K9Leader

This thread on C5 MPG (http://forums.corvetteforum.com/c5-t...6-avg-mpg.html) included many comments citing MPG that forum members have gotten on their C5s. I've seen many, many other posts with claimed MPG numbers. It appears that most of these cited MPG figures are based on the readout from the DIC. Only occasionally will the poster note that the cited MPG was based on calculations from miles traveled divided by # of gallons used.

I calculate MPG on every fill-up and also note the DIC readout MPG. Typically, the DIC readout is notably higher than the manual calculation (actually not manual - I use an online MPG tracking site that does the calculations). For example, the last few months of fill-ups show this:

Calculated MPG/DIC MPG Readout (Difference)
25.0/26.5 (+1.5)
16.3/19/4 (+3.1)
24.1/25.9 (+1.8)
19.1/22.1 (+3.0)
15.5/14.2 (-1.3)
17.2/19.1 (+1.9)
22.6/24.6 (+2.0)
18.5/21.4 (+2.9)
22.2/24.8 (+2.6)
19.2/22.1 (+2.9)
17.9/20.0 (+2.1)

The DIC MPG is consistently a couple MPG higher than the calculated MPG, which raises doubt as to the accuracy of the DIC MPG and which, therefore, calls into question all the claims of 30, 31, 32 MPG here on the forum. The best MPG I've gotten in the 3.5 years I've owned my '99 (auto, 3.15) is 26.3. My father had this car before me and the best he ever got was 29 (although that was a fill-up, get immediately on the interstate, drive 130 miles with top and windows up, get off the interstate and immediately fill up again).

So, a number of questions are raised. Why are the DIC numbers so far off the actual? Is this just my car? Hard to tell as I suspect the vast majority of C5 drivers who post MPG claims are basing those claims solely on the DIC readout. I would be interested in hearing from others who have similar comparative data.

K9Leader
Red cars are faster.

jcgunn

I also do the math at each fill up. The DIC is always 3-5% optimistic. One thing you can check is your indicated speed (MPH) versus actual (use GPS). So, if the speedometer is a mile or two high due to tire size, there is your answer. The DIC thinks you drove farther than you really did. (FWIW, the DIC fuel flow readings are super accurate, that's why the AFR #s on these cars is so good)

K9Leader

Speedometer compared to GPS is dead on - always consistent. Wheels are the Magnesium Sports and tires are the OEM sizes, so unlikely that is an issue.

K9Leader

racebum

mine is fairly accurate if i reset and do a specific style of driving.

where it will be off is if i do 200 miles on the freeway and get 26mpg on the display, then, do a bunch of in town driving

it usually doesn't fall quite as quickly as it should

most the time my DIC says 17.5mpg or there about {all stock 02 z06}

if i drive 175 miles it's almost dead nuts 10 gallons to fill

QCVette

My DIC is usually optimistic too. Usually about 1.5 to 2 mpg.

The mileage is different between an A4 with 3.15 than a 6M. The 6M has a 0.5 6th gear and really does well on the highway.

I am one of the ones that claim 30+ mpg on the highway. As an example I took a 1700 mile trip this summer to the Corvette Museum and the Tail of the Dragon. Although mostly highway at 70 to 75 mph there was some in town and some lower gear playing on the twisty roads. When I reached home the DIC showed an average of 32.0 mpg while calculated with the miles and gallons showed 30.5 mpg for the whole trip.

I have done better with only highway driving, and when I really tried for mileage with lower speeds, less acceleration, etc. I got quite a bit better.

Mine is lightly modded with a Vararam, Fast intake, LG longtubes, X-pipe, B&B route 66, and an HP tuners tune. I don't know if those changes make any difference to the accuracy of the DIC or not.

TastyBacon

I calculate mileage for every fill-up from the odometer and the receipt. My best tank this summer was 28.9, and that was with the AC running most of the time. The DIC reported over 30. I don't track the DIC readout religiously (because as shown in the original post, it varies so much vs. reality) but I find it's about 1.5 high on average. I've had cars that were way bigger liars, the Corvette is actually pretty honest

wcsinx

An automatic has significantly more drivetrain loss than a manual. I can easily break 30 mpg on the highway as long as I keep it <= 70 mph. I have done it many times and verified the DIC at the pump.

zz28zz

My DIC consistently indicates 4-5% better than calculated. I also calculate at every fill-up.

pickleseimer

I find mine to be accurate enough to not feel the need to double-check the car's reported average against a separate calculation. Using 25 mpg as an example, a 4% error represents a difference of 1 mpg. I'm not expecting the car to be able to be super accurate - if it reports the average within an accuracy of 1 mpg over a tankful, I'm pretty happy with that. As mine is not a DD I don't drive this car thinking much about calculating fuel economy when the car does a decent job of doing that for me.

SumToy

I have a 03 and it will get better if you keep in 70 to 75 then 60 to 65. Now it is a 6 speed car. I think it lug a little is why.

Gordy M

When I had my runflats on, my mpg was very close to the DIC--about 0.5 mpg lower, when I changed to aftermarket tires I was consistently about 2% or better on the DIC after I corrected the PCM for the difference in tire size.

Corvette_Ed

Those with manual transmissions get better gas mileage than those with automatics for the most part; this is common knowledge. Also, most of the people, such as myself, who make claims of 31~ mpg are basing this off of both the DIC readout AND manual calculations of miles traveled divided by number of gallons. I regularly get 31 mpg on the highway at 75-80 mph, and this is based off of both the DIC and manual calculations. My DIC is .4 off on average from the manual calculations.

