I have a 1962 F100 uni with a stock 1973 302 and an AOD from a 1983 T-Bird. The 9" is a 3.70:1 gear. I currently get about 14 MPG. This is to be my daily driver so, I want good gas mileage. I don't care about jumping off the line in a drag race.
I drive mostly 55mph and higher wherever I go.
What do you guys think about putting a 3.25:1, a 2.75:1 or even a 2.47:1 gear ratio in it? Do you think I can reach 20 MPG? Will it be seriously sluggish off the start?

 

14 sounds good to me, of course you could get more, but at what cost? Sorry no help on the gear question, I'm running 3.50 with a 390 C6.

 

The AOD's overdrive ratio is 0.67, so when in OD, you're 3.70 axle ratio magically becomes 2.479!

I would say that if anything, you need steeper gears, because an early seventies 302 2bbl engine made maybe 150 hp......

I had an '81 F150 SWb with a 5.0L Roller motor out of an '88 Mustang(~225HP), along with the trucks original C6, the truck had 2.75 axle ratio, and was great on hwy, but was NOT HAPPY around town, so I swapped in a 3.25 pumpkin, and it was much better around town, and still decent on hwy.
Your engine is ~75HP down on the roller motor, so it's not going to be happy trying to lug your Uni down the HWY at some ridiculously low RPM, it needs to be in the sweet spot of it's torque range to get optimum mileage, and I think when in OD, it's well below that.
Try going for a decent length hwy drive in DRIVE (ie: not in OD), and see what happens, if test mileage improves from what you currently get, consider changing to an axle ratio that puts you at your DRIVE cruising RPM while in OD

 

Well, this thread has dried up and turned to dust???........................................... ...

 

No, Sir.. It's not dried up. Sorry about that. I'm just new to posting and lost how to get here for responding. I'm still not sure if this will be correct and you read it.

Thanks for the great input. I have a 3.25 that I will try. I'm currently turning about 2500 -3000 RPM at 70 MPH with the 3.70. That RPM sounds low. I know. What is the RPM "sweet spot" for my 1973 302?
A guy I know said he burned up his trans pulling a small 21 foot boat behind his 1977 F100 which had a 2.75 gear. He only went about 25 miles. He thinks the 3.25 is as far as I should go down.
I haven't touched the truck in three weeks. This Memorial Day weekend, I hope to get the 3.25 in it. I'm swapping in "easy ride" springs from a 1969, too, while I'm at it....and all new brake system (rearend only). I already painted all the parts. It's an extra rearend so, I was able to access it easily. Now, it's time to put it under the truck.
I'll let you know how the 3.25 works.
I'm sorry for my lack of response. I'm not too good at participating in these forums, yet. I do, however, read a lot of them.

 

You have entirely missed my point...........reread my post

 

Naaa. I read what you said about going down the interstate in "drive" vs "OD". You say I may need to go the other way and get a higher ratio for higher RPM. I will be installing the 3.25 (from the current 3.70) this weekend, hopefully. I realize that will make the RPM even lower. That was my intention. Maybe, it's the wrong move. IDK. If it doesn't get higher MPG, I'll make a change. Even with the anticipated improved MPG, it may be too sluggish for the occasional in-town driving.

I guess what I don't understand is "This thread drying up". I guess that was a pun that went over my head.

In any case, if I can actually get the rearend swapped, it should only take maybe 45 minutes or so to then swap in a different gear. If the 3.25 doesn't work for me, I can try the other way as you suggested. I just don't have those other gears....yet.

If you recall the 70's, there was the oil embargo. Vehicle mnfrs moved toward saving MPG. I figured in doing so, they started putting in the 2.73 gears in the big vehicles. That's why I was headed that direction. Not knowing the RPM "sweet spot", this is my guess on getting improved MPG.

 

Hey, if you have the parts, go for it. That's how we figure stuff out, right? In my uni at 70 mph I'm running near 3800RPM, at 55mph I'm running 2600RPM, and I get roughly 13mpg, which I consider good considering the engine setup I have, anyhow let us know how it turns out,

 

No it's higher than it should be with an AOD, what size are the tires? Are you certain it's going into OD? There should be a pretty large rpm drop when it does. Maybe that pumpkin has even steeper gears in it.
2000-2500rpm. The 302 in general makes no useable power below 2000rpm and your copy is probably the weakest ever built, so lugging the motor does nothing for performance or fuel milage. Best milage is achieved at he highest speed attainable with the least throttle input even if that results in a bit higher engine rpm... within reason of course, 2500rpm and barely touching the throttle will consume much less fuel than 1800rpm at WOT.

