The MPG in my firebird is driving me crazy. In one day I drove down the same highway 4 times (2 passes in either direction at 150km each pass): 15.38, 18.12, 15.70, and 20.08 mpg. This is actually quite normal for the car, with just highway driving it ranges between 14.6 and 25.2 mpg without and good reason for the differences. Anyone have a clue what could cause it to fluctuate this much?

Is this on a newer car with the digital read out of your mpg?? Or is it your 1971 Firebird if it is the first thing that I would look at is the vacuum advance. You can just plug off the line and see if your mpg stays the same if it does I would lean towards your vacuum advance not working properly.

Is this on a newer car with the digital read out of your mpg?? Or is it your 1971 Firebird if it is the first thing that I would look at is the vacuum advance. You can just plug off the line and see if your mpg stays the same if it does I would lean towards your vacuum advance not working properly.

This is my 71 Firebird, the only car I have. When I bought the car it had the vacuum advance disconnected, and it suffered the same fluctuations as now. I have also replaced the vacuum advance unit with a new one, so I don't think it has anything to do with it.

Anyone else have any ideas what could cause this much variation in efficiency? Could it possibly have something to do with poor wiring or electrical, that is one thing that is really poor shape on this car...

I have already tried replacing the cap and rotor on the distributor, and replaced the spark plug wires, and plugs. I have been trying to figure it out for the past 6 months, I guess I just don't understand enough about older cars to properly diagnose the problem yet.

Do you calculate the MPG accoding to the Fuel gauge reading? Most of the fuel guages are not reliable....and diffrent driving habits and methods can cause diffrent MPG ....I guess you may want to consider the average MPG of the ride in the highway... and yours seems decent,,,, (even 15 MPG)

Do you calculate the MPG accoding to the Fuel gauge reading? Most of the fuel guages are not reliable....and diffrent driving habits and methods can cause diffrent MPG ....I guess you may want to consider the average MPG of the ride in the highway... and yours seems decent,,,, (even 15 MPG)

I calculate my MPG as the number of miles traveled (accurate odometer) divided by the amount of fuel purchased. I don't worry about what mpg I am getting, just that it varies so much that I beleive something isn't working properly.

I consistently fill up with the same grade of fuel all from Shell gas stations so that eliminates more variables in the calculations. I just can't understand why with no reasonable changes in terrain, weather, or traffic how my mpg could change so much.

Sounds like it might be in the amount of gas you are putting in the car. How are you filling it up, are you relying on when the pump clicks or are you forcing it to top off? I would also think the more miles in between fill ups the more accurate you will be on your calcs. Might be something to consider.

Sounds like it might be in the amount of gas you are putting in the car. How are you filling it up, are you relying on when the pump clicks or are you forcing it to top off? I would also think the more miles in between fill ups the more accurate you will be on your calcs. Might be something to consider.

I always fill it up until it clicks off, but since I almost always use the same filling station that should be relatively close each time, I don't think that could make the difference between 14 and 25 mpg anyways. I used to do 4 hour drives home every weekend, and the numbers just seem to randomly bounce all over the place. I have been keeping a very close eye on my mpg since I got the car, to see what changes had the most impact (Overdrive transmission works well).

If waiting for the pump to shut off is not accurate, is there a better way to check it?

The wind can make a huge difference in MPG also. More than one might think.

I would also check the speedometer cable and the gears in the tranny. It's possible one of them is not turning at the right speed. Cables get dry and wore out after being in or cars for 30 some years. The gears in your tranny could be wore out also. They are just made of plastic and are very easy to take out and replace.

I would also check the speedometer cable and the gears in the tranny. It's possible one of them is not turning at the right speed. Cables get dry and wore out after being in or cars for 30 some years. The gears in your tranny could be wore out also. They are just made of plastic and are very easy to take out and replace.

Actually I upgraded to an electric transmission (4L80E) and have a programmable electric speedometer, so there are no plastic gears to worry about.

Does anyone else see fluctuations like this in there mpg? Is it possibly normal for these cars?

The wind can make a huge difference in MPG also. More than one might think.

I will second this. I never really noticed something like this until this summer. Pulling my boat in one direction and through the city region on the main highway which winds around into a different direction and into the open fields. Man did my gas gauge drop when the open fields were present. Most other times it is not bad like this. Could quite possibly be the wind against you or helping you.

I suppose you could be right, it may just be the wind. I never would have guessed that it could have that much effect on the car. I know I never had this problem with my 1990 Firebird, but I guess it was alot more aerodynamic than the 71...

Just a thought but it probably could be air quality too.