Burb questions..

[manualburb]

So, I just bought a 1990 Suburban. This is my second one. First was a 1/2 ton auto (700r4) with tailgate style rear door. I liked the 1/2 ton but really wanted a manual transmission. So, long story short I found the 3/4 ton and it had the much better barn doors and a granny four speed. What I know so far is that it has an 8 lug 10 bolt front end and a 14 bolt rear 350 tbi. I guess what I'm asking is, what transmission is in my truck, (it is factory, the only one I have seen however) is there a tag on the differentials that will tell me gear ratios, and how do I tell the difference between a NP203 and NP205 transfer case? Thank you for the help.

[zieg85]

SM465 is the transmission.  They were available but are very rare I am finding out.

[bake74]

     First off, welcome to the site.
     Transmission is a SM465 most likely, in production until 1991.
     There should be a tag on one of the top bolts on the rear pumkin (if still there) that will tell you gear ratio.  If not, the only true way to tell is open it up and count the ring and pinion gears and do the division to get your gear ratio.
     Transfer case guide to tell you the differences.  (just in case you do not know this, green words are a link to open another page)
     NP203 transfer case

     NP205 transfer case.  Notice the circle with 3 bolts in the middle, easy way to tell a NP205.

     NP208 transfer case.

     This is the typical shifter for a NP203.

     This is the typical shifter on a NP205.

     This is the typical shifter for NP208.  Notice you have to pull the shifter to the right and down to enter into 4WL.

[rich weyand]

If you have a tach, you can calculate the gear ratio pretty easily if you know one speed/rpm combination in high gear.  If you don't have a tach, connect a timing light that has a tach readout to a pigtail on a cigar lighter plug, and run the spark plug lead out the vent window and under the hood.  You can clip it to any cylinder for tach; #7 is easiest.

Let's say in high gear you get 2500 rpm at 60 mph running tires with a 30.7" diameter.  (like BFG All Terrain T/As in 31x10.5R15)  If the trans was connected directly to the rear wheels, you should be going:

((2500 rpm)*pi*(30.7 inches)*(60min/hr))/((12 inches/foot)(5280 feet/mile) = 228 mph

But you are only going 60, so the axle is 228/60 = 3.8.  Since your readings were approximate, you can figure you have a 3.73:1 rear axle.

Here it is simpler:

Axle ratio = 0.003 * RPM * tire rolling diameter in inches/speed in mph

That gets you close enough to figure which factory ratio you have.

[manualburb]

Thanks for the info, I can tell just buy the shift pattern on the T case that it is a NP208 and if I remember right those are the chain driven ones. I will crawl under it when the weather dry's up (I live in Oregon) LOL, and check for tags on the differentials. Once again thank you, I'm sure I will have many more questions as my ultimate camping/hunting project progresses.