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I recently bought an EVIC from a member to replace my factory chronometer in my Turbo Z. I wired it up properly, and all functions work. Last night I finished my first tankful and compared the computer readings to calculated at the pump.

Computer showed gallons used as 11.2. I pumped the car and it clicked off at 7.2, which usually means I can get 2 more gallons in. I pumped it to the cap and got 10.0 gallons in. So the fuel gauge interpretation was off that much. The gas gauge had read 1/4 when I started filling. Miles traveled (331), of course, is the same on EVIC as the trip odometer, since they have the same source.

So the EVIC gave an average reading of 29.6 mpg, in reality I got 33.1 mpg. Has anyone else experienced such an error? That's 10%. Most people report that they are closer than this.
 

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My N.A. '87 is VERY close, didn't have one (navigator) originally. My '89 Shelby is off by maybe 1-2 MPG, I think that one came with one originally. I also have one in my '87 Shelby but I haven't been able to do get a true test of accuracy (for a variety of reasons), that one I installed myself. And I have one in my '88 Pacifica, also haven't been able to test that one out yet, never made a long trip with that car. Also have a 2 button that P.O. installed in my '92 2.5 T1, that one also seems to be within 1-2 MPG.

I consider 2-3 MPG accurate enough considering that sometimes I may get a fuller top off in that tank than other times. Even little things like if the gas lane next to the pump is on a slight grade will effect how much I can get in the tank.

Oh, wait - I now have +20s and a Cal in the '89, haven't done a long road trip with it since those upgrades, numbers might be different now.
 

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So the EVIC gave an average reading of 29.6 mpg, in reality I got 33.1 mpg. Has anyone else experienced such an error? That's 10%. Most people report that they are closer than this.
I know it's not the same vehicle or setup but my Ram's readings are usually off by 1-1.5 mpg or about ~10% compared to my hand calculations (miles driven/gallons used). Last fillup the average fuel mileage displayed read 19.6. Calculating it by hand yielded 18.46. It's been that way since I bought the truck new. On the other hand our old '00 T&C was usually withing 0.1-0.2 mpg difference and our Journey is usually off by only 0.5 mpg or so.

The worst comparison I noted in the Ram was on a 600 mile triip a few summers ago. Took my son to summer camp in western NC. By the time I arrived home the average mpg display read 24.5 mpg whereas my actual calculations showed 21.02 mpg (442 miles / 21.03 gallons). It had read 22.5 mpg when I stopped to fill up.
 

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Over the years I've had them read low by a couple MPGs and high by a couple MPGs on different vehicles.
The oddest was my first Dakota R/T I bought new in 1999. Just me in the empty truck and the readout was low. Add a passenger and some cargo (probably 500 lbs) and the computer was accurate.
I haven't checked the one in the new Challenger yet, but the 300C was dead accurate.
 

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Discussion Starter #5
The error is entirely due to the gas gauge reading by the EVIC. It said I was down by 11.2 gallons, when it was 10.0. That's a big error. I fill it at the same station, same way, every time. It gives me just about 2 gallons beyond the first click-off to fill it completely. A one gallon error beyond filling it right to the cap is huge. So either it's not reading the signal from the gauge correctly, or the gauge signal doesn't truly represent the correct amount and is off by a lot. Nevertheless, the error seems larger than what most people experience. I'd be interested if those who have these units could check the 'gas consumed' reading just before filling, and see if it's higher than what really fits in the car.
 

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gas consumed is 'ALWAYS' wrong on mine.. by at least 2-4 gallons.. have done this comparison w/ my two 87 Shelby Z's, 89 Shelby, 86 C/C and 89 N/A. however the MPG, mileage etc, is always pretty darn close..

Also the Range is usually crap too, I could have a quarter tank of gas, it'll say 140 miles to empty, but will drop 5-10 miles off the range every few miles, and really only get about 50-60 miles to empty for instance.. I've even had it hit zero miles to empty and got about 35 miles and made it to gas station..
 

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I'd be interested if those who have these units could check the 'gas consumed' reading just before filling, and see if it's higher than what really fits in the car.
Yeah, I'm going to start putting notes on that in the phone app I use to keep track of the fill-ups.
 

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Mine has never been very accurate. Miles traveled is spot on with the odometer, as you'd expect, but fuel consumption and milage has always been off.
I do believe that both of those are counted by counting fuel injection pulses and their width, however, not fuel gauge reading. Distance to empty is calculated with fuel gauge reading, however, can can seen in my car falling off faster over time just like my fuel gauge.
 
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