Advice install 3.07s in 93 i/c auto for max fuel mileage?

[M37]

I wasn't sure which area to post, but anyway.... I have a 93 W250 with auto/OD and 3.54s. Just bought it and it is a sad truck. One front axle u-joint shot, the prev owner changed all the brake lines and then broke all the bleeders. I also have a 89 W300 or 350 (single rear wheels) with the 3.07s axles and I am planning on using everyting but the axles in a British truck conversion. MY QUESTION IS, WOULD THE 3.07 AXLES WORK IN THE 93 WITH THE OD FOR BETTER FUEL MILEAGE? The truck is bone stock otherwise.

[oldestof11]

Yep. Speedo wil be off tho.

[M37]

i was hoping that there would be a speedo drive ratio gearbox on the t-case that would correct that part

[burnt_servo]

i was thinking of the same exact thing with my truck ......

if it had a manual transmision , without a doubt yes ......

with a nonlockup auto transmision , especially with a stock torque converter , you may end up working the torque converter really hard and end up getting worse millage ( besides overheating the transmision ) .

you need to figure out the hieght of the tires your going to run , work in your axle ratio and the transmision in overdrive to figure out what rpm the engine will be turning at 60 mph .

that number needs to be higher than the stall rating of the torque converter for things to work out .

[Philip]

The speedo correction isn't in the reducer box. Its in the speedo drive gear in the 205 T case.

I have had my reducer box and VSS on a 3:54 stick shift truck. It reads the same MPH between both trucks. This was verified with another vehicle with a known accurate speedo.

[GLHS]

3.54 x .69 = 2.44 overall ratio which equates to 1590 rpm @ 60 mph using a 31" tall tire.

3.07 x .69 = 2.12 overall ratio which equals 1380 rpm @ 60.

A better converter is what you need FIRST!
When I had the old 93 with auto it made a huge difference.

[M37]

The truck isn't worth the cost of a new torque convertor. In this area the average speed on the highway is 75, I would like to cruise at 70.
What is the stall speed of the factory convertor?
I intend to keep the truck stock even the tire size (31")to try and squeez all the life possible out of it (at least until I find a nice affordable replacement).
It will be used pretty much just for transportation and not towing or heavy hauling.
Along with trying to maximize fuel mileage, the 3.07 axles are in great shape, the 3.54 need work.

[Mark Nixon]

IMO putting 3.07s behind an overdrive automatic is counter productive, because of both windload and with so tall of final gearing, the engine is working enough that fuel mileage will actually drop off.
Not to mention if you try to pull a trailer, or a heavy load, chances are you'll burn up the clutch packs in the transmission.

A deeper gear tends to be less stressful on autos, which is really why you'll never find an OVERDRIVE application factory installed with taller than 3.54 gears in a Cummins equipped Dodge.

A 5 or 6 speed is much more forgiving, and provides better selectivity, espcially if it starts to load down on a hill, or into the wind.

Mark.

[Richie O]

Don't forget guys that 1st, 2nd and 3rd gear in a 518 is the same ratio as the 727. The 727 and 3.07 combo was used for 2.5 years and lots of those trucks towed. If you were to tow just leave overdrive off. Mark is right on lower gears are easier on the tranny, but if you can keep it cool it will last. That being said I would rather have a 518 with 3.54's then a 727 with 3.07's.

[GLHS]

I'll say it again.
You want better mileage; get a better converter!

[Remps]

Stock stall speed is 1775 rpm for the 727. From a dead stop I can hit 2800 in under 2 seconds,and it BARELY drops on the upshifts.Climbing a steep hill I gotta add 3 or 4 hundred rpm to maintain speed. (Like from 2200 to 2600 to maintain 65 mph.)Now imagine the same converter tryin to maintain 75 mph at a way lower rpm.Not likely.Unless you are gonna drive 90 mph all the time, change the converter. Or I'm pretty sure you will be wasting your time if you plan to actually use overdrive.

[lazyjoe]

You want better mileage; get a better converter!

X2

[M37]

Thanks

[1STGENFARMBOY]

Like Mark said some times a lower gear makes better MPG,s, i know it sounds wrong but i had this happen to me, my 95 cab chassy 5spd 4:10 ratio got 3 or 4 MPG better than my 97 5spd with 3:55, go figgure. I know were comparing apples to oranges with auto,s and 5spd,s but you get my point.

[M37]

I have a 84 Mack R model with a 300. The nice part is Mack tells you in the owners manual what RPM gives peak fuel economy, with that engine it is 1550 RPMs. If I keep it around there, I can squeeze 10-11 mpg out of a dump truck (single rear axle). So I am just thinking about all the low end Cummins torque and how to utilize it best. Will have to stop at the Cummins dealer and see what specs they have.