About 2 years ago or so I installed an AOD into my 71 F100 behind a very mild 351w. I also put in some 3.70 gears. I was after better mileage and performance. The performance is OK and I do love the way the thing shifts with the PA street strip valvebody. The better mileage is'nt happening. I used to be able to squeeze 17-18 out of it with the 351w 2 v 3 speed manual gear and 3.35 gears on the interstate running around 60-65. Now I just got back from a trip to the mountains and averaged 12.5 now that was running about 70-75 pushing alot of air out of the way.</DIV> <DIV>I have tuned and messed with this thing plenty and just don't think it's gonna get any better. i would be happy if I could get 15 mpg. I have access to another c-6 that probably needs rebuilding and thinking about giving the rebuilding part a try and going back to a c-6 with a 3.25. </DIV> <DIV>I know that performance and mileage are not gonna go hand in hand but should'nt a 3800 lb truck going 70 mph and turning about 2 k RPM's be able to get better? Could it be my power valve coming in all the time? If I drop my main jet sizes it does'nt run as good and gets spark knock too easy. I doubt I'll change anything for quite a while due to money but I would like to get alittle better mileage out of it just for spite now Any suggestions from the gurus?
lint

 

Are you sure the odometer is still accurate? New gears and new tranny may mean the odometer is significantly off. Just a thought.

 

Yea, I swapped out the speedo gear at the time of the tranny swap plus I've checked it against a mile marker.
thanks though
Clint

 

Try putting in higher rear end gears, and keep the AOD.

 

I have access to a 3.89 center I thought about trying. I have 3.73's in now. I'm thinking maybe with the 3.73's it's lugging the engine too much pulling in the power valve too often
Clint

 

No I said higher (numerically lower) I'm not sure why if you were going for milage you put 3.73 in there. I would have left the 3.35 in there because in 3rd gear the AOD and a C6 are the same, 1:1 and a .86:1 ratio in 4th. Not a heck of a big difference and not much of an economy advantage with those lower rear end gears. But on the other hand the gears did help with torque and acceleration. So the way I look at it is you have choices, either you got performance or economy, or some compromise in the middle. Just to let you know I have a 77 F-100 with a bone stock (complete with breaker points) '69 351W 2V, C4, and 2.75 gears and the best highway milege I ever got was about 13mpg.

 

Well for one thing right now if I am cruising at 70 mph I'm onl turning about 2 k. if I had left the 3.25's in there I would'nt turn much more than a high idle. i don't think I will get any better by going to a higher gear but thanks for you responses.
Before I did the automatic swap I had a stock 351w 2v with a 3 spped stick, 3.25 gears and I could routinely get 17.5-18 on the interstate if I kept it below 70 mph. I think it was just setup right. Now I have gotten away from the sweet spot it was set at and am trying to get back some of it.
I believe the AOD is around a .67 in OD so I just feel like with anything below a 3.73 I'll be really lugging it.
Clint

 

well one thing you could try is driving around in D instead of OD for a tank of gas see how that changes your mileage. I had a guy ask what kind of mileage he could expect by switching from an AOD to a C6 and I told him to drive around in D and see what kind of mileage he gets. I have heard that even though the RPM's will be higher, the engine is more thermally effeicient and you end up with better mileage. If you want to calculate RPS and stuff try this link, good luck

http://www.ring-pinion.com/calculato...latorsind.html

 

that's what I was getting at when I said I might try the 3.89's just did'nt know the right words for it.
I know that an engine has a sweet spot where it likes to run and I feel it might get better mileage if it's allowed to run it it's sweet spot. With my old setup it liked 65mph on the interstate, if I was'nt paying attention to my speed it would seem to end up at 65 all the time.With the AOD it is now 75 that it likes to run. I'm pushing ALOT of air at 75 mph.