Matching tire size to gearing/drivetrain?

[Tony_SS]

I'm in the market for new tires. But I dont want to mismatch anything. Right now I have 33" on a 6in lift. I was thinking 35" but I'm afraid of messing up mileage/driveability etc.

How can I tell what gears I have? Glove box code? What are the ideal combos? I have the 700-R4 trans too.

[tvblazer78]

Tony did we not talk about this on your ride to long ago..

The way I do it, is get the back tires off the grond and make one turn with the tire/wheel and see how many times the drive line tunrs and that will get you close.

[msgross]

pull the back cover off the diff, look for the ratio on the ring gear (i.e. 41:11 = 3.73 etc) doesn't hurt to change gear lube anyway.

If you have anything lower than 4.10 then you might want to swap, if you have 3.08's then your engine will really be bogged down.... Bigger tires need a lower gear (higher number)...

[Gemp]

I would bet their 3.42's....It should be on the glove box door. My Burban has them. It revs around 1500 at 55-60 with 33's.

[Jonboy]

Look in the glove box first, and see what you have. Axle codes will usually start with a "G" (GT4, GU6, etc.). From experience, 35's with 3.23's are a beating (hey, at least it looked good.....).

[Tony_SS]

I found GT4 in the glove box, so 3:73's it sounds like.

Think 35" tires are OK with them?

[lakeroadster]

Originally Posted by tvblazer78

Tony did we not talk about this on your ride to long ago..

The way I do it, is get the back tires off the grond and make one turn with the tire/wheel and see how many times the drive line tunrs and that will get you close.

To check rear end ratio jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.

[Prerunner1982]

http://www.sierrabronco.com/TireGearRatio.htm

Sounds like 3.73s might be a little high for 35"s, however the lower first gear of the 700R4 may be ok, but the OD won't like it.

[tvblazer78]

Originally Posted by lakeroadster

To check rear end ratio jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.

Yep you did a better job on giving info. Thanks, Trav..

[Tony_SS]

Originally Posted by Prerunner1982

http://www.sierrabronco.com/TireGearRatio.htm

Sounds like 3.73s might be a little high for 35"s, however the lower first gear of the 700R4 may be ok, but the OD won't like it.

You think it'll "chug" the engine pretty bad on the hwy? It's saying around 2100 rpms at 65mph with 3.73's and 35's. I already keep in in D with the windy, hilly areas around here.

I might just try it, and if its too bad, I'll pay someone big bucks to swap 4.11's front and back I guess?

[USSkoval]

I have the same current combo as you on my '88 Burb, it's far from ideal. I wouldn't want to go up to 35's if you like to use OD. If you do get 35's and you pay someone to re-gear, just go with 4.56's. Your 700r4 will love it, and your mileage may actually improve.

[Tony_SS]

Originally Posted by USSkoval

I have the same current combo as you on my '88 Burb, it's far from ideal. I wouldn't want to go up to 35's if you like to use OD. If you do get 35's and you pay someone to re-gear, just go with 4.56's. Your 700r4 will love it, and your mileage may actually improve.

Thanks for the feedback. Its either that or I stick with the 33" and go down to a 4" lift so it looks right.

[78swest]

i got 38s on my burb with 410s and a th400 i get 14mpg highway