Rear Axle Gear Ratios

I have been searching for the past several hours on the internet to find the differential gear ratios for our trucks, but to know avail. I used both service manuals I have, but no luck there either. The reason for all this started last summer when I was asked where someone could find a T-34 transmission to get overdrive, and/or better fuel economy. What I realized later,was a direct drive 5th gear style (T-35 or similar) might work if the gear ratio was changed. If, for example you could go from a 4.09 reduction to a 3.00, the reduction will still allow a "granny gear" with the 5 speed but the change in rear axle gear change will help drop the engine RPM when in top gear. This might be helpful for a 4 speed transmission, as well. But will it help enough to justify the cost of doing this? Will any problems be developed? Such as the drivability of the truck or anything else, for that matter. If I knew what ratios were available, that might be possible to think through, and that is exactly what I can't seem to find. Any one have any good ideas? Thanks, Bill

When the ratio at the rear is less (faster speed , 3.23 etc) the overall reduction in the trans must be increased to get the same starting ability. HOWEVER this increase the torque on the driveshaft and U joints. One of the reasons O/D were used "back in the day" was to put less torque thru the driveline.
The is a friction penelty for gear up at the trans, then gearing back down at the rear. With todays push for fuel economy,many heavy trucks now use "direct gearing" in the trans with very high rears (lhig 2's to low 3's) but this required much heavyer driveshafts and U joints which we not avaiable back in theday,and engines with gobs of low end torque.
As with everything it is a trade off. The transmission must be able to stand full engine torque in the lowest gear without spitting gear out the side.

Why don't you just use the MT-118 parts manual at Irish Mikes??? If you have the axle code,you can find the ratio by looking at the ring & pinion sets.

Just remember,if it's 4wd both axles must have the same ratio.

The T-34,35 & 36 can handle the torque,it was used with the RD engines and D354 Perkins diesel in addition to the SV8's.

Scottso, I have a T-34 bolted to a V-345 installed in a truck now, you're quit right. It still needs a new driveshaft between the transmission and transfer case. The T-34 is longer than the original transmission. Plus, a special bracket for the transfer case I haven't made as of yet. Additionally, the u-joints are not the same size. Close, but the T-34 is a little bigger. That truck will need the wiring replaced before I drive it, but I'm getting a little done every now and then. Since I was asked where to find a T-34, and have only seen T-35 style gearboxes, I wanted to see if there was another way to do the same basic thing. Proved to be more of a headache than I thought it might be. I have found the references you mentioned and noticed there are many ratios possible, depending on the size truck you started with. I started with a 908, up to a 1300 series. The 908 started with a 3.31 to 1 ratio to a C-130 that ended with a 6.16 to 1. These seem to be a deeper reduction than more modern ratios I've heard of. I still have to find both the circumference of the possible tires, plus several engine horsepower and torque curves, if I can. Right now, it seems just as likely to get a good match as one that wouldn't work out. Also, after I posted this, I read an article from GM stating that changing gear ratios won't help that much, just how much work the truck might do. I would like to find some simple formula to help, but that seems elusive right now. Wouldn't happen to have one, would you? Bill

Bill, this may help, may not, also, do you have danas axels in your truck? Check it out,
http://www.ringpinion.com/calculators/


I have a 240BDT---32 inch tire--3.55 gear in a dana 60, I can pull another gear, not looking for millage, but instead of, 2700 RPMs, at 70 or so MPH, I would prefer 2200 or 2300 for my application. This is an example only.
kevin in MN

Kevin, thanks, it does help. One thing I happened to remember was that, at least 20-30 years ago, IH dealers had a small adapter that went between the speedo cable and the trans. output. Also, they had a chart to tell you which adapter to use. I wonder if that is still available? That to, could help these old trucks.
Bill

Bill, Every IH parts manual has the charts for the ratio adapters and driven gears (the one in the trans.) They are in Group 10 (later books),Speedometer & miscellaneous.
With the transmission code (or transfer case) find the corresponding chart from the index. Using the rear axle ratio & tire size locate the line that matches. The line will list digits followed by a letter,the digits are the ratio adapter,letter the driven gear. Next step is to the "reference index" ,the left column is the adapter pn's,right driven gears. Using the digits and/or letter (key) locate it and the part number is to the right.

Here's a link for Group 10 in the MT-118 ~ 900 to 1500A,B & C models.
https://app.box.com/shared/otkihx6yoa/1 ... 03087934/1

Here's an example...
Truck has a T-15 / code 13015 ,3.73 rear ratio & 15x7 tires.
Index says go to Fig. 10-4, I find the tire size in right column and ratio in top. Gives me 17F.
Going back to Fig. 10-1,Fig.10-1A gives the adapter pn for "17" = 63362H. Fig.10-1B gives the driven gear for "F" = 63287H which it shows has 14 teeth.

Simple really,just take your time.

Scottso, thanks. You are, as usual, a big help. Bill