62 GP Gearing Questions
 

Gentlemen,

I have replaced the tired 389/Slim Jim with a 455-TH400 in my 62 GP. Now my speedo is off by about 15 MPH at 55 and she really twists up on the freeway just cruising the speed limit. It seems the TH 400 has a lower final gear ratio than the slim jim?

If that's the case I would like to change rear end gearing to solve the problem. I read that 62 GP came with a 3.23 for A/T. So if the above is the case, what rear ends are available (if I could find one) for my car, I would like the highest available as I do a lot of long distance freeway driving. BTW I'm only looking for an open rear end.

Thanks,
Mike

2:69 open rears are plentiful and worthless and would be easy to get.
I'm sure someone in your area can help or check junk yards for an
automatic catalina 61-64.

The slim Jim and the TH400 both have a 1:1 high gear, so there is no difference in gearing. Perhaps the new trans or the torque converter is slipping.

As stated by b-man, they are both 1 to 1 in high gear. Is the 400 shifting twice so that you can confirm that it is actually in 3rd ? Is the converter stock ? You don't mention specific rpm's so I am assuming you do not have a functional tach and that you are gauging the rpm by what, sound ? If the swap was made with no other changes and it is getting into 3rd with a stock converter, there should only be the most negligible difference in rpm. The correct driven speedo gear should be available from www.thegearbox.org.

If you replaced the slim-jim with a TH400 and your speedometer is off - it's not an issue of rear gears or torque converter - the issue is with the gearing with regard to your speedometer cable - you can purchase the correct nylon gear that goes into the transmission to properly tweek your speedo where your speedometer cable plugs into the transmission.

If you are having issues with RPM's being too high.... then you may have a transmission or rear gear situation.

Normally, an A/T car '61-'64 came with a 2.69:1 (GP was 3.08 I believe) and went down from there depending on what was ordered... a manual transmission came standard with a 3.42:1 and went down from there depending on what was ordered.

Gentlemen,

It looks like the next step is to hook up a tach and actually measure RPM and accurate speed. That will tell all. More on that in a few days.
Thanks

My 62 GP came with 3.23s,I went to the 2.69s and love them.My car is a 4 speed with a 3.42 first ST-10.tom

You can also figure out if you are in 3rd gear by shifting down to 2nd while you are at speed limit speed. You will not hurt the trans. That much is certain. Or... just try shifting it manually.

If you are not used to it, it would be very easy to be in 2nd in a TH400 and not know it if you have 2 series gears in the car already. The main tell is the vehicle holding back when you let off the gas and the secondaries being open/ed when you step on it at the Hwy speed. If you have 3.42 or 3.23 you would be wound way up in 2nd b/c even in 3rd w/ a TH400 you will run around 3k with 3.42 rear.

In addition to the above, my reaction is this: provided you don't have super short tires, 3.23s shouldn't be revving to the moon.

Get a reliable tach in the car and see what your Rs are.If you're at 3000 at a true 70 mph, I wouldn't sweat it.

Find your final drive ratio and install speedo years or an adapter to correct it.

Guys,

I have found that my car speedp is running 95 mph at a true 70 mph and the tach is at 3200 rpm.
I think it is due to smaller tires. Does anybody know the calculation of gain in mph on speedo if I change fron 3:23 to 2:69?
It looks like that is what I plan to do. Im just wondering what my new true mph adjustment would be at 3200 rpm with that gearing increase.

Also, if anyone has a 2:69 from 61-64 I am interested if you are within 500 miles of Central California.

Thanks,
Mike

What size tires do you have on it? Your effective gear ratio is about 3.55 by those numbers, so in guessing 205/70/14 ???

I would guess the 400 came out of a car with a 2.73 rear and has a 39 tooth speedo gear in it, you have to much error to get rid of it with a speedo gear change, you would need an adapter as it sits.

Changing to a 2.69 rear will get it in the ballpark, close enough to get rid of the rest of the error with a speedo gear change. best guess is you will have to go from a 39 tooth to a 41 tooth and you will have to change the housing to the one for the 40+ tooth gears.

As far as speedo driven gears, housings, and the o ring goes, you best source is your local GM dealer. They still carry all the parts...

Assuming your 70 MPH true number is correct and was taken at 3200 RPM with your 3.23, changing to a 2.69 rear gear would give you 84 MPH true at the same engine speed.