I don't see anything that suggests they use any hyper-mileing techniques. The only thing that might be construed as supporting your position is the relatively low speeds that were used for the highway rating up until 2008. But even considering that, the article also states that the ratings were reduced by 22% to more accurately reflect reality. But again, I've never had any problem beating the EPA estimates in every car I've owned.

What speed? Are you good about keeping your tires inflated? When was the last time you got an alignment?

I checked my car the first summer after I bought it. It was consistently within 0.5MPG but it was on the high side.

FYI, the 6-speed and auto C5's were re-rated 26mpg & 24mpg highway.

You should try looking at any Canadian fuel economy ratings. What a load of crap they are. For example, the most efficient Ram is rated almost 31MPG highway in Canada but only 25MPG highway in the US. We must have fuel that goes further....

The F150? The bosses truck displayed 12L/100km on the first trip with it. The truck got 15.6MPG calculated. No wonder they didn't bother with the decimal place. Being out by 4L/100km the decimal doesn't matter.

Reasons for real MPG differences and optimistic DIC MPG readings:

One of the variables involved in MPG is Altitude . I am at 5200 to 5800 feet above sea level and all of the fuel injected vehicles that I have owned get 2-3 more MPG than at sea level. But then- the DIC should take altitude into consideration when calculating . But Fuel injected vehicles will always get more MPH at higher altitude.


The fuel injection system lessens the amount of injector duty time when altitude is increased.

All stock speedometers are calibrated optimistic for stock tires with new tread depth at the highest possible tire temperature and maximum tire pressure,at maximum load- that the car will experience. As the tire wears-the speedometers will become more true.

Many vehicle tires are not correctly inflated and manufacturers want the speedometers to show optimistic or near correct speed-even when the tires are low on pressure. Optimistic speedometers are less likely to cause a speeding ticket.

I have checked my DIC mileage on a number of occasions against full tank, refill, miles calc. It has always been well within 2% accurate, and I have GPS checked my odometer and it is dead on. I have been very impressed with the accuracy of all the gauges in my 2015. They all seem to be within 1-2%. Weather, temperature, winds, terrain, tire pressure, traffic and driving habits all make a significant difference, as does un-adulterated gas.

I you ever change the fuel injectors to other than OEM Stock, you will need to go into the PCM and readjust the calculated parameters based on injector pulse width.

Bill

Hmmm. I must be a strange specimen in the corvette lab. I don't calculate my MPG every fill up. Sometimes I don't fill up, honestly. Sometimes it's cold and windy so I just put in four gallons to get home. Guess I just don't see the point in meticulously calculating MPG. Either that or I just have a life. Probably the first option.

I average 13-14 mpg city and freeway. But then I like the go pedal and the sound of the BOV. Rod

There's a coupla things in your 'discussion stimulator' that it seems you haven't given *heavy* consideration--terrain and driving style. When the car was new to us, I used to track the MPG vs. DCI religiously, just to see what the comparison would be. For me, the DCI was off about 5-7%. If I was heavy on the pedal the discrepancy would be even larger.

All the numbers I quote below are hand-calculated, not DCI... I average 26 mpg from tank to tank--that's anywhere from 25.7-26.3 for "local" driving depending on weather, traffic, and destination.

If memory serves, my best is 26.8 for a 6,000 mile round trip drive from podunk, Mississippi to San Francisco. The 26.8 overall figure includes 2 weeks of metropolitan heavy commute driving. Excluding the 2 weeks (about 800 miles), the freeway-only number was 28.3 (that's not memory, that's my recorded number), rolling hills and all. My freeway speed averaged 73mpg (just under the 5-over limit in most states--yeah, I could go faster, but you wouldn't bleeve how many cops love to hassle me because of the MS plates; after all, I'm just a trespassin' dumasped furrin rednek); allow spurts to ~90 for passing, and a "constant 85mph" when I was blessed with a scout car I could pace behind. There's only two places in the country that I've driven where I can recall driving long stretches (~100mi or more) of flat ground in the Vette--AZ, and FL. On one of my trips, once, I got a DCI-claimed 33-something mpg, but that was only for the time I was stuck behind an 18-wheeler through 25 miles of a one-lane road 55mph construction zone once in flat west Texas. We were cheating at 62mph.

If you look at the EPA estimate, it's only 28mpg for the later model c5s, so 25 or 26 are prolly the numbers to expect. Skin out the AZ, FL, OK, KS, NE, etc responses for their "flat" terrain, and you'll probably see that most results tally with that 'real-world' EPA numbers. But I'd stipulate that a 28-32mpg claim from some of our flat-land writers ain't all DCI smoke.

Wow did i get transported to the Prius forum? Gas mileage who cares

Haha, true, but some of us are just nerds

I calculate most of my fill-ups in the vette as well. It's a sanity check to me, but also just another source of info to check up on my efficiency with the car.

Best I've ever gotten was 28.0 (hand calc'd) and 30.2 on the DIC. That was from Gatlinburg, TN to western KY (not sure on mileage). That's in my 02 z06 running between 65 and 80. Full bolt-on car with mail order tune. It varies but is always between 1-2.5mpg high.

HMMMmmmm... "I do, I do" says the nerd kid with his hand up at the back of the class... Pure fact:

I traded a Prius for a Corvette. No lie. I just now figured out gas is costing me an extra $282/year (for 20,000 miles of freeway driving). Insurance costs are the same believe it or not; $20 more for the same coverage per year. Who'da thunk it?

I *do* miss that Prius fuel display: "44mpg" in street driving, but then, I shouldn'ta traded the second Prius for a Suburban. (one accident in the Prius vs. a GM 4x4 convinced us to get out of them) And I just figured out what driving an 'armored car' costs vs. a Prius--$2100/year for gas. A piddlin'.

