Determine what ratio gears one has in a 915 Transmission

[Bavaria911]

I was wondering if one could determine, without opening the transmission case, what gear ratios are set for 1st through 5th gear for a early 915 5 speed for a 1972-73 2.4 MFI. Is there a certain MPH your drive in each gear and record the RPM then use a math formula to get the ratio for each gear.
Anyone that knows, please chime in.
Vr,
Scott

[Flieger]

You could do that. Need to know engine rpm, road speed, and tire overall diameter.

[Bob Harriman]

Can't the version and serial #'s reveal that?

http://www.blueskymotorsports.com/in...temid=6http://

Those are 901's but wouldn't there also be a manual for the 915?

[chris_seven]

Any errors - please let me know

[Jon B]

Can't the version and serial #'s reveal that?
http://www.blueskymotorsports.com/in...temid=6http://
Those are 901's but wouldn't there also be a manual for the 915?

Standard gear ratios for 72/73 model transmissions:



Jon B.
Vista, CA

[Jon B]

Any errors - please let me know

Chris,
All transmissions in your chart are for later cars except 915/02 and 915/08.
915/08 is a 5-speed; someone omitted the 5th ratio.

Jon B.
Vista, CA

[chris_seven]

I didn't type it in and will correct and try to add the earlier information - Thanks

[Bavaria911]

Flieger,
What speed must I drive in each gear to record the RPMs? Can you provide the calculation steps, an example please.
The shop that rebuilt the motor believes the transmission has non-standard gearing.

Thanks,
Scott

[Flieger]

All you need to do is have the road speed and rpm stable. You find your tire rpm and divide by your engine rpm to find the overall gear ratio. You will most likely have a 7:31 ring and pinion but it could be 6:29 (very short) or 8:31. So multiply by 31/7 to find what your gearbox ratio is (approximately) and compare to the tooth ratios of the available gear pairs. If you find a second gear ratio that is way too tall, then you might have an 8:31 final drive. Second gear is cut on the input shaft for all but the 904 mainshaft so it is most likely a common ratio there. If you have the 904 mainshaft then anything goes.

Now, finding tire rpm is where the most error creeps in. You want the speedometer to be accurate so try using gps instead.

Next you need to know tire effective rolling radius. This is not as simple as it seems, since the tire deforms under the weight of the car but centrifugal force causes the tire to grow at speed. Add to that the fact that if the tire is transmitting a lot of thrust (anytime the accelerator is floored at a somewhat high engine rpm), the driven tires (rear) actually travel further than the free-rolling tires (fronts) because there is a tiny amount of slip that is the mechanism for generating grip.

All that said, just try measuring from the ground to the top of the tire. Multiply by pi (3.14159...) to get the circumference. If you measured in inches then divide by 12 then divide by 5280 to get miles. If you are using metric then take cm and divide by 100,000 to get km. Divide your road speed by your circumference to get the number of tire revolutions per hour (if you started with mph or km/h). Now divide by 60 to get tire rpm and continue with the process outlined in the first paragraph.

Addendum: The tachometer and speedometer should be most accurate in the middle of their ranges. If you use gps then you can still do the speed limit but try to keep the tach up around 4,000. If you get an odd ratio pair then it may be worth doing a bit of measurement uncertainty analysis to determine what the + or - is on the final answer.

[Jon B]

Second gear is cut on the input shaft for all but the 904 mainshaft so it is most likely a common ratio there. If you have the 904 mainshaft then anything goes.

Max, this only applies to 901-based 5-speed transmissions.
In a 915 transmission, 1st gear is the integral part of the input shaft.

Jon B.
Vista, CA