Gear Ratio and Tire Questions!

[tx72k15]

First off, I did a search and it didn't really return anything specific that I saw. Hopefully I'm not beating a dead horse but if I am someone please kindly direct me to a thread that will help. Thanks in advance.

Now for my questions:

I have a 1972 gmc 1500 mounted on a 1982 or 1983 suburban 4x4 frame. It's already body lifted 6" and has 35" Mickey Thompsons on it with about 50% tread left so I've been pricing new tires. I'd like to go up to 37" tires but I don't want to tear up my gears. I've always heard that 35" tires are as big as it's safe to go with stock gearing. Is this true? If so, what ratio would be safe for 37s?

Also, the tires I have are 35x12.5x15. I'm not exactly sure what width the rims are but how do you know what will fit? Is there some golden rule? If a 12.5 width 35" fits should a 14.5 width 37" fit?

Thanks for everyones patience with my rookie questions,

Doyle

[Critter]

Like you mentioned 35s with stock 1/2 ton running gear is about the biggest that you should go as a rule of thumb. However I think that rule is based on at least a moderate amount of trail use. I know a lot of people that have mounted 37s and bigger on half tons but they are purely street cruisers. Even then you could have trouble especially if you have a hopped up engine creating a decent amount of torque.

A little more info would probably help us out. You mentioned an early 80s frame, is it an early 80s drivetrain also? In other words do you have a 700r4 overdrive auto transmission from the suburban or something else? Are the axles from the suburban or are they 72 GMC parts?

[tx72k15]

It has a turbo350 trans and the rest is stock suburban.

On a side note, I notice you're in Bulverde. Thats not too far from Floresville. I'm new to the area, are there any good places to go wheelin anywhere near me or you?

[Critter]

If its all stock 1/2 ton suburban drive-train that means its corporate 10 bolt axles front and back. You mentioned looking for a place to go wheelin, personally I wouldn't run 37s on those axles if you plan on "playing" in the mud and rocks. ESPECIALLY if you also drive it on the street. Last thing you want is to weaken a component in the dirt then have it break on the pavement at 60mph on the way home. A lot of people would/do run those tires on those axles and get away with it, but eventually something is going to break. Just my $0.02.

As for a place to take your rig I'm probably not the best person to ask. I take mine off the pavement from time to time but its usually to the lake, beach, private land for hunting/fishing etc. There are quite a few clubs around here though that would probably be what you are looking for. Check out San Antonio Off Road. I've met a few of the members and they seem to be a nice group.

[Critter]

Also check out this link for some gear ratio figures.

http://www.grimmjeeper.com/gears.html

I'm not even going to tell you what gears I would run because I generally gear higher than most would.

[tx72k15]

Thanks for all the help!

[tx72k15]

Okay from what I've read, stock gears and 35s make a truck a turd. Suburbans consistent with what I have came stock with 3.23s or 3.42s. My truck is no turd. It'll burn the tires up from a standstill on dry concrete. This leads me to believe that I have something beefier than stock. How do I verify this?
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[Critter]

Only sure way to tell what gears you have is to pull the rear cover and count the teeth. The only other option would require a

1) GPS unit (a handheld like a hunter/fisherman would use or one made for a vehicle like a TomTom, etc)
2) a tachometer
3) a tape measure

Get on the road in 3rd gear and use the GPS to establish a consistent speed. Make note of what your RPMs are at that speed. Then park the truck on concrete and measure from the ground to the center of the hub on the rear wheels. Use that info to work backwards to arrive at a likely gear ratio...or give me the info and I will calculate it for you. Once again that method isn't 100% accurate, but it should be close enough to determine a ratio beings there were only so many possibilities.