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I understand there are 2 different 4th gear sets, and then obviously the 3.88 vs 4.12 or whatever it is. What real world rpm drop do you see? We plan to drive our Speedster and are considering this swap. Has anybody done this? I have a 1776 in it now, and it feels like it's out of gear by 55 mph!
1956 CMC(Speedster)
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I can't remember at 55, but at 70 mph, it Is turning just at 3,800 rpms... To keep from getting run over, literally, we sometimes have to run 75, which is damn near 4,000. Even on back roads, it's an awkward gear, 4th is bogged, 3rd is wound up by mid corner. If this were one if my race cars, and I knew it had a 4.12, I would put a 3.73 or so gear just for starts.
A "normal" 4th gear is .89, which as you can see is an over-drive. A "normal" 4th gear is a 4.12, and a "freeway flier" (properly built) has a 3.88 R/P. Generally speaking- the smaller the ring gear, the stronger it's going to be. A fire-breathing 2332 with a 4.11 is not the best way to do this.

There is a .82 4th available, as well as a 3.44 R/P. Rancho's online catalog has pretty much all the information you could want (as far as what's available), and an on-line gear calculator will do the rest for you.

You can do pretty much anything you want with these transaxles for about $1500 or so- custom gearsets included. If you want a TBD, it'll be another $1500 or so, and the only true LSD is a 40 year old ZF part that sometimes shows up on "the Samba".

I had a .82/3.88, and the engine to pull it. It wasn't the do-all/end-all you would think. I've ridden in speedsters that had the .89/3.44- it wasn't perfect either. A .89/3.88 is common, and really as good as you are going to get without going to a 5 speed of some sort (Berg conversion or 915). Life is full of trade-offs.

As an aside- everybody talks about "the 4th gear problem", but IMHO it's first and second that are worthless. Unless you are pulling stumps, or are a drag-racer launching at 7000 RPM- first is ridiculous.

Wow, Keith 3,800 RPM at 70 MPH and near 4,000 for 75 MPH is really spinning that engine. At least it must run cool with all that air
over the engine!

At 70 I'm looking at 3,200 RPM and at 75 it's at 3,450 RPM so I'm a pretty satisfied guy. CHT is never above 300 and oil temp is usually 185.

R/P is 4.120 with a .820 4th gear---super for interstate highway travel.
You can look up your transmission code to get all the stock ratios here: Plus if you click on the chart icon on the left of the row, it gives you a nice rpm/kph gear ratio chart/calculator you can use to see what possible ratio changes will do for you.

The stock ratios do have good spacing even if 1st is pretty low. So change the R&P if you can. Just changing 4th makes that 3-4 gap pretty big.
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