1968 911 5-speed ratios

[Autosport]

I am looking for optional Ring & Pinion and ratios for a 1968 911 - 5 speed. Ideas?

[Grady Clay]

Welcome to the Forum.
You will find a lot of help here.

First, why do you want a change? What is your goal? How are you going to use your 911? What is the type number of your transmission (form 90x/xx)?

That said, there are two R&P ratios; 7:31 used in almost everything and 6:29 as a race (hillclimb) part and in a ’69 912. The 6:29 is very fragile and unacceptable for most uses. That effectively limits you to a 7:31 ratio.

The gear pairs themselves are generally available and special ratios very expensive.

The ’68 911 typically has a 902/02 geared:
1st – “A” 11:34 = 3.091
2nd – “F” 18:34 = 1.889
3rd – “M” 22:29 = 1.318
4th – “S” 25:26 = 1.040
5th – “X” 28:24 = 0.8214
R&P - 7:31 = 4.4280

This is actually a very desirable set of gears for street and track with a 2.0 or larger engine. A common rearrangement is “AFJOS” which retain the more expensive 1st and 2nd gears and uses the 4th gear as 5th. The racing gear ratios “J” 3rd (21:31 = 1.476) and “O” 4-5th (23:28 = 1.217) are difficult to find and expensive.

A critical issue is your ’68 has a “Simplified differential” and should be investigated for the update. If left unattended it can entirely destroy the transmission. Do a Pelican Search on “Simplified differential.”

Again, welcome.

Best,
Grady

[Autosport]

Thanks Grady........
This is for a customer who now likes his '68 so much he is using it as his daily driver. He spends most of his commute on the interstate around 80mph. He does not use it for anything competitive and wants to bring the revs down in top gear. I was hoping there was a taller CWP but a taller 5th or 4th and 5th would do it. DO you happen to know if there is anything avail?
Thanks again,
Steve

[Early_S_Man]

Steve,

The following thread has several gear charts in it:

901/13 gear question

[Grady Clay]

Steve,

Yes, the common taller 5th gears are available almost for free. The usual ratios are “Z” 29:23 = 0.7931 from some ’69 transmissions, a “ZA” 29:22 = 0.7586 from the ’70-’71 type 911 transmission and the “ZD” 31:22 = 0.7097 from all the 914 transmissions.

I recommend you have him keep the “X” 5th as a spare part. He may decide to reinstall it at some point. At 80 mph the “X” has the engine turning 4050 rpm, “Z” 3750 rpm, “ZA” 3600 rpm., “ZD” 3450 rpm. The issue with these high gears and a 2.0 is the engine torque isn’t sufficient to comfortably accelerate up a slight grade. He may find himself downshifting to “S” 4th at 4900 rpm. I would have him start with a “Z” 5th. That will feel like a big change.

Have you researched the “Simplified differential”?

Best,
Grady

[dd74]

Grady - I tried to PM you, but your mailbox is full...

[dd74]

Thanks, Grady. I emailed you at gradyclay@hotmail.com.