MQRTRS C5

This last weekend on my way back into town, I averaged 36.9MPG according to the DIC in the car. This was over a 220 mile distance with the cruise set at 80. I couldn't hardly believe it was that high, so I filled up as soon as I got home to check. Doing the math, I 'only' got 29.2 mpg. Thats more than a 7mpg difference. While I'm not complaining about getting 29.2 mpg, it sure wasn't very accurate. What gives?

Is that normal, for the DIC to be that inaccurate?
How about the 'range' setting and average speed? Are they just as inaccurate?
:seeya

indyblue98

While I would find it hard to believe 36+ mpg, it seems possible to have a pocket of air in the tank so that a single fillup could possible take more than normal. Just a 1 gallon difference would account for this discrepency. I have only put 6,500 miles on my car but have always fill as much as possible and I have kept track of the fuel used tank to tank and the resulst are usually within 2 mpg of the DIC. :seeya

tipcapman

Based on a very small database, I find my DIC to read about 10% high.

Range is based on fuel remaining in tank and average fuel mileage, so yes, this number would be just as inaccurate as the average MPG.

Average speed is based on speed while the vehicle is running, i.e., if you are caught in a traffic jam the clock is till running, but other than that it is very accurate.

L :seeya

rhoeven

Did you have or had the fuel gauge problem? This might account for the difference.
Just last week, I did check my mileage for the first time in over 4 years of ownership. According to the DIC reading my mileage was 19,9 mpg and the actual math calculated 18,8 mpg. About 1 mpg off which could be explained due to long idling while adjusting my newly installed T84 Euro headlights.
Driving was about 40% highway / 60% city.

I feel the Dic fuel reading is quite accurate.

:cheers:

LoneStarFRC

I have checked my actual mileage versus the "DIC" mileage ever since I bought mine new in '99. Over the last four years, the DIC mileage consistently averages about 7-8% on the high side. As I understand it, the algorithm used in computing the average mpg is based on the previous 20 gallons. So, in other words, the DIC is continuously computing fuel usage (unless you reset it) on the last 20 gallons used. Since our cars only hold 19.3 gallons of gas, maybe this accounts for the difference.

Robert


[Modified by LoneStarFRC, 8:56 PM 9/9/2003]

Y2KMRC5

My DIC reading is about 10% off in the optimistic direction. It doesn't seem to matter whether it is highway driving, city driving or a combination. I always reset it when I fill up so it starts at 0.0 idling at the gas station (and the range says 'low'). It seems like GM could make it more accurate, but as it is, if the owner doesn't calculate it all the time, he thinks his Vette is VERY good on gas mileage!

Dave :D :p:

abrimberry

From archived thread: http://forums.corvetteforum.com/zerothread?id=575268

For every tank, I track some information from the DIC. I also track actual MPG. Below shows a comparison between actual MPG and what the DIC displayed. For comparison, I included the accuracy of my girlfriend's 350Z.
Auto / Actual MPG / DIC MPG / DIC Avg. MPH
1999C5Coupe / 23.34 / 25.38 / 39.25
2001C5Coupe / 23.94 / 24.74 / 42.53
2003C5Coupe / 21.70 / 22.26 / 38.43
2003350Z / 18.89 / 19.42 / 29

Terry34

Mine is consistently at least 10% optimistic.

i_york

There's no way in the world that my Euro-spec C5 holds 73-litres of fuel!!!
I've run the car until the fuel gauge is resting on its end-stop and the range on the DIC has been on LOW for miles and the most I've ever squeezed into the tank has been 63-litres.

Does anyone know if the tank(s) on Euro-spec cars are any smaller???

Oh, btw, my average mileage is 21 miles per US gallon. That's based on 70% highway, 30% urban driving cruising at 80mph-ish on the highway.

Cheers,
Ian.
:cheers:

waytofastvette

yeah I dont know but this is what i was averageing yesterday

Heretic

I agree that the DIC is optimistic on MPG.

The fuel gage or capacity has nothing to do with it. The PCM computes fuel useage from the pulse width its giving the injectors. So since it knows that and it has the 4000ppm signal from the speed sensor, instantanious MPG is easy to compute. The DIC averages these numbers vs the odometer for average MPG.

The difference is because fuel usage due to idle, startup and evap losses are not accounted for, whereas they are with the miles driven/# of gallons pumped method.

tipcapman

Heretic...

You are right about the way the DIC measures fuel flow rate and calculates instantaneous MPG, but the average MPG is calculated based on total fuel flow and miles driven. There is no inaccuracy due to idle or startup. The amount of fuel that evaporates is small, but you are right, that is not measured.

Regards...

L :thumbs: :seeya

larrysb

Mine is right on the money. Eerily accurate.

I RESET it every fillup though, along with the trip odo's and the range calculation.

36 mpg seems a bit suspicious to me, unless it got reset.

I fooled a couple of friends of mine once by resetting the MPG computer and cruising on a long downhill freeway for a couple of miles. Pulled right into their driveway and showed them the DIC, proudly declaring my C5 a 41 mpg vehicle.

I'm usually around 29-31 on a long freeway ride, depending on where I get gas. California oxygenated stuff gets lower MPG than stuff I've bought in other states that don't do the reformulated fuels.

LoneStarFRC

Can't speak to Euro versions, but US models hold just a little over 19 gals (US) according to the owners manual and the GM shop manual.

Robert

vbdenny

The first couple hundred miles on my 16 Z51 I was astonished to see 28.9mpg when I wasn't even trying. This was on I-64 and I-95 between Norfolk and Washington. I really never even considered the fuel consumption when I bought this Corvette. Should I ever consider hyper-mile (as opposed to my hypo-mile habits) it's nice to know it is possible.
So far the range has been between 17.3 and 28.9. The 17.3 was justified though.

Bill Curlee

There are SEVERAL things that can adversely effect the accuracy of the Calculated Miles/Gallon reading;

1. Its based on a completely STOCK engine. If you add a bunch of modifications that effects air flow through the engine, the CALCULATION algorithm will be inaccurate.

I know that EFI Live Tuning Software has a table that lets you adjust values in that calculation to make it accurate when you change engine parts. When I did a Heads Cam Swap to my 02ZO6, I installed #42lb/hr injectors and that calc was way, way, way off. My tuner adjusted it and put it back in spec.

Macleod52

I've been keeping track of mine since I got it three years ago. On average it's off by no more than 1mpg. I have had it off by as much as 4mpg.

On the most recent trip the DIC said I got 33.3MPG but when I calculated it it was 32.9. My DIC is generally on the higher side.

Macleod52

Sorry, I get a little carried away when talking about Corvettes.

93 lt1 vette

21.2 average over 1000 miles on DTC, actual 17.25. Almost 4 miles difference.

Bill Curlee

I was asked by one of my buddys to help a forum member with a persistent problem so, I asked that person to call me.

We discussed the issue and I asked HER,, if she could pull up the DTCs in the DIC.

Its something that you say a LOT if you have a C5 so, its a NORMAL TOPIC of conversation.

We both had a good laugh when it was all over..