I just got another K5 which I'm gonna put 37's on. It's got a 4spd auto with overdrive and 3.42's. I can't decide between 4.88's or 5.13's. It will see mostly local driving with very little highway and no offroad. (the body is mint and no rust which is hard to find in the northeast so I want this one to last). Which gears would be better? I'm thinking 5.13's would be the way to go.
CK5
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gears for 37 inch tire
- Thread starter Jeff Carr
- Start date
No problem
, check out the gear calculator >>> http://pitbulltires.com/calculate.php plug in your possible choices to get an idea of rpms at different speeds.I would start with the "find new gear ratio with new tire size" chart to get a base line idea.
Bama72
Registered Member
Most small blocks like to run between 1800 to 2800 rpms. With 4.88 and 37 at 60 mph your rpm should be 1860. With 5.13 and 37 at 60 mph your rpm should be 1957. Since you don't plan to offroad I would look at what your average highway/cruise speed will be and use the already listed calculator or the one on ringpinion.com to find the rpm you would like to run.
Personally I would use 5.13 just to reduce stress on the automatic transmission.
Personally I would use 5.13 just to reduce stress on the automatic transmission.
OK , so that would "in theory" keep you where you are at now, not accounting for the additional weight and wind resistance. In reality 3.42's are too low for a stock rig, 3.73's or 4.10's are much better. That being said, if you do the calc. using 4.10's as your factory gear with 31's the new ratio comes out to 4.88's.
I had an 85 K5 with a healthy 355 roller motor and a built 700 trans, I ran 4.56's in it with 35" tires, I was really happy with it wether on the highway,around town or some medium wheeling. 4.56's with 35's - go to a 37 and you get a 4.82 on the calculator. If you want some more "snap" off the line go with the 13's , it really comes down to how YOU want to use the truck
blowedupmotor87
1/2 ton status
Unless you plan on running bigger tires, I'd go with the 4.88 gear or maybe a 4.56. On my Jimmy my stock gearing was 3.73 (IIRC), when I went to 37's I went to 4.56 gears which actually kept my highway rpm almost the same as stock (% increase in tire size vs % increase in gearing virtually identical). Thus, my speedo was pretty much right on still and my engine was still happily in its power band. One nice thing is 4.56 gears are standard in CUCV 1&1/4 ton axles so, can kill three birds with one stone! (beefy axles, better gearing, and a rear locker!) Just a thought! 
If you are running stock 10 bolts I would steer clear of the 5.13's as the pinion is going to be really small on that gear set!
Since you are going to be mostly using the truck on road, there's really little benefit to switching to drastically lower gears, just go low enough to keep your engine in a happy rpm zone. If ya go way low on gears, you'll just end up sucking down more fuel and performance may actually suffer since you may put the engine having to above its optimal rev range per given mph.
Sounds like you've got a nice K5! What's the full specs? Pics?
If you are running stock 10 bolts I would steer clear of the 5.13's as the pinion is going to be really small on that gear set!
Since you are going to be mostly using the truck on road, there's really little benefit to switching to drastically lower gears, just go low enough to keep your engine in a happy rpm zone. If ya go way low on gears, you'll just end up sucking down more fuel and performance may actually suffer since you may put the engine having to above its optimal rev range per given mph.
Sounds like you've got a nice K5! What's the full specs? Pics?
Adventurous 4x4
1/2 ton status
4.88's would be as tall as I would go with what you described. What axles are you planning to run? If it is street only then the 10 bolt axles should be fine.
Another one of those myths that everyone believes but there's really no proof to back it up.
Yes, I certainly agree that the really low gears could, in theory, make a weaker ring and pinion setup. My own experience and what I've seen makes me believe that the ring and pinion generally won't be the weak point on that axle regardless.
Stomis
Professional Amateur
Yeah the ****ty carriers are
I would go 5.13s with an OD trans. What difference is 150 RPMS really gonna make to you for the exchange of low end you'll get.
IMO when talking about an OD auto trans you should always go for the steeper gears whether its a built 454 or a stock 305 the 700r4 doesnt care whats driving it when it starts hunting OD and 3rd on an upgrade.
blazer88
Registered Member
I've got 4.88's
I've got an 84 blazer with a four inch lift and a three inch body lift. I bought it that way and it has 4.88's. I drive it daily on the freeway(hwy 99 in Fresno) and we are full of idiots, and I have plenty of power and room to pass all of them on the road. Driving on the city streets it is just fine. I'm not the fastest rig on the road but big enough that people get out of my way and give me my space. I don't have a tach installed so I cannot tell you my rpms. But if you got any questions fire back. I am also running the 700r4 transmission and stock axles.
I've got an 84 blazer with a four inch lift and a three inch body lift. I bought it that way and it has 4.88's. I drive it daily on the freeway(hwy 99 in Fresno) and we are full of idiots, and I have plenty of power and room to pass all of them on the road. Driving on the city streets it is just fine. I'm not the fastest rig on the road but big enough that people get out of my way and give me my space. I don't have a tach installed so I cannot tell you my rpms. But if you got any questions fire back. I am also running the 700r4 transmission and stock axles.
I have 35's and just did 5.13 and absolutely the best choice. I do some towing and these gears helped a great deal. I also change to a 14 bolt axle for the larger ring and pinion. The 4.56's I previously had grenaded in the 10 bolt and were just crap for towing. Without the boat in tow the blazer is snappy with 5.13 and feels like a much newer car at least as far as getting of the line goes.
Stomis
Professional Amateur
Philly87
1/2 ton status
Do it once... 5:13's!
blazer88
Registered Member
37x12.5'sx15 guess i forgot to mention that
Stomis
Professional Amateur
5.13...nuff said
chulisohombre
1/2 ton status
ive been told that anything above 4.88 on half ton stuff makes the pinion small and weak.from stock tires to 37 and 3.42 to 4.88 would be a good change also.you would still be in a good gearing for overdrive and still have some better strength in the pinion shaft.but if you already chose,good luck.
