Hi guys, I just purchased a 91 and have already modded it to help towing. I am hoping you can help me with some general questions.

1. I have a 3 spd auto. What do I do to get the best mileage. Change Transmissions, change gears in the current transmission, and/or change the gears in both the diff and transfercase. I am pretty maxed for speed at 70. Can I do both, what do you suggest? I use this truck to tow my race car 3,400 trailer and car plus camper weight if I put it on, and for hunting in the winter. Mileage is of primary importance. I really do not want to but the aftermarket overdrive because it is so expensive. So I am looking for your help.

2. I have also upgraded the turbo and the housing has a 2 1/2 in diameter outlet and my crossover pipe (no intercooler) is only 2 in. It seems to me that no matter what turbo you do it will be restricted by the 2 in crossover pipe. So does anybody make a larger pipe? Or do I just put a different connector to go from the 2 1/2 outlet to the 2 in crossover pipe?

Thanks for your help.

Spank

 

Forgot to say that I want to stay auto. It is really nice not having to shift.

Spank

 

Welcome!

About your only options for mileage are: 1. Slow down (not happenin', I know ) 2. Trans swap, either to a later automatic or to a manual. Depending on how lucky you get and how handy you are this is the 2nd cheapest way to go. 3. Aftermarket overdrive. 4. Lift and big tires.

The truck already has 3.07 gears, which are the tallest made for the Dana 70.

If you want to go faster, the 3200 RPM governor spring will let you go over 90 (with lousy mileage).

There is very little aftermarket support for our trucks since they are so old. Guys who have upgraded their turbos either home-built a new crossover, adapted to the old crossover, or put an intercooler on the truck and did away with the crossover pipe altogether.

 

Best way to be able to make use of a turbo upgrade is to add an inner-cooler, which will eliminate the cross over. You can also put in the later radiator, but you'll need to trim the core support and shroud, unless you can track down donor parts from a later truck. Your gonna have 3.07 gears in the rear. You won't be able to improve mileage from these, BUT when they go they're gone. You can't re-gear that case, it's specific to 3.07. You can't get replacement gears or carrier either. How many miles on your truck? Might as well change the oil back there, and make sure the little lock screw and guide pin for the spiders look straight and tight.

 

Guys I cannot believe I have such low gears. The truck simply is not very fast. My friends cruise at 70 in a gasser and I simply cannot. Are you sure I don't have 4.10?

Also if I change to a different auto, any suggestions?

Again this truck simply does not go very fast at all. My 7.3 ltr ford non turbo had far more top end than this. I need to lower the RPM's so I can get better mileage. I only cruise at 65 so I just cannot believe that with 3.07 is what I have. I will look on the pumpkin tonight to see if it says. Again it has NO top end at all. So anyway my goal is to cruise at 65-70 and get 18 mpg or more with just my racecar. Total weight is 3400 that is car and trailer.

Thanks

Spank

 

if its still there there is a tag under the hood that tells what gears you have,,it will prolly say 3.0 not 3.07 , theres also a tag on the rear end that identifies the gearing,. mines a 89 dually with the same auto and 3.07 gears, 76 mph is hung out for me, so yours is in the same range. gear venders makes an o/d unit but its high,, theres a company in dallas that will custom build you an o/d with a lock up for 2500.00 with the converter. and they claim it will hold up to the cummins and they back it up with a 2 year warranty.

 

check you linkage to make sure you are getting full throttle and ajust it if needed.I have 3.07 and had a leg cramp from trying to do 70 + with every thing ajusted right 80 was my max speed .I have since put the 3200 GSK in and have a hard time keeping it under 70. the rad mod is a cheep mod if your handy and can fab your own parts and can find a fan shroud from a late 70s big block dodge truck. and as far as a 7.3 having a high top end it also revs 1000 rpm higher then the cummins. I get 17 mpg at 70mph any faster and it drops just as fast these trucks have the airodinamics of a barn. the 3200 gsk ($20) is the best mod for the $. you dont have 4.10s if your doing 70. with 4.10 x 2500rpm = 58mph you have a slow turning industrial motor in there liven it up for $20.

 

Thanks rebal, I'm glad someone did some math to show the 3sp auto guys that they don't have 4.10's. Low reving Cummins, 1:1 direct drive top gear trans, times 4.10= would equal SLOWWWWW. Dodge used the 3.07's with the 3sp for some highway speed. Without the spring, you should be able to do 85mph wide open. You need an overdrive transmission.

 

I'm sure they also used the gearing for emissions, so you probably couldn't special order anything else. Like Rebel said, adjust the throttle linkage till you get out the slop and have full travel. Make sure the linkage meets the stoppers both ways though, so the throttle doesn't get slammed. Also, get all the slop out at the pedal, either bend the pedal stem up to take up the slop, or put in a decent (safe) spacer. Then, you should be able to hit 80-82 on a level freeway with the pedal on the floor.

 

My truck hits 76 mph holding it wide open. Where can I pick up one of the 3200 rpm springs?

 

I got mine from PDR but any pump shop should be able to get it or have it.

jiMMy