HELP: School me on 2 speed rear end ratios

[Bigdually]

I'm having an issue finding info on the internet, so here's my issue.

Recently bought a 79 C70 with a Spicer 5 speed/Eaton 2 speed rear. The SPID label lists the rear gear ratio at 6.50. Is that the high or low side ratio? What's the other ratio?

I've attached pictures of the SPID label, the stamped numbers on the differential, and the plate on the 2 speed shift unit.

Thanks for any help you guys can provide, although I've been around larger trucks my whole life I've never had one with a 2 speed rear.

[CarolinaHD]

That’s the high side, low should be around 8 or 9:1.

My c7500 lists the ratio as 6.57 and here’s the list of ratios. Yours will be close.

[Bigdually]

That's what I was afraid of, and honestly what I was suspecting. Thanks for the info. I wish it was one of those higher ratio combos so I could use it, but I'll have to stick to pairing the 427 and Spicer from this truck against the existing 4.63 gearing in my GMC.

[350TacoZilla]

The 2spd is nice if you have a more modern OD trans but with the old 1:1 4th or 5th it really limits the trucks. They obviously didn't plan on you taking a MDT dump truck on interstate @70mpg back then though, 50-55 was probably their desired top speed with 25-45 probably being more of the normal operating range for cities and back roads.

Its more work but the 4.63's are kinda desirable so you might be able to sell off some unused parts and swap in a newer OD trans with little bit more cash outlay .

[Bigdually]

Originally Posted by 350TacoZilla

The 2spd is nice if you have a more modern OD trans but with the old 1:1 4th or 5th it really limits the trucks. They obviously didn't plan on you taking a MDT dump truck on interstate @70mpg back then though, 50-55 was probably their desired top speed with 25-45 probably being more of the normal operating range for cities and back roads.

Its more work but the 4.63's are kinda desirable so you might be able to sell off some unused parts and swap in a newer OD trans with little bit more cash outlay .

I bought the truck the SPID label on the first post belongs to simply for the 427/5 speed combo to go into the truck in my signature. It currently has a tired 366/4 speed. I regeared the 15k single speed Eaton in that truck to 4.63 and the top end cruising speed is about 70 in 1:1 4th. I don't really want to go any faster, I just need more power and more gears and thats where the donor driveline will be nice. I've looked up the ratio table for the 5252 Spicer and I think it will suit my needs just about perfectly. I was just curious about the 2 speed unit in the parts truck, it's low mile and I'll probably sell it if I can rather than scrap it.

[350TacoZilla]

Yeah I just mean if you went with the 6.50 2spd rear a OD trans would pair up decent. The current 4.63 with the spicer 5spd swapped in will work good but you won't have that low axle ratio to get stuff moving. My dad had a single axle dump with 366\5spd and 2spd rear and it was a good combo but again 50mph was about where it wanted to live with the deep rear gears.

Side note I remember when you could buy the 366's for like $50 out of the bus or MDT's but you had to watch because a ton of people just thought tall deck = 427 and it would be a 366. Personally I like the 366 but they are never goin to compete with the 427 for power.

[Bigdually]

I'd definitely be looking into an OD trans or swapping the center section to a higher geared setup if I wanted to keep the rear out of this truck. My GMC had 6.17s originally and it was wound tight at 55 in 4th.

I've been on the hunt for a 427 for a long while now. I had an older guy who'd owned a few MD trucks tell me he always felt that a 427 truck ran as good loaded as a 366 truck did empty. I don't know there will be *that* much different but I'm sure the power gain will be noticeable. Not to mention this 427 is about 100k miles fresher than my poor old 366. Plus the very worn SM 465 shifter in the GMC has so much slop it wiggles like it's in neutral when it's in gear