[leftybass209]

Originally Posted by summitflier

I'm not entirely sure the gear ratio determines the carrier series.

For example....
This page says 3.08 to 3.90 is 3 series.
and 4.10 and higher is 4 series (for cars) but says 3:42 and lower is 3 series for trucks and 3:73 and higher 4 series for trucks.
http://www.jegs.com/p/Eaton/Eaton-Po...363&Ntt=750876

This says it's for GM 12 bolt truck axles and supports 3:73 but is a 4 series.

http://www.summitracing.com/parts/rmg-4902801/overview/

This says it's for GM 12 bolt truck axles and supports 3:73 but is a 3 series.

http://www.summitracing.com/parts/rmg-4900721/overview/

I suppose it's possible that JEGS and summitracing are wrong but...

What you are seeing is correct. Since the development of the OEM gearsets and the 3 and 4 series carriers, aftermarket manufacturers have developed gearsets that are thicker and thinner so that you don't have to purchase a gearset and carrier. Currently it's possible to purchase new 3 series 4.11's or 4 series 3.08's, among other oddball combinations that don't follow the OEM rule.

Don't confuse yourself or waste shipping money buying the wrong gearset. If you are certain the carrier is an OEM piece for a 67-72 carrier, the rule as stated in my first post applies.

I hope that clears it up.