Sooo... the moral of the story is, a C5 Z will only cost ya about another $300/year ifn it's your freeway cruiser (with a 20k mile cap). The Prius only got 37.5 across country; its' strength is commute traffic. So now ya know. Yeah, yeah.... I know; you wasn't interested in the first place. I'll go away now.

In EFI LIVE these are B3501 and B3502
My C5Z always reads 7% high. Just this morning I calculated 14.4 MPG and the DIC was 15.5 MPG. I don't know exactly how the calculations are performed, but I am going to change the B3501 scalar from 1.0 to 0.93 and see how that averages out.

I with the group that monitors "smiles per gallon"

Especially now that I'm boosted

My DIC is usually very close..

But I believe that some of the optimistic figures may be accounted for
by the amount of ethanol in your fuel..
Remember, ethanol does not provide as much energy as gasoline.
So even though you used 10 gallons of gasoline to X amount of miles,
it may take 10.2 gallons if you have 10% ethanol.
Also, I don't know if the C 5's have an ethanol fuel map.
The Air Fuel Ratio Stoic of gasoline is 14.7. The Stoic of
gas/ethanol is lower ( i think something like 14.3 ish).
So in my opinion, this may account for mileage being "off".
I try to get NO Ethanol fuel, but around here I can only find in 87 octane.
So I have to make a choice of no ethanol OR add octane boost - which most have alcohol in them, so I guess it's a toss up

I do not really drive my Z much anymore now that I have a VW for my commute. I did however start using fuelly to track my fill ups. My mileage these days is always 15-17 mpg in the Z. I mainly just drive it around the city and goof off when I am in it. If anyone cares to start tracking their mileage Fuelly offers a great site, and you can look at other cars as well.

http://www.fuelly.com/driver/sigforty

I completely top off the tank every time I fill up. Why? Because if I experience a dramatic reduction in mileage, I have an indicator that something is wrong. Sometimes, a gas mileage "hit" is the very first indication that something is wrong somewhere in "there."

I calculate, manually (in my head - I AM a CPA, after all), my gas mileage but never pay any attention to the DIC's mpg data. In my judgment, the DIC is incapable of accurately accounting/adjusting for the number of variables that occur. Actual miles divided by actual gallons (using a calculator if you wish) is always CORRECT.

One other thing I do (with all three of my cars) is check the miles I have driven just before filling the tank, then predict (based on the "normal" mpg I get for each car) how many gallons I'm going to put in the tank. Never get surprised in my C-5.

In my daily driver, I occasionally get surprised when a fill-up requires more gallons than I predicted. But virtually always, that discrepancy is accounted for by the fact that my wife drove my car during this tank-full. She loves to start the car, then let it idle in the driveway while she screws around with girly things (hair, make-up, cell phone calls, etc.). Particularly wasteful when the air conditioner's running, and she typically leave the AC "on" even in the winter.

I rarely have the AC on at all (unless I'm using my car in "commute mode"). I'd rather have the windows down, and listen to the mechanical music my car makes. After all, that's why I own it.

Lastly...I absolute WON'T drive my car in cruise control mode. WTF? My 600+ rwhp C-5 is for driving, for goodness sake. Plus, on Texas highways I can't drive at a constant speed for any long stretches. I always fudge and go +10 mph whenever I can; and I invariably run up on a pink-haired lady, or out-of-state licensed car, who persists in going 2 mph UNDER the posted speed, and in the passing lane. (In my judgment it's why the right to bear arms was one of the first amendments to the constitution, but that's another thread).

The Lizzard

If you rely on Computers you are out of your mind. It ain't no question, you are wrong, simple as that. It is virtually impossible for computers to calculate precision at this level. If you think so, show me.

The computer should be able to calculate the MPG to a reasonably high degree of accuracy.
The computer knows how many revolutions the rear axles are turning from the speed sensor on the differential. This should be as accurate as the agreement between the computer's programmed value for tire size and the actual circumference of the tire. Hysteresis of the rubber might cause some minimal fluctuation in the diameter of the tire and centrifugal force will cause a very slight increase in the tire diameter with increasing speed, but I would estimate either of those variables to be much less then 1% at highway speeds.

The computer can also keep track of the individual pulses of the fuel injectors, and here is where a much higher order of variability is encountered. When in new or freshly serviced condition, injector flows rates are well known and match closely across a set. After years of service flow rates drift and vary, nozzles become clogged, and the pressure regulator may be off from its initial rated value. The injectors spray fuel into a manifold that can be in a state of flux pressure wise, causing variations in the quantity of fuel delivered. Voltage fluxuations can increase or decrease the pulse width time. Changing fuel temperature changes the density of the fuel, I think that is totally unaccounted for in the programming.

What does it mean? How accurate can the computer be?
I think it is best determined empirically.
My C5Z DIC is off by 7% consistently. 2% from different sized tires and 5% "other". Once I "true" the fuel economy scalar in EFI live, I'll can track the MPG and see if it matches more closely with my calculated number.
How do I account for fuel temperature and corresponding density change from fuel delivered at the gas pump?!
Maybe I'm putting a little too much thought into this.....

Last week, I made a 100 mile run up the New Jersey Turnpike. Filled it before I left and filled it when I got back. I have a 99 6 spd. I got 25.5 mpg dividing gals used against miles traveled. My DIC said 21.3.. Bruce

Same with the gas station pump you are using as "gospel"..............are you really positive that that gas pump is telling the "truth"?

Are you getting the correct amount of fuel you are paying for (I know, "state certified" and all that)?

And don't forget that 10% ethanol results in about 5% less fuel economy......

Plasticman