 

I think that your current combo is about right. These old trucks have a lot of frontal area to push at highway speeds. Working on engine tune might bump your mileage up a bit. Also check what water temp your donor engine is designed to run at, an engine running cooler than normal will get lower mileage.

 

Conansky and Chiefdave,
Man, y'all really know my truck. The tires are 6.00x16. When Coker Tire has their show next year, I'll try for some wider tires. I have a pair of 16"x7" wheels (widened originals) for the rear.
I think I am running about 2000 at 70 MPH and I thought it couldn't be correct. The RPM gauge is rather shaky and the speedometer doesn't work. The gauges are old ODI units. The fuel gauge is off by half (must be the wrong ohms). I found an original cluster to back in place until I can get Classic Instruments for around $975. Yikes!

I've been using an app on my phone to tell the speed and distance and just always filling up to check the MPG.
I have the 3.25 ready to go. Now, if the Georgia rain will hold off this weekend, I'll be good. It sounds like I may be putting the 3.70 pumpkin back in it...after I clean it up and paint it.

The passenger axle is bent on the 1962 rearend and a leaking brake flex hose and all around the pumpkin. That's why I went with the 1971 rearend. "Rebuilt it" in the backyard to slip it right in place.
If the 3.25 doesn't work, at least, I'm prepared for it.

Side note: My wife HATES my truck. Do y'all have that problem?

Another side note: Conansky, I may hit you up sometime to see what, if anything, I should do to beef up the perfomance of that timid 302.

All you guys responding are awesome. Thanks for everyone's help. I'll definitely keep you informed about the gears.

This is just too cool.

 

2,000@70mph is right in the zone. Your engine was originally equipped with a 195 degree thermostat, If your running cooler than that you might want to try changing it and bumping the timing a couple of degrees. My '17 F150 with 3.73's runs just a tick under 2k rpm @ 70 mph. I run the same Coker tires on my '49 Tudor just for originality but you might get a slight bump in mileage with radials. Best of luck to you with your project.

 

It sounds like you already have the right combo with an overdrive and 3.70. IMO, If you must have more speed, no higher than a 3.50. Anything higher will be lug city for that engine and trans.

 

I would say the first thing to do is make sure the readings you are using to figure the MPG are dead on, just a little off will give bad feed back.
I would try and get the speedo working and dialed in and a tach so you can make sure the above are on the money.

As was pointed out you should make sure the AOD is going into OD (over drive) but based on the MPH & RPM and someone pointing out the final gear ratio of 2.47 you are most likely in OD.
The only thing I have to go by is my 81 F100 Flare Side with a 300 six / T18 / 2/75 rear gear and I am only pulling mid 14"s MPG, hit 15MPG once in 4000 miles since Nov. 2019.
At 65 MPH I am turning about 2500 RPM the 300 sweet spot is about 1800 RPM

I hear the 302 has higher RPM sweet spot than the 300 six. Someone pointed this out and said to not use OD. I think this is a good test to see what way to go with the rear gear ratio.
The other thing is to slow down the speed as this will lower the RPM. I did this with my truck and try not to go over 65 MPH even if the speed limit is 70 and most people are doing 80+ MPH.
So doing both for a tank of gas each will tell you what rear gear you may want to switch to.

The other thing about slowing down is that brick you drive trying to push all that air out of the way.
Ever try and run in a pool and how hard it was? Then when you walked it was easier it is the same when driving thru air at high speed.
I don't know if you can but if you can add a small air dam on the front down low to stop air from going under the truck will help with MPG.
If the under side was smooth so the air could slip by would help so the next best thing is to stop the air with the air dam.

So there are some things you can do to get more MPG. I have heard of guys getting close to 20 MPG with a 300 six in the 80-86 trucks
There is also 1 member who just got his 84 flare side on the road with a little built up 302 and OD stick and 3.08 rear gear getting 17 MPG and he has not even tried to "tune" for more just yet.
good luck
Dave ----