watching it and learning how to drive to get the best economy has really improved my overall economy!Steve



Ditto. #1 reason to have one, even if injectors or other mods have caused accuracy to be way off.
 
The instant MPG is like an old Vacuum gauge on a gasser, If the needle was around 15 inches, you were good, if it were down to 5 or less, then WOT. 22" was decell.



Now it is read in MPG 5= WOT

99 = Decel.



The average is the reading going up and down and the average of the MPG over the disdance of the miles. the longer the miles, the closer and slower the readings are.



I have a bombed truck with 2 boxes. It is true that the boxes fool the computer, but if you are driving easy, it usually stays pretty accurate.
 
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