TB1300

My experience matches what is reported in most of the replies to this post.

My DIC is consistently 0.5-1.5 mpg too high vs calculated mpg.

Running through a tank of gas, with 95% freeway driving at 70 mph returns a calculated 29 mpg.

Running through a tank of gas, with 100% freeway driving at 80 mph returns about 27 mpg (calculated).

My best tank was a long leisurely drive on back roads with few stops, at 50-60 mph. This returned 33 mpg (calculated)

My car is an M6. It definitely gets better mph at 60-65 mph than at 70-75 mph. 60 mph in 6th gear does not lug the engine if you are cruising at a steady speed.

K9Leader

Yes, I know that manuals usually get better MPG than automatics. I also did not mean to sound as though I question the 30+ claims solely because my car has never gotten that high - I know that a sample size of one means nothing. The intent was to question the accuracy of the DIC MPG, and, in this and other instances in which it has been discussed, the anecdotal indicators are that it is usually more optimistic than the actual.

Also, I have to disagree that "most" who make MPG claims are calculating it based on both DIC and manual calculations. "Most" people aren't that conscientious (****? OCD?). I spent the first 34 years of my driving life meticulously recording the MPG on every car I owned and strong-armed my wife and children into doing it. My wife rebelled and stopped it on her car several years ago and my children as soon as they got their own cars (as opposed to driving one that I owned). Two years ago I gave it up on everything but the Corvette. Just needed to simplify the minor details of life.

The "claims" to which I was referring are the many toss-off claims you see on the forum - "dude my vet is so allsome! grate looks and performanc and 32 mpg you cant beat that!!!!!!!!!!!!"

I am reminded of the old story about the car manufacturer that advertised its newest model as being able to hit 60 mph. A customer just could not get his to get past the high-50s so he took it into the dealership to complain. They promised to look at it. When the customer got it back, he took it out and was pleased when the speedometer needle pointed at 60 mph. Satisfied, he went on home. What crucial adjustment/repair did the mechanic make that got the car up to 60? He adjusted the speedometer - customer happy and none the wiser. Is the optimism of the DIC similar to this - make the customers happy by giving them something that allows them to make 30+ MPG claims by fudging the numbers just a little bit? Notice that the DIC MPG is higher the overwhelming majority of the time.

Do some Corvettes hit actual numbers of 30+? Sure, those that have all the optimum factors - manual transmission, favorable running conditions (optimum temperature & weather; long, flat stretches of open, traffic-free highway), a meticulously-maintained Corvette that operates at top efficiency, and a driver who knows the techniques that will maximize MPG. Ed, that may be you and a few others on this forum, but it is not most people, not even most Corvette owners.

K9Leader

marco383

My understanding of the workings of the DIC system is that the fuel usage portion of the equation is driven by injector duty cycle with the stock injector size being assumed. Being heavily modded, my C5 is running 80 lb/hr injectors vs. the original 26s (I think). Probably because of this, my DIC mileage is WAY off the calculated number. I'd be a happy camper if I could actually achieve mileage in the low 20s as indicated by the DIC.

wcsinx

You do realize that the EPA estimates have the C5 at 28 MPG highway, right? And that rating is not made with the assumption of ideal conditions. Personally I have been able to beat the EPA estimates in every car I've ever owned, and 30 v/s 28 is only a 7% difference.

You seem like you have an axe to grind. I've made many trips to Orlando and back (~120 mi) on I95 and 528, and I've never done anything other than set the cruise control at 70 mph, lean back and relax. I don't draft 18 wheelers or use any other "hyper mileing" techniques. Yet every time when I fill up after that trip, I'm invariably between 30 and 32 mpg. And I'm NOT LOOKING AT THE DIC. Take that for what you will. These are my real world numbers.

QCVette

Glad to see I am not the only one.

I guess being an engineer and a six sigma black belt, I always look for data and analyze it.

Bill Curlee

K9Leader

If you own or know someone that owns EFI LIVE Tuning Software ( I don’t know if HP Tuners has it or not) you can FINE TUNE the reported DIC MPG and AVERAGE mpg reading.

For those of us who have changed fuel pressure or changed fuel injector size, we have to use those tuning tables to RE-ADJUST that calculation.

So, it’s possible for you to get it more accurate.

Bill

K9Leader

EPA estimates are really only useful for comparison purposes (and only then, comparing cars that were all tested under the same form of the test EPA was using at the time) and, in fact, do assume/utilize some pretty specific high-MPG producing techniques - see this: http://www.caranddriver.com/features...-mpg-estimates

Not sure why you took such offense - it sounds as though you are one of the few who has the supporting data.

I'm not complaining that my Corvette does not get 30+ - it's a fun car for me, not my DD. The 25-26 it gets in highway driving is almost as good as my last couple of DDs have gotten and I think is pretty good for a 1990s designed performance car. The overall MPG I have gotten over the 3.5 years I've had the car, including all driving, around town and highway, is 18.8 - again, I'm okay with that for my fun car.

I just wanted to initiate a discussion regarding the DIC MPG and its accuracy/inaccuracy. I also don't have a problem with the DIC MPG not being exactly accurate and see it as a way to compare a vehicle's MPG over time. I will, however, continue to take with a grain of salt any MPG based solely on the DIC.

K9Leader

K9Leader

Ah, but you, Bill, are a wizard, the Gandalf of Corvette Middle Earth. I'll just live with it as is - just thought I'd raise the question on the forum and get a discussion going.

K9Leader