TECH QUESTION..............Gear ratios

[Mark 69 911]

I just purchased another 69 911.

When cruising on the freeway at 60 mph the tach reads 3200 rpm.

The trans is a 901 dog leg 5 speed.
The rims are 6x16 fuchs with BF Goodrich comp T/As 205 55 16 and are 24 1/2 inches tall.

What rear end/transaxel gear ratio do I have?
Is there a method to calculate the ratio without getting into the transaxel?


Any information is very much appriciated.

[RarlyL8]

My guess, 7:31.
You can use rpm, speed, and tire roll out to calculate your final drive ratio. You must also know your individual gear ratios to determine the differential gear ratio (if 5th is 1:1 this becomes easy). Don't have Bruce's book handy so I can't say what stock would be for a '69 901.

[Early_S_Man]

Most definitely 7:31 r & p, The 'Type Number', 901/xx and serial number, stamped on the underside of the transaxle, would help pin down all of the ratios that were standard for that version.

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Warren Hall
1973 911S Targa

[Roland Kunz]

Hallo

It is possible to calculate it out but very hard to calculate final drive ratio and gear ratio from that.

A base parameter for the general transition is that Porsche makes 2 m street per rotation.
Now if you have 3200 revs per minute this equals 6,4 km/m x 60 = 384 km/h or 238 mls

Your transition is 238/60 = 4/1

As your speedo is not correct and the 225/55/16 have 1,95 m rolldiameter the nubers are over the thump

Now geting to your R&P and gear ratio to achive that is a other story. It can be long gear an short R&P or short gear and long R&P.


Grüsse

[911pcars]

What kind of 901 do you have? It should be stamped under the trans. housing (901/0, 901/02, 901/03, etc.). If you know, I can supply you with the 1-5 & R ratios.

The standard final gear ratio on 901s is 4.43:1 (7:31), although I once had a 901 with a 4.83:1 (6:29) ring and pinion (actually p&r). Engine speed at 60mph was around 4000, but the 1-2 gears and the low rear ratio actually surprised a lot of cars under low speed acceleration with my then-stock T engine.

Sherwood Lee
http://members.rennlist.org/911pcars

[josh912]

Try this: http://www.pelicanparts.com/techarticles/mult_tranny_ratios/mult_tranny_ratios.htm

Also, check the accuracy of the speedo. Who knows, you gears might have been changed. On my 912 I have 205/60 tires with a 27:25 5th gear ratio resulting in 3550 rpm at 65 mph!

------------------
Joshua Harrison
1968 912 Coupe
www.geocities.com/jkh912
56 days till I'm reunited with my 912!!!!!!!!

[Mark 69 911]

Thanks for the help.
Also, get this, when shifting up through the gears from second up to fifth the RPM's only drop 200 RPM's per shift. Talk about close ratios. What have I got here, a real racing geared transmission?

[Early_S_Man]

Mark,

You meant 2000 rpm, of course!

Find that type number and serial number that always begins with 7 or 5 ... and we can tell you for sure ...

------------------
Warren Hall
1973 911S Targa

[Mark 69 911]

No Warren not 2000 rpm but 200 rpm.
I'll get those numbers in the morning.
Thanks Pal.

[stlrj]

Here's an easy way:


http://www.thegrid.net/thedrivetrainpage/rpmcalc.htm

Cheers,

Joe