mpg's all over the place

[az1966]

200 ci 6 poper in a 66 stang , 3spd manual tranny, 1 week 20 mpg the next 15mpg,,, have a notebook in the car marking all settings and gallons at the pump,, to make matters worse I drive the same way and route day after day,,, rebuilt the carb ...double checked timing

HELP!!!! it's driving me nuts

 

[The Plankster Prankster]

what brand of gas are you burning? same every time or different?
bad gas cuts your mileage farther than you would believe!

do you notice a difference in power between times of good and bad economy?
something like the timing failing to advance would cause poor economy and power loss

are there any signs of theft?

how is the ignition system?
my truck has plug wires come loose every now and then that hurt my economy a bit

any other variables you can think of?
could it be something as simple as one gas station's pump cutting off a couple gallons early, leaving you to believe the previous tank had great mileage and the following tank was poor...check out the simple things like that before you start throwing parts!

 

[BUZZ]

I'm leaning towards bad gas or the gas pumps not reading the same.Do you use the same pump every time?5 mpg is a big differance

 

[Harte3]

Back in the late 60's-early 70's I had quite a commute to and from work. I was using "slut" gas...the cheapest mini-mart stuff available and got four round trips out of a Ranchero with a 170. There was a Shell station next to where I usually filled and I started filling there...got 4 1/2 round trips out of a fill and the inside tail pipe color changed from black to tan and the spark plugs ran clean too...cured me of using the cheapest gas available.
While all gas comes out of the same "rack" the majors add additive packages at the rack or at time of delivery to the gas station...and it does make a difference.
If you are in an area that mandates a winter formulation gas, (E10?) mileage and power goes to pot. My '83 F150 300 damn near refused to run on the stuff prior to the rebuild. It's bonus time around here for repair shops in the winter. People with the older carbed vehicles line up to have there "carburetors" fixed when the problem is the gas.

 

[BUZZ]

I thought about it some more last night and other than the gas being your problem I was thinking about your fuel lines. I'm sure you've already checked them once but maybe you should look at them again. I have a truck that would leak one day and not the next, it gave me a devil of a time trying to find it. It turns out that the gas would swell the line and stop the leak untill I let it sit for a few days then when I drove it later it would start leaking again untill it could swell back up. It turned out being my fuel return line and the rubber line going to my filler neck.
Also you are using the same grade of gas each time right? I'm sure you know that straight gas and an ethonal blend burn differently but I've some times woundered about octane grades also. It seems that a higher grade octane gas would not burn as well in a lower compression motor as a lower grade but I'm not a fuel expert. It's just a thought.

 

[wheelzzs]

I had the same problem years ago and it baffled me until I finally figured it out. On a fill up your gauge always goes above full so it must be full right? wrong check to see how far you go until it goes to the F mark on gauge from tank to tank. My tank had a dent in it and when I gassed up on an incline the tank did not fill all the way. If you are missing 2 gallons of gas your millage could vary 5 mpg.

 

[kukm66]

Do you have more data besides the 2 concurrent weeks? Calculations can be off significantly from 1 week to the other due to how much gas actually went in. Display results from at least 6-8 weeks in order and take an average.

Causes of bad mileage:

carb running rich
brakes out of adjustment
timing off
plugs fouled
leaks in gas lines
stop and go traffic
excessive idling in town
jackrabbit starts
bad gas

most of these were already displayed in the threads above

I would take a few road trips on the highway and take some readings, then compare

 

[pikesan]

How far is it to your work? Does it vary by temperature at all?

Does the car have vacuum advance? Make sure the vacuum diaphragm's working. (just suck on it and see if the advance changes)

Try to take out the variables:

• use the same gas station (same pump too if you can)
  check the tire pressure every day before you leave
  log the ambient temp *just for giggles*
  Time of day
  water temp when you leave the house

what else did I miss?
(did you double check your math? just kidding!)