3.88 freeway flyer VS 4.12 rpms?

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!

Running tall rpms at 55, you probably are running the 4:12 R & P yes the 3:88 is the better trans for hwy. driving, tire size also dictates rpm.

What rpm's are you currently turning at 55 mph? Then it's just arithmetic to calculate your rpm drop with different gear ratios.

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.

with the 3.88 at 3000 rpm you will be going 65 mph, for about every 500 rpm over you willl pick up 10 mph. in my car on the freeway i cruise at 78 thats a bout all you can get away with, it at 3700 rpm. bill

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.

Well, I do understand that the fan is blowing faster. However Heat is a byproduct of RPM! it runs 230 in 90* heat. It's 180* oil in 75* weather. Anyway, I believe it's a 4.12 gear... It's not fun watching the closure rate on I65 and your already ata proven GPS 75 mph at a zillion rpms..

You can look up your transmission code to get all the stock ratios here: http://www.keversite.nl/motoren/vcode.php 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.

Jack..I like your ratios for highway cruising and while my engine is in limbo, I'm looking at various options around the car. Might even paint the bugger !. Are you still running a 25" tire to get those specs above ?

Actually 1st isn't really all that short. Comparing my AT code transmission to a stock subaru 5-speed, 1st is 6% taller than the subaru and 2nd is the same between the two.

David---I emailed you off-site.

Great info site, Justin. Thanks!