gearing.....

Well I'm buying a 250 I6 to build up as a fun project, to eventually replace the 200 I6/C4 that I have now.
I have available to me a AOD so with the torque of the 250 I think it will be fun to build and a high MPG road cruiser...I like long road trips, and right now at 75 mph I wish I could shift down.
I have 2.79 rear right now with tall tires so I have to do some figgerin'.....

 

Mavman72, Joe Dirt...
What rear end ratios do you have to get that 1700 rpm and 2000 rpm at those highway speeds and what size are your rear tires?
Thanks,
-M

 

I currently have 2:79s' in the diff.With the aod and a 26" tall tire The engines turning the affore mentioned 1700 RPM.

 

I used to have the AOD behind the 351w in my F100 before I went 460/c6. I never could get any MPG with either a 3.25 or 3.70 gear. I got the best with the 3.70 but really felt a 3.89 or higher would have given me the best mpg.
I got better with the 3.70 than the 3.25 though. That was with 235/70/15 tires.
I could squeeze 15 mpg with the 3.70 but it dropped to 11-12 mpg with the 3.25's
I get a sold 15-16 with the 460/c6 and a 3.25 OR 3.0 gears now.
I am saying this because I feel like the AID needs a higher gear in the back to really get the best MPG with a carbed v8.
My reason for this is that if the engine is lugging most of the time the power valve in the carb is allowed to open thus pouring more fuel into the sytem and making the tune too rich.
What do ya'll think?
clint

 

The power valve opens when manifold vacuum is low, you'd have to lug the engine with a nearly wide open throttle to open the power valve. I hope no one here does that, the main and rod bearings don't appreciate it.

 

Mine is different I guess. When I was tuning my carb I ran around all over the place, up and down hills, foot on the floor and just cruising with a vacuum gauge ran into the cab. At idle I would get around 18". Cruising on flat ground about 50 mph It would drop to about 15"
Start up a long grade and give just enough gas to maintain my speed and it would steadily drop to just about 5". The power valve would come in around 6.5 and if I was really paying attention I could feel it open up.
clitn