3/4 semi floating axle to full floater swap

[SirRobyn0]

So my 1984 C-20 350 W- 700R4 has the semi floating rear axle and I'm looking to swap it to the full floater we are talking the factory full floating diff, not some other diff. I believe that it is very likely the current axle is 3.08 unit, but I've not removed the cover to count. RPO code indicates that it is the "standard axle" with no reference to axle ratio, and really I should and will find out my true axle ratio before proceeding.

My main questions, I "think" that the axle housing is the same size so I "think" I won't need to shorten my drive line, but I could be wrong, if someone knows that info that would be great.

I know some of you are going to get whinny about this, but fuel economy is a concern, for me. Look I know most of these trucks aren't mileage makers, but I don't want mine to be any lower than it is. I spend a good amount of time loaded, but also have some long freeway drives to get some of those loads. One of my concerns is that most of the stock full floating diffs I've seen seem to typically have axle ratio of 3.73 or 4.10

I'm thinking if I look for one with a 3.73 ratio that should be ok, with the 4.10 probably being a bit lower than I'd want to go, really I'm just looking for a second opinion / other thoughts from ya'll.

 

[Bextreme04]

A 1984 C-20 would not have come with a 3.08 as standard with any power team according to the GM Heritage booklet. What are the Actual RPO codes for your truck? An LT9 with Trans Code MX0 And Axle Code GQ1 would be a 3.73. For the same engine/trans code but GT5 axle code you would have a 4.10. If you have Trans Code MX1, then GQ1 is a 4.10 and there is no GT5. You can look up all the standard power teams and options from the 84 trucks for yourself here: https://www.gmheritagecenter.com/do...its/Chevrolet-Trucks/1984-Chevrolet-Truck.pdf

 

[SirRobyn0]

A 1984 C-20 would not have come with a 3.08 as standard with any power team according to the GM Heritage booklet. What are the Actual RPO codes for your truck? An LT9 with Trans Code MX0 And Axle Code GQ1 would be a 3.73. For the same engine/trans code but GT5 axle code you would have a 4.10. If you have Trans Code MX1, then GQ1 is a 4.10 and there is no GT5. You can look up all the standard power teams and options from the 84 trucks for yourself here: https://www.gmheritagecenter.com/do...its/Chevrolet-Trucks/1984-Chevrolet-Truck.pdf

I appreciate that. As your probably aware I'm mechanic, but that doesn't mean that I'm up on specs at all. So unfortunately my RPO code sticker is in pretty bad shape, but I was able to make out the axle code of GQ1, and I just looked my trans code is MX0, so that would make my axle a 3.73 as it is or was from factory. I sure appreciate you setting me straight on that. And that being the case I'll have to consider if I should look for FF that is 3.73 or 4.10, I didn't want to take to big of a jump, but a slight increase to help with the heavy loads might be a plus. I'll have to think on it a bit.

 

[Curt]

I just swapped the full float 14 bolt for the semi float 14 bolt.I had to have my driveshaft lengthened 2”.

The semi float has a pretty long nose.Standard axle on this 84’ model K20 was 4.10 gears.

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[highdesertrange]

if you want better fuel economy. slow down. wind resistance is the single biggest factor in MPG. after all it's like we are driving a barn down the highway. highdesertranger

 

[MikeB]

if you want better fuel economy. slow down. wind resistance is the single biggest factor in MPG. after all it's like we are driving a barn down the highway. highdesertranger

Yes, slowing down helps, but it's not practical or safe to drive much slower than the traffic around you. GM didn't put 2.56 and 2.73 axles in our trucks to help them carry heavier loads, they did it for MPG ratings.

 

[SirRobyn0]

if you want better fuel economy. slow down. wind resistance is the single biggest factor in MPG. after all it's like we are driving a barn down the highway. highdesertranger

Yes, slowing down helps, but it's not practical or safe to drive much slower than the traffic around you. GM didn't put 2.56 and 2.73 axles in our trucks to help them carry heavier loads, they did it for MPG ratings.

This is an older thread and I've pretty much decided to keep my semi-floater for now and re-gear it. Yes, I would still like to go full floater one day, but I bought an 18 foot trailer that can haul 7K so re-gearing the existing axle has become more important. I'm not complaining, but there is another thread outlining my plan. To summarize it turns out that even though my RPO code says I should have a 3.73, I actually have a 3.41. So probably the differential was swapped at some point.

As far as gas mileage is concerned I get that these trucks are not economy machines, but when I do a feed run and drive 2+ hours on the highway empty to get there, and 2+ hours loaded with a ton of feed back home I need the best mileage I can get and ability to haul. Because you know gas cuts into the savings on those long runs. I don't mind feeding the gas tank, I just want it to be the best it can be with my current setup. And now I need the ability to tow a loaded trailer which may have 3 tons of weight on it at times.

Since I have the 700R4 I've decided to go with a 4.56 regear. I've spent some time doing the calculations, and taken advice given in that other thread and I believe it will be a good setup for the hills here.