white93vette

I just got back from a road trip to Phoenix. I filled the tank four times for the entire round trip. I calculated mileage based on odometer miles divided by gallons of fuel put in the tank (the old fashioned way). I got 24.2 MPG based on that method. For the same time, the "Fuel Information" computer on my dash showed I averaged 23.1 MPG. I was just wondering if others have done this comparison and what their results were. Also, does anyone know what type of instrument is used to calculate fuel flow to perform this calculation?

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scorp508

I did this when I went to Bowling Green last year. Over the 1100 miles getting down there the car said I averaged I think 26.5mpg. I forget the exact #, but I remember it being within 1 of my own calculations.

FoolCrzy

My mileage thing doesn't work..It has a couple times intermittently. I keep it turned off now... any ideas?

Tom Piper

The computer calculates the amount of fuel used based on the time the injectors are open and the intake manifold vacuum (intake vacuum adjusts the fuel pressure by means of the fuel pressure regulator). Keep in mind, all fuel pressure regulators are not exactly the same and can affect the actual fuel mileage.

Tom Piper

SunCr

On digital clusters, I don't think vacuum, per se, has anything to do with the readout. The ECM uses throttle angle - TPS signal; Odometer signal, Engine Load (which in one respect, is the electronic equilivant of vacuum, but it's not generated by the FPR), Air Intake - MAF, Engine Temp - Engine Temp Sensor; and the Fuel Level Signal - Fuel Level Sender. My mileage isn't totally accurate, but nor is my fuel gauge (is it on any car?). So I've always figured that the Fuel Level Sender was the weak link.

Marcho Polo

Pretty accurate statement I think.... interestingly, I can change a constant in the chip and make your readout say just about any mileage I want!

On my '85, I tweaked this to get my mileage to be dead on with manually calculating it. On my '90, the digital readout may say 17mpg for around town driving, and when I fill up and manually figure it, it comes out around 15.7 or 16.0, so my digital readout is a little higher than what I'm actually getting. I haven't been on a trip to do highway mileage yet. The few times I've been on the freeway and look at the instant mpg, it stays around 24-29 depending on conditions. So I'm guessing I'll get around 23-27 actual mpg when I fill up.

MTVette

I usually get about 1 to 1/2 mpg less than the display indicates. No problem, though - when the gauge shows 'empty', it takes a full tank.

Tom Piper

I was suggesting the computer is sensing the intake vacuum from the MAP (manifold absolute pressure) sensor not from the actual fuel pressure regulator vacuum -- the fuel pressure is assumed to be a certain value from the manifold vacuum reading. This depends on the fuel pressure regulator being somewhat accurate. If someone uses an adjustable fuel pressure regulator and changes the setting, the calculated fuel could be wrong.

In the end, all the sensors the computer uses are taken into account to control the injector pulse width. The pulse width of the injectors and the fuel pressure determine the fuel flow.

Tom Piper


[Modified by Tom Piper, 1:12 PM 10/19/2001]

LT401Vette

As usual Tom is right on.
If you install an AFPR and adjust your fuel pressure different from stock, your indicated fuel mailage will vary. The computer knows it had the injector open x ms. and assumes under the stock pressure and injector size. It can then calculate how much fuel was used. If you increase fuel pressure or injector size, you will have injected more fuel than the computer realizes, therefore see a higher than expected MPG.

So if you feel better seeing high mpg's on the dash, just increase fuel pressure or injector size and recalibrate the tune for it :) .

In Hoover with 36lb injectors, the dash will tell me I am getting over 30mpg on straight highway trips, 22mpg long term commuting to work. For a real number take about 33% off.

dtorc4

If my gas tank shows empty, then I ran out of gas 30 miles ago :D :cheers:

Seriously, if my gauge gets to reserve, then I really am out of gas. It happened to me twice last week. I was going to school, and the reserve light came on w/ 2 bars worth of gas. When I got back to my car and started it up, it still showed 2 bars. As soon as I put it into gear, it died. I didn't even make it out of the parking space!! :cuss

Is there any way to fix the fuel tank sender?

FrankLT4CE

On my LT4 my actual mileage is 1 to 1.5 mpg less than the avg mpg shown on the display. I have also corrected for odometer errors when I did my actual mpg calculations. FrankLT4CE

65Z01

As I remember the fuel level detector looks a little like the float in a toilet tank. I believe that if you pull the assembly and bend it a little you can correct the readings. You might also be able to use a resistive divider to correct it electronically, at least at the low end where it matters.

LT401Vette, sounds like you are saying that my current 48psi vs stock 44psi FP has caused my mileage reading to error by about 4% on the high side, since the O2 sensor now signals the ECM to cut injector pulse width in closed loop mode. In that case my current readings of 31-32mpg highway are realy like 29.6-30.6, which is more in line with the stock readings. Well maybe slightly higher.