Generally, you would think that gearing something to run at a lower rpm would result in better gas milage.

In my case my F100 has a (not very stock) carbed 302 with a C6 and 2.75 gears. I'm guessing I get around 10mpg. The engine really doesn't feel like it's breathing in third gear until you're going 75mph.. or if i'm running in 1st or second gear.

I honesly think I can get just as good or better milage with much more aggressive gears. Thoughts?

 

I agree with you and think if you got lower gears, you would get better mileage. I have a 86 with 33s and it had 3.55s in it and with the 302 efi and AOD trans I would get about 10 or 11 mpg. Over the summer I did a 9 inch rear swap and put 4.11s in it at the same time. I also put headers on it at the same time, but after that swap Im getting 15 to 16 mpg. I think the headers helped that some also but mainly the gears is my guess. Plus you get alot more 'seat of the pants' feel with lower gears.

 

Every motor has a sweet spot where it gets the best milage and generally it's the rpm where the motor is nearing peak TQ. For the 302 that's up around 2500rpm so the solution with this motor is always more gearing.

 

Did you change the speedo calibration after the gear swap? If you put taller gears in without changing the calibration, it'll appear as though you're getting better gas mileage because the drivetrain will have to turn more times per mile.

 

Very good point.

 

Is your timing being advanced? What RPM does a tachometer show at 60 MPH?

 

1. I don't know how to tell.
2. I haven't hooked up a tach yet.

The truck sat for about 5 years and i'm still working out the bugs.

 

Do you have a timing light?

 

Negative..

Edit: Wait a minute, a timing light is that thing that has been sitting on my shelf for three years? Which happens to not be there when I need it?

 

Seems you should buy some tools & equipment, dude.

I wouldn't know how to determine if the timing is being advanced without a timing light.

 

In response to my above edit I found a timing light.

http://ecx.images-amazon.com/images/I/51t62xss2CL.jpg
(looks like this)

 

Yeah, that's a timing light!

OK, hook it up, make sure you can see your timing marks. At idle, I'd expect it to be at
around 10 BTDC but this will vary, look on your engine sticker or Haynes/Chilton's to see
what the specs say.

While watching the marks, rev the engine to about 2500 or so RPM, the timing should
advance by several degrees - say, to about 22 or so (BUT this will also vary).

 

Is the truck in question Duraspark-II?

If so you need to first check the timing as ctbutis describes with the vacuum advance unplugged, then check the timing with the vacuum hose plugged in.

This is because the DS-II distributor has centrifugal advance, and the DS-II module will also advance the engine timing slightly. Unpluggin and pluggin in will tell you if the vacuum advance works or if there is a problem.

It will also tell you if the centrifugal advance is working or not.

 

I probably should have thrown this in before, I have a Holley 4180 (4bbl)
carb. Maybe I should go to a 2bbl. It runs great, and is pretty fast once it's put in 2nd gear. (Not that it's bad in 3rd gear, it just doesn't have much kick)
Will do, I have been busy so I didn't get to it yet..
I'll try that when I get around to it. Yes it's a Duraspark-II system.

 

Since the carb is not a factory installed item, make sure the vacuum advance is NOT getting full manifold vacuum at idle. That would make it work backards, retarding the timing as you open the throttle.
Holley has a vacuum port on many of their carbs that is located on the side oposite the throttle cable, kinda up about 1/2 way. Many books label it as a "Spark port", and that's where the vacuum advance would hook.

2.75 gears are rather tall. Depending on tire size, you can see as low as 1700 RPM @ 55MPH. Not exactly a great range for power. Swapping to a 3.25 ratio rearend would certainly liven that truck up. I've got 2.75 gears in my truck, and plan to swap to 3.25 or 3.50 in the near future.