differential gearing
 

I've got a 79 k20 camper special 350/th400/np205. Before I converted to the 205 my max speed was around 60-65 without worrying about blowing the motor. After the swap I'm down to around 50-55. The 205 came out of a 80 k30. Just wondering what my rear gear ratio is? I read they came with 4.10 or 4.56 rears. Reason I ask is im looking at adding 4-6" of lift and 33 or 35s on it. I'd rather not replace the diffs but I'd also like to be able to do 70 on the highway. Any help is greatly appreciated.

Re: differential gearing
 

Just a tcase swap will have no effect on the rpm.
But to be sure, take to diff cover off the rear and read the numbers on the ring gear. They will tell the ratio. Then you can determine where your rpms should be when. Then determine if you have torque converter issues or something else.

Re: differential gearing
 

Keep this in mind. Bigger tires effectively need more gear to get rolling. One thing some forget is the bigger tires have an effect like an overdrive gear. Effectively for each revolution of the tires, you are going further than you would with a smaller tire. So if you already have 4.10's already you will be ok with either 33's or 35's. 4.56's with the 35's is the sweet spot if you got them.

Still, like Obijuan said, pull the cover and read the numbers on the ring gear. You'll know for sure. I've found the 4.10 ratio is more common on the K20 vs the 4.56 gear, 3.73's are probably the most common.

Re: differential gearing
 

Where will the numbers be on the ring, the outer edge I assume. I'm guessing that the 205 I swapped in has a lower output gear ratio than my old 203. Torque converter is brand new along with a quality rebuilt transmission. Thanks for the help guys

Re: differential gearing
 

With the diff cover removed, don't worry if you don't find the numbers. Just count the teeth on the ring gear and divide by the number of teeth on the pinion gear.

As to your two t-cases, low range is different but high range is the same at 1:1.

Re: differential gearing
 

I would check your speed against a GPS unit, I bet the speedo gear is different between the two swapped transfer cases.

Re: differential gearing
 

That's how I got the speeds. The speedo adapter took a dive on me so I've based my speed off 3 different speedometer apps. I'll check the diff this week. Thanks for the help

Re: differential gearing
 

Definitely not the tcase for the difference. Unless you are in low range. But it could be the speedo drive gear is different .