I have a '72 F-100 with a 390 engine, it's got an automatic transmission (C6 maybe- I am not too familiar with it since I have not had it for very long or get a chance to work on it yet). I am getting about 6-7 mpg, not sure if this is normal or should I be getting more than that, and if there is something I should look into replacing. She is my weekend truck, so not looking to get the gas milage like those crazy prius or whatnot. Just trying not to waste too much $$$ on gas, especially with this price nowadays.
Also, Cannot get a very acturate reading on my odometer since the dash lens appears to have been scratched or fogged over. Tried to replace them, but seems like the screws are reversely screwed in (I think), not sure the right approach to that, hope someone maybe have done this before and can point me toward the right direction. Like I said, I am pretty new at this old truck thing, so please excuse me for using the wrong terms. Thanks in advance.

 

sounds like your not getting a good reading you should get at least 10-12 mpg

 

Do you think it may because of higher speed (60-65mph) on high way? or could it possibly be due to the carburetor? Thanks gliderider!

 

Well what gear ratio do you have? Older trucks typically run 3.73's.

Do you have an electric fan or is your fan bolted straight to the pulley.

Your carb could be out of tune, lazy timing, some combo of both.

 

Do you know what your axle ratio is? That might be a big culprit.

 

I have no idea, how should I go about checking for my gear ratio? unfortunately the older trucks don't display the rpm, so...kind of out of luck there. I will have to check on the type of fan it is behind my radiator, will let y'all know, thanks guys.
Joe

 

Well, all I was saying is that you might have something like 4.10's while gliderider is running a 3.55 or something numerically lower. That would explain why you are getting so much less mileage. If it is all original, you should be able to decipher your vin number and data plate. It will tell you the gear ratio. There might also be a tag on one of the bolts on the differential telling what ratio it is. Last resort if the other two don't pan out is to pull the differential cover and count teeth.

 

Bear in mind I have a 71 work truck and couldn't see the gauges thru the cloudy/scratched dash lens. I had a box from a toy boat with a large see thru plastic window in it. I cut that out of the side of the box and with my dash front on the bench I took out the old plastic and patterened the new piece using it and reinstalled and it looks like brand new. That was a couple years ago. You can't tell it from orig.

 

The only way to accurately gauge mpg is by filling the truck, noting the mileage on the odometer and then doing some subtraction and division next time you fill up: miles traveled divided by gallons pumped/purchased. Either your odometer is off or you have a sick 390. My 69 F250 with 360 and C6 and 4.11 rear end gets 10.5 to 12.5 mpg, mostly in-town driving. I would check timing, do all the tune up stuff, clean the carb and run some Sea Foam through a tank of gas...then try to gauge the mpg.

 

merlynr, how were you able to remove the dash lens, when I took the screw off from the from, seems like there were screws around the back of the panel I needed to remove before I can get the panel off. So that's where I am stuck. Thanks

 

Wow! 12.5 mpg's with 4.11's? There is hope for me after all! Please tell me that you have a 4x4, tmcalavy.

 

Sorry, it's 2WD...bone stock with more than 100K on the dial, running a Holley 2 brl. carb. Wish it was 4WD though. If I drive like a granny, I can get 12.5 mpg...10 is closer to reality though.

 

Somewhere many years ago in a car magazine I read about this trick. I never tried it, but it makes sense.

Block all the wheels and put the transmission in neutral. Jack up one rear wheel and spin it one full revolution. Count the revolutions of the driveshaft as you do this. If the driveshaft spins 8.2 times, you have a 4.10 (double the ratio due to the differential and one wheel turning). If it spins about 7.5 times you have a 3.73. If it spins 7 times, you have a 3.54.

This all assumes you don't have a limited slip/positrac. Like I said, I never tried it, but it beats pulling the third member and counting teeth.

 

Hey Alamo, that is actually a pretty good idea. I read that, too....somewhere.

 

Try some plastic polish on your IP to get rid of some of the scratches and the haziness. You can find some at NAPA or other stores. Then follow the advice of plastic aircraft modelers and coat the lenses with Future floor polish for a good shine. It would make very clear lenses.