4.10s, 35" tires, 1:1 final drive ratio is similar to 'stock' performance, depending on how your truck is equipped. Personally, I'd rather have a truck that is at it's peak efficiency @ 55-60 mph with room in the power band to pass, especially climbing hills. I don't know what the obsession with speed is, but if it's to keep your engine free of carbon, you might as well set it up to run on the pavement @3000+ rpms at speed. Otherwise, conserve your fuel and let all the idiots pass on the left and make your rig 'green'. I can forsee speed limit changes back to the days of the oil crisis in the 70s and 80s again if things don't start looking up

 

The 3.50 gears I have currently do a great job on the highway with the 36.5 tires on it with top speed in excessive of 100mph, but I'm converting my bronco to an all mud truck.

It is a 1986 4X4 Full size Bronco with a 4.9L in it. It has a 9inch rear with a mini spool, and a IFS Dana 44 front end with a welded locked front end that is a 3.51 ratio. The tranny is a Bog Warner 4-speed manual granny tranny (not sure on the ratio). It also has a New Process transfer case (not sure on the ratio eirther).

The combination I'm looking for is for all mud and climbing, MPG and highway speed is no concern for me. The first gear in the tranny tops out at 10-15mph (or when I usually shift between 2,000-3,000 rpms), and 2nd gear tops out at 25-30.

I'm looking into putting in an automatic transmission, over a short throw shifter because every time I go through some thick mud, or a long steep hill the time it takes me to shift from 1st to 2nd kills my momentum/power band that I'm in.

So I guess what my question is what transmission set up should I go for, and would gear ratio play a huge role in this decision?

Thanks,

Ryan Whipple

 

Well, that extra bit of information answers everything!

First off, over 100mph! Woohoo! Especially with 36" tires. You at least gotta be glad knowing your 4.9 is running in pretty good standing. I will admit, with my 3.00, 30" tires, and a speed limit of 75, I have a habit of looking down and realize I'm doing 90+. I love it.

However, if you're converting to a mud truck, don't care about gas mileage, and don't care about highway, the answer's pretty easy. You're going to want 5s or more to get a lot of torque. You could also go with the high 4s if you want your tires to spin a little faster (I hear that's good for mud too).

If you want an automatic, the C6 is the way to go. You won't really need an overdrive, so I wouldn't mess with the AOD or the E40D. The C6 is known to be almost as bulletproof as the 300, so it's a good combination. It'll give you a 2.46 1st, 1.46 2nd, and a 1.00 1st. Keep in mind, however, that the downside to most automatics is that they have higher first gears than any manual you can put in there. The stock manual has a 3.29 1st with a 6.69 granny gear. Serious crawling right there.

Your transfer case is probably the NP-208, which, when in low range, divides all of your speeds by 2.61 or so. (If you're looking for a stronger one, go with an NP-205, it doesn't have as high of crawl ratio, but is gear to gear instead of chain driven.)

Have fun.

 

I may not have any power left, but I never have to come out of OD to climb a hill - and I don't lose speed. Actually, my little old truck leaves all the other vehicles in the dust when it comes to hills. They fall behind while I'm just tickin' on at 60 and 65.

I should have mentioned this when I said I didn't have any highway power left.

 

Ryan, if you're using "L" or "1st" in mud and not on rocks.... which all the granny 4-speeds have a 6+:1 ratio in "L" or "1st" (some variants use a ~4+:1 ratio)... then I don't know what to tell you. I personally would never use an automatic for mud or off-pavement use (mainly due to the 'short' 1st gear ratio, unless I had 4.88+:1 gears in the diffs). I could probably find a C6 auto for you to trade for your granny 4-speed. You should be using "1st" (2nd )and "2nd"(3rd) for the mud, and maybe "L"(1st) for the really steep climbs. I used to drive a '74 F-250 352cid FE-block truck (my grandfather's) and could speed shift it on and off pavement all day long, without a short-throw shifter... heck, I don't know of anyone who makes one for the T-18/19, NP435, or SM456 (GM/Muncie) transmissions. As far as gear ratios, I would go with 4.10s or 4.56s, given the powerband of the 300 I-6, and even that is a stretch and would lean more towards 4.10s.

Bill, I kinda figured that was your situation - all depends on what the truck has as far as gears in the transmission, differential(s), and tires and where your rpm's are at while at speed in relation to the high end of the power/torque curve.