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What crawl ratios would everyone "like" to have. I'm running a C-5 auto. Can I get too low? If I use stock info on my BII I get a crawl ratio of 22.755 with 3.73 gears. If you say that is with 27" tall tires and figure for new gears with taller tires I get 4.56's with 33" tall tires and a Crawl ratio of 27.819 and with 36" tall tires and 5.13 gears a Crawl ratio of 31.297.

Reason I ask is I'm going to put in a doubler t-case. And I have a set of Explorer axles under it now with 3.55's in them. That would put my Crawl ratio at 53.711. I have a full width 9" with 4.11's also and that would give me a Crawl ratio of 62.184.

I did some checking on some heep rubi info and with the stock 31's it has a Crawl ratio of 46.68. Corrected for 33's it's 49.756 and 36's it's 54.187.


I ran the 3.55's with 31's in another BII with a 302 and it seemed about right for the highway and did well off road. On this BII I will have 33's to wheel and maybe 36's(if I can get them from myson). I also have a set of stock wheels and tires. The BII will be driven around town. And will make a trip or two to Utah and Colorado.

So should I go with the 3.55 or 4.11's? 3.55's with stock tires will put me close to 4.11's with the 33's for in town and highway. Is a Crawl ratio of 62 with an auto tranny too low for 33's?

Thanks for looking.

I have a 302, NP435, Dana 20, 4.56, and 36's. At 75:1, I'm happy. It's perfect for 99% of stuff, however, it's not quite low enough for some things. I would like to get over 100:1. Keep in mind, that's with a V8.

6.69 x 2.46 x 4.56 = 75.05:1

There are two things you really need to consider, your hiway performance, and your offroad performance.

Offroad performance will almost always benefit from lower gears. You can't really go too low where axle gears are concerned. With automatics, there is a point where your brakes will not be able to stop you, and you'll have to kick it into nuetral, where as with a manual, you can just push in the clutch. But you won't get to that point with axle gears alone.

For hiway performance, a small change in axle gears or tire size can have a pretty significant effect on performance. For this reason, if you spend time on the street, I recommend matching your axle gears and tire size to your engine, so you cruise in your engines powerband. Generally for most engines, this is between 2500 and 3000rpm. If you have overdrive you can get way with a little lower axle gears on the hiway. If you have a V8, you can run a little taller gears. Also keep in mind that most older torque converters slip even at highway speeds which will make your cruise rpm a little higher.

Then general rule of thumb is
31's = 4.10
33's = 4.56
36's = 4.88

You can of course tweak these numbers one way or another to best match your engine and your use. 70% street / 30% offroad etc. (plan for the future!)

If your bronco will NEVER see the street, then you have nothing to loose by gearing it out. to 5.13 or whatever. Although the pinion gears start getting pretty small.

The last thing to consider is the torque converter. They are good for approximately 2:1 reduction. I think this is a pretty generous estimate and depends on your torque converter. Unfortunately, this reduction comes at a cost of heat with kills autos.
(2tc) = torque converter
(2.48db) = BW1350 doubler

2.48 x 2.48 x 3.45 = 21.22 x (2tc) = 42.44 ----x (2.48db) = 105.25:1
2.48 x 2.48 x 3.73 = 22.94 x (2tc) = 45.88 ----x (2.48db) = 113.78:1
2.48 x 2.48 x 4.10 = 25.22 x (2tc) = 50.44 ----x (2.48db) = 125.09:1
2.48 x 2.48 x 4.56 = 28.04 x (2tc) = 56.08 ----x (2.48db) = 139.08:1
2.48 x 2.48 x 4.88 = 30.01 x (2tc) = 60.02 ----x (2.48db) = 148.85:1

Since you will run 33's and possibly 36's in the future, and you run on the street, I would run 4.56's without question. If you go straight to 36's and have a stock motor, I would run at least 4.88's.

I think the doubler is a good idea as it will push you over that 100:1 mark. I wouldn't let the doubler affect my decision on axle gears though.

Too much unless info....ahh well. It's friday and I killed some time.

I agree, you should step back and look at what you are using the truck for. my EB is going to be a DD/wheeler so I will stick to the stock gearing. my axle code says 4.10s and I am gathering for the NP435 swap now. that with the D20 and I think I am set. the doubler is a good option because it can be shifted so you don't even know it is there.