Im looking for info on what a stock 89 dodge 6bt stock gear ratio is, with a TF727 4x4. And also what the engine RPMs are at, at 35mph and 65mph. And what stock size tire these trucks came with. Im trying to figure something out and need that info to do it..If there is a chart somewhere with the rpms and speed that would work. Thanks ahead ..Mike

 

First gear: 2.45:1, second gear: 1.45:1, third gear: 1:1. Factory tire: 235/85/R16.

3.07 rear end would be 1139rpm @ constant 35mph in 3rd gear and 2115rpm @ constant 65mph in 3rd gear.

3.55 rear end would be 1317rpm @ constant 35mph in 3rd gear and 2446rpm @ constant 65mph in 3rd gear.

4.10 rear end would be 1521rpm @ constant 35mph in 3rd gear and 2825rpm @ constant 65mph in 3rd gear.

 

You most likely have 3.07s
and maybe this will help.
Calculators

 

If you want to be a man and figure it out on your own, multiply axle ratio by vehicle speed by transmission ratio by 336.13 and divide the total by tire diameter in inches.
(axle ratio x speed x trans. ratio x 336.13) ÷ tire size

So, (3.07 x 35mph x 1 x 336.13) ÷ 31.7 = 1139.3 rpm

 

stock tire size on my 1990 is 235/85/16

 

Thanks, Johnny-on-the-spot.

 

Thanks... I had another thread up basically asking If anyone had a 89 or so dodge diesel running 40 inch tires. And if so what ratio they are running. That went nowhere to help me...I have 4.56s in my truck from the gas engine, which is too low for the cummins. Im trying to figure out what ratio I should be running to put this engine RPMs back to stock. Im sucking down a "S" load of fuel because of me running at higher RPMs around 35 mph or more. And since I dont have a tach, I dont know what Im running at that speed. When I move out of this crappy state hopefully soon. Im planning to go back up on 44s. Those calculators dont help me much due to the fact I dont know what RPMs Im at on those speeds. The Beards breakdown of rpms,ratio,and speed help, along with Fluffy nuts tire size helps. Im just trying to get the RPMs close to what a stock cummins is supposed to run at any given speed. I cant take this on the highway for anything long, cause I dont want to rev it out and suck down fuel. I still want to be able to drive on the highways if I need too. I want to change ratios once, Not keep swapping them out to find what works the best. Thats why I found it hard to believe, in my other thread that NOBODY ever is running 40s or 44s on an 89 or so diesel. And had to change ratios to match.

 

With 4.56s and 40" tires, you're turning a God-awful 2500rpm @ 65mph. By switching to 4.10s you could bring it down to 2240rpm. Keeping your current gears and adding overdrive would net you 1718rpm.

4.10s and overdrive would bring you down to a very reasonable 1545rpm (if highway driving on 40 inch tires can be reasonable).

BTW, you're turning 1350rpm @ 35mph with your current setup.

 

Overdrive is not an option due to the short wheelbase of my bronco. I dont have the length to run a longer trans. I already took a jeep shorter shaft and tail housing and installed it on the 727. The setup is done and perfect for me.. Changing gears is my only option. As for high way use,believe it or not. I had no problem taking a 2 hr drive all highway to Rausch Creek Off road park with the gas engine and 40s. It was a smooth easy drive. I was thinking running 3.73s since I dont have overdrive. BUT If I go back up to 44s, Should I just go to 4.10s or stick with the idea of the 3.73s. It sucks cause, just local driving at 40 mph eats fuel and revs up. I like how quick off the line I am turning those 40s,But I need to get some of my higher speed back. I need to get the RPMs down, Which should bring my MPG up since I wont be running that high.

 

Smaller tires or taller gears are your options. 3.73s on a 40 inch tire will have you turning around 1250rpm @ 40mph. You just have to decide what you can live with. I personally couldn't justify anything bigger than 35s for a DD unless I lived in central or South Florida or the gulf coast. Maybe 37" Goodyear military tires, but only because they're so damned cheap.