Well what should I start to check far as the fuel system? There's no codes what so ever, no check engine light, no service vehicle soon on the DIC. Only determining this based on the fuel gauge, trip meter and the amount of gallons put in the tank at the station. So I have a 19.1 gallon tank and after resetting the trip once a full tank was put in I've gone about ~57miles (mostly highway driving) and I'm down 1/4 tank. After calculations that would mean about ~228miles per tank

I've had problems with mpg since I've bought the car (about 3 months ago) but it's time to get to the bottom of this.

 

base your calcs on what you actually add/what you drive over 10 tanks or so. Don't use the computer data, and make sure your speedo is correct by using a gps to compare speeds with your odo.

 

Relax, my friend.

The gauge on these cars is not like "normal" gauges, in that it doesn't really reflect how much fuel is in your tank. It looks at the average fuel economy since the calculation was reset, and it adjusts the gauge accordingly.

in other words:
You fill up, then sit in traffic or idle your car or drive it like you stole it. Your MPG will be pretty low from any of those or a combination of them or whatever. Then, you park it and shut her down and come back. The fuel gauge will adjust at that time based on your average MPG so far, along with the Range/Distance Until Empty. Since your MPG is so low at that point, your gauge will sit lower and your Range will be short. If you drive on a highway at normal cruising speeds, your MPG will improve and your gauge will adjust. You could get in the car and reset the MPG calculation just as you reach your cruising speed, and your MPG calc will be MUCH better. Of course, the real way to keep track is with the distance driven on the odometer divided by the fill-up quantity.

Corvette Gurus, feel free to correct me. These are my observations in several months of ownership of a C5.

Hope I helped some.



edit:

also, i have noticed that what "kills" the MPG on these cars is idling. because when idling, your MPG is 0.0 because you aren't moving! i will be lowering my idle speed ASAP with HP Tuners to help. there are also some ways to lean things out a bit with HPT or your choice of aftermarket tuning software and improve MPG.

 

ya that is the way i have been doing it... and it just doesn't seem right. oh well, dyno tune within the next month should straighten things out

i had no clue that the fuel guage adjust depending on the driving habits I normally just cruise on the highway and seldom in town, but I guess I will just have to live with it.

 

I'll humbly disagree with gauge operation. The fuel level gauge gets its information from the BCM which gets its information from the PCM. The PCM gets its information from the 2 fuel level senders (one in each tank). The PCM calculates total fuel in the tanks by the information recieved from each sender and sends that information forward to the BCM. So, the fuel gauge should be actual fuel level (total for both tanks). There has been some problems with the senders and GM has a bulletin out on it. Seems sulfur in the fuel causes the senders mess up.

Anyway, if the car feels right, I think I would make sure I have what I think I have. Altering tire diameters or rear axle ratios without "fixing" it in the computers will show incorrect miles traveled. I would always use actual gallons pumped also.

 

One other thing. The gauge ain't accurate enough to calculate from 1/4 tank indicated on the gauge. Fill the tank, reset trip, fill at 1/4 tank or less (whatever you feel comfortable with), note miles travelled and gallons to fill and run your calculations. Do this over several fill ups and you'll have a good idea of mpg.

 

ya this is what i plan on doing... it's better to re-fuel at a 1/4 tank instead of empty?

 

As noted, whatever you feel comfortable with. The "experts" say the further you go, the better the measurements and calculations. I personally don't trust any gas gauge to empty. The point is to not use only 1/4 tank of fuel and then fill and calculate.

I rarely look at the DIC fuel stuff anyway. It's fun on a trip to see the mpg go up or when you are driving spirited to see it go down. But, it doesn't give the real number I'm looking for. FWIW, I've seen as low as 17mpg for true city driving and as high as 29mpg for true highway (on a 1300 mile trip, lots of opportunities to check).

You didn't buy a vette for the gas mileage, did you? (j/k)

 

have you upgraded your exhaust recently? most people report their gas mileage is significantly reduced immediately after an exhaust upgrade! you can ask ANYONE about this and they will verify!

if this isn't your daily driver, you might want to pull your injectors and see if they are all functioning correctly (not sure where or how to get this accomplished, but i'm sure a shop that can flow match a set of injectors can certainly help you with this)

is your air cleaner clean? how about your plugs and wires?
if it were me and i just bought the car i'd change plugs, wires, and maybe the O2 sensors on a long shot, and buy a new filter (or clean your reusable one)
make sure the pressure in your tires is correct also - this can cause a decrease in MPG as well.

 

i get 12 mpg in town in my 04 coupe with a4. but i have h/c/i stall and gears. also on my fuel gauge each tick mark is one gallon of fuel. count the tick marks down from full and that is how many gallons it will take when i stop for gas. it is linear and never off by more than 1/2 gallon that way.
Dave

 

I think the confusion here is between the gas gauge and the DIC mpg calculation and they are different readings from different sources. jss812 gave a good summary of the fuel gauging. In my experience its quite accurate providing you don't hit the dreaded gas gauge problems. I also try to refill at 1/4 but I've seen the HUD/DIC low level warning at 60 miles to go a few times.

The DIC has 2 functions but takes dynamic information to make its calculations. The instantaneous mileage is exactly that. Look at the figure when you accelerate hard and it'll read as little as 3 mpg. If you're coasting it can be as high as 80 mpg. The average mileage figure is an average over the time since you reset the calculator. By that I mean if you have Average Mileage on the DIC, hit reset and then drive 60 miles at a constant 70, it's an average highway mileage over 60 miles. Set average mpg and hit reset when you're cruising and the figure will be high. On average at a steady 70 with my A4 it'll be about 28. If you do the same when you are accelerating hard the figure will be low - maybe 5 or 6. Over a few trips it should average out and in my case it settles at about 20 mpg.

As you rightly say, the best way is to do a manual check on the gas mileage between top ups. If you really are at 11 mpg, either your driving is "spirited" or there's a problem. I'm surprised though that you have no codes if the car is running smooth but using gas at that rate.

 

First of all, you cannot say anything even near to accurate based on fuel level gauge reading

Please correct if I'm wrong what I have understood about MPG calculations. It measures the air taken in the engine and based on that assumes the amount of fuel used, so it doesn't meter the actual fuel flowing in the pipes. So it cannot be as accurate as manually calculating. But anyway this shouldn't cause 11 MPG Especially because most often the on-board computers are optimistic in gas mileage Of course if you have a problem with the fuel level sensors, it could show the wrong fuel level (but I think in most cases it should then also have some code for this problem). E.g. if one sensor sends wrong data, but another right, could it show wrong value in gauge but still not having any codes?

Anyway, the "best" way (the accurate one) to calculate the actual fuel consumption (already referred before):
1) Fill up the tank. Write down the miles run until that.
2) Drive e.g. 2000 miles (or whatever you want but long enough to have several refuels done). Refuel when needed, write down each time the gallons refueled.
3) After 2000 miles, refuel again.
4) Sum all gallons refueled (not including the first one before starting this "test drive"!).
5) 2000 miles (or whatever exact miles at last refuel) divided by sum of gallons => you will have very accurate average MPG

If you don't want to wait until you have this 2000 miles driven by your everyday runs, do it e.g. during weekend and drive this 2000 miles at once Have some fun driving around your state