Obviously I have a 78 bronco with an 89 6bt with the 727 trans...I have 40 inch tires with 4.56 gears in my dana 60s. Now,,This was fine for the gas engine that was in the truck,,But now with the diesel my highway speed is limited. I have no problem doing 65-70 but Im on the throttle a good amount with the RPMs higher than I would like to be. The other downside to this is the trans runs up to 3rd quicker than it normally would...Im debating on changing my gears to 4.10s or maybe back to the stock 3.53s or maybe 3.73s . Im not used to dealing with diesels and the gearing..If it was a gas engine Id have a good answer. I dont want to kill the torque that I have to move this truck but at the same time need to lower my RPMs on the highway. So I figured to ask for peoples input that might have a similer setup like mine..ie. bigger tires,auto trans,..I have an idea of going to 4.10s but I dont want to do this and have minimal effects and end up having to go higher. Thanks Mike

 

Limited RPM range always hurts any compromise in gearing, one way or the other. You have to give up on one end whatever you gain on the other. Best solution for a street-driven Cummins 4x4 is to add gear in the trans area with a Gear Vendor OD or something similar.

 

Food for thought: Your engine/trans combo would originally have come with a 31" tire and a 3.08 gear. 2200rpm would net about 66mph. With your 40" tires and a 3.73 gears, 2200 rpm would net about 70 mph. I think that's probably the gear that would be off the shelf that would get you as close to the trucks original final drive ratio. I am also a big proponent of overdrive as a solution, though it is a much more expensive route. I think if you want the best of both worlds, being good low gear ratio and good highway performance, overdrive is the best bet. If you are looking for a cheaper solution, lowering the gear ratio will be best, although you will loose considerable low end grunt. The following link contains some calculators that will help see what gear ratio's will provide what kind of driveability. Calulators from www.4Lo.com

 

Overdrive is out of the question...One reason I went with the 727 is that I put a jeep 727 tailshaft and housing on the diesel 727 to shortin it up due to it being a short vehicle as it is. At this point its either a set of 4.10s or 3.73s. I just got to decide how much of my low end do I want to give up.

 

An alternative would be to put the 2.7? low 1st gear set in the 727, then run a taller final ratio, like 3.54s.

Mark.

 

Im not messing with the gears in the trans. Im kinda going towards the 4.10s. Id like to just ask the simple question of has anybody with a 6bt put a set of 40s on or close to it and what ratio are they running. I guess maybe not too many people are running bigger tires on the dodge trucks? I thought maybe someone has and would have an answer. I guess we will see. And since I dont have a tach anymore ,its hard to figure out what RPMs im running at..Im thinking about looking into one of those tach kits for these engines..

 

4.10s with 34" tires was too deep for highway driving in my 1st Gen. That's with a G360 5spd tran w/OD 5th. That's also why it has a Gear Vendors OD installed. Just based on your screen name, wanting 40's and with the 727, I'd go with something much deeper, forget about driving it on the road, and be ready to start fixing and beefing up stuff after it breaks.

 

Da Beast has 4.11s behind a NVG4500. Even with the o/d he's been considering 3.73s I don't recall how tall his tires are. Seem to be in the 35" range, though.

 

You might be surprised how much difference 5 more inches of tire makes.

 

Im not too bad on my trucks,even when wheeling. Anything can break though. Ive owned this truck 7 years now. The previous owner which im friends with had the stock 3.54s turning 44" Ground hawgs. Sure I could do a hundred down the highway but the bottom end was gone. I have dana 60s front and back and havent had an issue with them unlike the dana 44 I had in the front,I kept replacing bearings every few months,It just couldnt handle the bigger tires.And up until a year ago,This was my daily driver. I drive it on the street more than I go wheeling anymore.Its good on the street. Rausch Creek is a 2 hour drive from me. With the gas engine it was no problem cruising that long,But the diesel doesnt need all that gearing apparently. I dont want to do a hundred,Just have a nice 65-75mph cruise with out reving it out. I just dont want to go with 4.10s and still be too much for it. But like you said,,,5 inches of tire can make a big difference...