Mechanical Speedo calibrated to Ring and Pinion?

[ARCSinAK]

I have a 7/31 transaxel in my 74 911 just recently replaced the cable, wondering if any mechanical speedometer will work with this R/P as I have an extra mechanical spedometer with a functioning odometer. The original odometer is not functioning, will the speed be accurate if the other mechanical speedo came off a 8/31?

We're all mechanical drive speedos based on 7/31? Did the electronic switch come with the 8/31?

[Walt Fricke]

You could confirm all this with the parts manual (which Porsche has kindly made available in PDF format on line). But your '74 came with the 7/31. '75 and on until the G50 had the 8/31. '72-73s had the 7/31 also.

Some, but not necessarily all, earlier transmissions (901, 911)had the 7/31 as well, so it gets trickier - you need to establish the year and perhaps model (914? 912? Sportomatic?), plus for early cars you could get "airport" gears and hill climb gears - at least five different gear sets, some of which maybe had an even shorter R&P.

[Matt Monson]

75 had a mid year final drive change. Any of the 9154x series gearboxes one would need to check the final number and spec.

As Walt mentioned, pet can provide speedo part numbers for which speedo unit is calibrated for which final drive.

[ARCSinAK]

Walt- thanks for your input, I understand what R/P my 911s have and history of 915 changes. I just don't understand the process of calibrating a mechanical speedo.

Matt- Checking PET will give speedo part numbers that will be calibrated to gear box numbers?



My 70 T also has a 7/31 915 so much better gearing for a small island than the 8/31 in my SC.

[ARCSinAK]

[911tracker85]

my 78 930 conversion had a 73-74 915 with a 7/31 and a mechanical speedo. sold the 915 to a friend and got rebuilt, short bell housing 930 gearbox. with several sets of tires planning a GPS based speedo.

good luck

[Tyson Schmidt]

The speedos are the same.

The mechanical drive 8/31 had different ratios in the speedo drive mechanism to compensate.

[ARCSinAK]

If I understand correctly the speedo compensates for the R/P? Different ratios in the speedo? If the extra mechanical speedo I have came from a 8/31 it will be calibrated for the 7/31 in my 74?

[Matt Monson]

I went looking in the PET this morning after Tyson's comment. He's correct. The speedos themselves are the same. The change in "gearing" to mate to the speedo cable comes from the transfer gears that spin the cable.

a 915/06 from , 74, as well as a 915/43 from 75 use one set of gears. The 915/40 and 915/44, and 915/45 from 75 use a different set of gears.

Short answer, it's in the gearbox, not in the gauge.

[Matt Monson]

parts number 27 and 31 in the diagram.

dC AutomotiveOver 140,000 square feet of new and used Porsche parts

[ARCSinAK]

Hi Matt-

Thanks for your time and input.

I understand this to mean that essentially the speedo gear "Transfer Gear" in the gear box is different for each R/P, spinning the cable at the same rate for the all years of mechanical speedometers, hence any mechanical speedo should register the correct speed.

The Goal: To change out my original speedometer with a spare I have in oder to have functioning odometer.