taller 5th gear for 915?

[Porsche_911s]

This might be a strange question, but is there a taller 5th gear for a 915 tranny other than stock.

Reason I ask, I finally got my 911 on a short trip (800 miles round trip) and my 7:31 R&P yielded 20 MPG with my 3.2. I have driven my current setup with an 8:31 and I perfer the 7:31 but wish I had a 5th gear that would run about 500 RPMs lower than the stock gear.

Any thoughts?

[john walker's workshop]

there's a 0.72 out there.

[MBEngineering]

HI Porsche_911s
what ratio have you in the box at the moment???

you could change to;
SN 28;23. 0.8214
29;23. 0.7931
SM 28;22. 0.7857
TM 29;22. 0.7586
TL 31;22. 0.7097

regards mike

[Porsche_911s]

I do not know the ratio I have currently. It is a '74 tranny with a 7:31. It was refreshed about 6k ago and I did not think to ask for numbers.

I had driven a friends '79 with a US 3.2 which had an '84 engine and matching 915. I was comparing his car to my '78 but mine has a Euro 3.2 and '74 tranny. I like the way mine drove but it is possible that I was just enjoying the additional horse power and not the transmission. His car seemed to lug a little in 1st gear.

Thank you for the comments!

[RWebb]

at a steady & accurate speed (test it) in 5th gear, what rpm does the motor turn?

[Bill Verburg]

'74 US 911 would be a 915/06 which already has the tallest 915 5th(21:29 0.724) that I can find listed anywhere

there is a ZD .7097 but that is a 901 gear, don't know if it can be used in a 915

Mike, where did the TL come from?

[Porsche_911s]

My current speedometer was rebuilt before I bought it and appears to be off by about 5 MPH at 70. So here is my best guess:

~3500 @ ~ 70 MPH

I could have the wife drive in front of me with the cruise to verify this, but this is what we came up with last time I checked.

[Bill Verburg]

915/06 w 225/50x15 is theoretically doing 75mph@3500

[Porsche_911s]

What would a trans with a 8:31 run? Theoretically?

[Bill Verburg]

your trans w/ 8:31 same tires as above will be doing 86@3500, 74@3000, 61@2500

[Porsche_911s]

Oh wow.... no wonder I enjoy the 7:31 so much around town. That is a much bigger difference than I thought.

If I could find a taller 5th gear, would it stress the '74 tranny?

[Bill Verburg]

Originally Posted by Porsche_911s

Oh wow.... no wonder I enjoy the 7:31 so much around town. That is a much bigger difference than I thought.

If I could find a taller 5th gear, would it stress the '74 tranny?

You are stuck in the street/track dilemma, I like to look at the top speed in top gear just to keep things simple.

a good track trans might top out at ~ 140-150, a good street trans 170 -180.

for dual use but still on the sport side I like ~160


yours depending on tires and rev limit w/ 7:31 is ~154, w/8:31 ~176

some try to get away w/ sporty 1-4 w/ a long 5 to compensate, I find that to be a really uncomfortable trans to live w/. But to eacj his own.

As far as stress, the thing to avoid is smaller and fewer teeth

[Hansv]

Hi there Porsche_911S;

A reasonably accurate way of checking your actual speed is by using one of those portable GPS/satnav thingies. My Tomtom taught me that my Beamer's speedometer is off by just 8km/h(5mph) at an indicated 250km/h(155mph) that turned out to be an actual 242km/h(150mph). Traffic made further examination (car was stil gaining speed) impossible. Be sure to have a friend read the satnav at speeds like this because you will need all your attention with everything approaching very, very quickly in front of you....
Disclaimer: don't try this at home.
Thank god for the last remaining unrestricted stretches of Autobahn.

[Porsche_911s]

Thanks for the tip! I plan on getting a GPS soon. 5 MPH at 150 sounds reasonable. I am sure I would be off about 15 MPH at 150. But has Bill pointed out, my car would never make it there!

A 4 speed with overdrive sounds like the perfect tranny for me. In Houston traffic its hard to ever get up to 70 MPH and keep it there for any length of time. I normally remain in 4th unless I am on a toll road or on the open road. Swapping gears or even a R&P would probably be too costly. I may wait until my clutch needs changed and then consider going to a later model trans to match the engine.

I think 3000 RPM at 70 MPH would be ideal. At least for my current setup.

[Hansv]

Hi there, Porsche_911S;

Houston sounds a lot like the western part of the Netherlands; over here it's also virtually impossible to reach anything over 75mph during daytime for more than a few seconds. And if you do rise to the occasion you stand a fat change of being treated as if you were committing a crime against humanity.
But at least we have the consolation of the German border at a good hours' drive....

[RarlyL8]

A buddy of mine installed a custom 5th gear set in his 915 for hitting the highway. Dropped it below 3K @ 70mph. The cost was prohibitive though at $600/set.

[safe]

I wish I had a tall 5th gear so I could cruise a little more comfortable at ~85-90, lowering rpm from 4000+ to maybe 3200.
My gearbox is a 4 speed, so it could really be a pure overdrive.

[MBEngineering]

HI safe
you have a good box to make a close ratio box from, if you pull the box out and strip the gears off the shafts and replace 3rd gear with 23:29 NT then fit your old 3Rd into 4Th, fit a 5Th of your choice and you will have a good box.

Bill Verburg.
"Mike, where did the TL come from? " it came from hear!!



and you are going to tell me (as I have not fully checked it ) that it is wrong??, as I have a TL, BUT!! not a 31:22??










there was higher ratios, as this chart has.







regards mike