my car runs great, maybe i need new injectors but still. when i drive on the highway (4+3) in OD, the car tells me i am getting close to 28 average MPG, most of my driving is highway 70-75MPH. i watch my mileage and everytime i fuel up i divide my miles on the trip meter by gallons of new fuel and i may get 18-19 MPG. when i first got the car in 05 i was getting about 23MPG when the car told me it was closer to 28MPG.
is the car off that far and why and what mileage is everyone else getting?
if i am wasting fuel where is it going?
if a fuel injector is leaking wouldn't it give me a check engine lite with a code for the oxygen sensor saying its too rich?
i have no fuel leaks outside the vehicle, no fuel smell anytime
how much benefit would i get with FIC bosh 3 injectors?

 

Some of the things that affect mileage are:
1. tire pressure
2. spark plug wires
3. compression
4. tire condition and rolling resistance
5. distributor cap and rotor/timing
6. engine temperature
7. aerodynamic drag
8. dragging brakes
9. torque converter/overdrive unit/fluid and filter condition
10. gear ratio
11. fuel used
12. fuel additives
13. oil type and formulation.
14. fuel injector type and condition
15. fuel pressure and supply
16. air filter and air supply
17. ambient temperature
18. chip calibration
19. The places you drive to has changed, ie: more hills, less flat roads

Now for a car that essentially nothing has changed, wear and tear items are the big factors.
The BIGGEST factor is that the fuel efficiency calibration for your car was based on fuel that was produced when your car was new.
Fuels have been losing BTU value for a long time now.
Your car is probably all right, but the gas doesn't have the same punch as it once did.
Better injectors would help along with higher fuel pressure for better atomization, but you just don't have the same gas anymore.

 

There are many things that can affect the mileage calculation that the dashboard does. Since you don't know the history of the car, anything could have been done. Different rear axle ratio and different flow rate injectors are two obvious ones...

Your 18-19 MPG is about what you can expect from this car.

 

I disagree with the 18-19 to be expected.

I had an '87 4+3 and it got really good highway mpg. I took a 1000 mile trip from Denver to Illinois and got just over 30 mpg (30.1) calculated at the fill ups. I used cruise at about 70 mph. The avg. mpg on the gage was pretty close to the manually calculated.

I also have an '85 4+3 and it has never been as good, but it gets about 23 or 24 mpg on the highway. The gage is also pretty close on this car.

There are also two different overdrives on the 4+3 depending on Z51 option or not. The Z51 cars have a .68 (I have read different sources say from 0.66 to 0.68) while the non Z51 cars have a 0.59 ( I have read 0.59 from the '85 service manual and 0.60 from other sources).

I think the difference between my '85 and '87 mileage was primarily due to the gearing difference. My '85 is a Z51 0.68 ratio and the '87 was a non Z51 0.59 ratio. There are also a few other differences between them such as the '87 is a roller cam and better electronics (tuning), but I think the gearing difference had the biggest effect on mpg. The '87 and '88 engines and tuning are basically the same. My '87 SLP tuning chip was listed with the same part # for both '87 & '88.

I also had an '88 automatic with the 2.59 rear axle ratio. It did pretty well on the highway too. I hit 30 mpg quite a few times on cruise at about 71. Normal was a little less with 28 or 29 pretty common for highway mpg.

I don't know why your car changed mileage between 2005 and now, but it sounds like something changed. It sounds like you might be on the right track in looking for something like the injectors leaking. I have heard that the ethanol in our gas causes the early C4's fuel injectors to deteriorate, but I am not sure what years are affected.

Good luck tracking it down.

 

I get 18-19 MPG combined city/highway driving. Highway alone on a 1500 mile trip to NCM i got about 26-27MPG. These numbers were backed up by measurement and dashboard reading.

 

I've kept track of every single tank of gas I've put in my car (great app called vehical) from day 1 and my avg is 16.7 50% highway. Btw I've spent $3340 in the year I've owned it. Mileage may vary :-)