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| Correct speedometer?/final drive calculation https://www.ausmini.com/forums/viewtopic.php?f=2&t=45982 |
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| Author: | bmacpiper [ Sun Sep 21, 2008 7:28 am ] |
| Post subject: | Correct speedometer?/final drive calculation |
Hello, Probably a simple one this time. The speedometer in the 1970 Cooper S doesn't appear in Parnell. Photo below; it goes to 120mph with no markings past that. The white arrows you see were just stuck on the glass and not on the face. The lettering reads: 54M2-1/12 and RPML 1312. Is this a stock gauge? Is there a way to tell final drive ratio from the lettering mentioned? Thank you, bmc 
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| Author: | albino235 [ Sun Sep 21, 2008 7:51 am ] |
| Post subject: | |
Quote: The lettering reads: 54M2-1/12 and RPML 1312.
Is there a way to tell final drive ratio from the lettering mentioned? Well I don't know if it's the stock gauge or not, but U do know that RPML 1312 stands for 1312 rpm per mile which, according to my calculations suits a 3.5 diff, and ten inch wheels... |
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| Author: | Mick [ Sun Sep 21, 2008 9:01 am ] |
| Post subject: | |
It seems the Mk2 gauges were marked only to 120, the earlier gauges carried onto 130. Parnell does describe this, look at the bottom two images on page 29. The english gauges were of differnet design to the Oz ones, but the markings carried over. |
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| Author: | bmacpiper [ Sun Sep 21, 2008 9:12 am ] |
| Post subject: | |
Albino235--thank you. The '66 has 3.444:1 final drive. Are your numbers based on a certain assumed tire circumference, i.e. that might have been slightly different 40 years ago? Perhaps the gearbox number will tell me more when that comes out? Mick--Thanks. Yes, I did see that in Parnell also. But all of the gauges in his photos have white concentric circles on the face, mine is just plain grey. The top one on p. 62 looks very similar to mine in terms of font and color, but obviously not the right mph markings. The numbers on mine don't appear in either MKI or MKII charts in the book, but I have seen another Oz Cooper S on ebay with the same speedo as mine. tx, bmc |
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| Author: | Mick [ Sun Sep 21, 2008 9:58 am ] |
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As mentioned above, the gauges were manufactured differently. Oz gauges do have the grey background where the english have a black. Also the indicator lights in the speedo will be different. English gauges had jewel like coloured coverings whereas Oz speedos have plain coloured plastic. As they are manufactured differently, it's not surprising that the silk screeniing might be different as well. |
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| Author: | bmacpiper [ Sun Sep 21, 2008 10:01 am ] |
| Post subject: | Miles = Miles? |
Just did some number crunching of my own, and measured the circumference of my 145sr10 tires in the garage. In the US, a mile is 5,280 feet, or 63360 inches. Is that true in England and Australia as well? If so: 5,280/1312=4.02 feet per revolution. 4.02x12=48.24 inches per revolution. The 145SR10 tire in my garage (brand new) measures 60.5 inches in circumference. The diameter is 20 inches, but even if you allow a full inch of compression for rolling radius, you'd have 18pi for circumference, which is about 56.5 inches. Obviously the tire would have to be a lot smaller to meet this. Assuming the tire's rolling circ is 56.5": 63360/56.5=1,121 revolutions per mile. At 60.5": 63360/60.5=1,047 revs per mile. Something doesn't add up. I also realize that I don't know how final drive relates to revolutions. Does a final drive ratio of 3.5:1 mean that for every time the tires turn once, the gearbox final drive turns 3.5 times? I think I've just confused myself. How do you calculate the final drive ratio from the 1312RPML figure? I guess perhaps the easier question would be, what are the units in a final drive number? Thanks...bmc |
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| Author: | Mick [ Sun Sep 21, 2008 10:19 am ] |
| Post subject: | |
Wow, you've killed me with numbers on this fine sunny Sunday morning in Oz. I don't think I can bear to look at them. Still, the gauge numbers match up eggzackery with the mk2 S gauge set I have here. Still won't prove the calibration until you have it calibrated by an instrument tech or compare with other cars out in the street. |
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| Author: | bmacpiper [ Sun Sep 21, 2008 10:41 am ] |
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Hee hee, sorry mate...go have a good cup of coffee. My dad was an old BMC guy and just told me that the 1312RPML relates to the fact that the speedo cable turns 1312 rpml, and has nothing to do with the final drive, etc. Thanks again, have a great Sunday. Raining here... bmc |
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| Author: | John Smidt [ Sun Sep 21, 2008 10:46 am ] |
| Post subject: | |
The lettering reads: 54M2-1/12 and RPML 1312. Your Speedo appears to be the correct one for a Australian cooper S Mk2 Aust Made Speedos are More of a grey colour on face, and the warning lights are flat (not diamond cut as on Mk1 ) the lights at rear of speedo plug into a flat surface ( Not Raised as On the Mk1 ) It is my understanding that RPML1312 is what the speedo is calibrated at,This means that @1312Revs Per Minute the speedo should read 60Mph |
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| Author: | 9YaTaH [ Sun Sep 21, 2008 10:46 am ] |
| Post subject: | Re: Miles = Miles? |
bmacpiper wrote: In the US, a mile is 5,280 feet, or 63360 inches. Is that true in England and Australia as well? If so:
5,280/1312=4.02 feet per revolution. 4.02x12=48.24 inches per revolution. Thanks...bmc Yes...but in Australia you would be looking at the soles of the feet 
You need to determine your diff ratio...because if it has been changed, the speedo will not read right.......but assuming it is original, then the only other things that could have changed is the the wheel/tyre combo, the little gearbox driving the cable (maybe) or the calibration of the speedo itself...might pay to have it overhauled and checked anyway. PS have you tried driving behind a modern car at a set speed to determine what speed is being indicated in the Mini? |
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| Author: | 9YaTaH [ Sun Sep 21, 2008 10:52 am ] |
| Post subject: | Cal Point |
bmacpiper wrote: Hee hee, sorry mate...go have a good cup of coffee. My dad was an old BMC guy and just told me that the 1312RPML relates to the fact that the speedo cable turns 1312 rpml, and has nothing to do with the final drive, etc.
Thanks again, have a great Sunday. Raining here... bmc relates to the fact that the speedo cable turns 1312 rpm....at the calibration point...look closely at the dial around the 50 to 60 MPH graduations for some little painted dots. |
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| Author: | sgc [ Sun Sep 21, 2008 11:03 am ] |
| Post subject: | Re: Cal Point |
9YaTaH wrote: look closely at the dial around the 50 to 50 MPH graduations for some little painted dots.
Is that what the I, II and III markings are for around the 30, 55 and 75 MPH points? Always wondered about those, thought they might be gearchange speeds... 
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| Author: | Mick [ Sun Sep 21, 2008 12:07 pm ] |
| Post subject: | Re: Cal Point |
sgc wrote: Is that what the I, II and III markings are for around the 30, 55 and 75 MPH points? Always wondered about those, thought they might be gearchange speeds... No, those actually were(are) gearchange points. BMC's cheap arse tacho... |
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| Author: | norton [ Sun Sep 21, 2008 1:22 pm ] |
| Post subject: | |
slight hijack but are all the speedo drives that attach to the side of the gearbox the same? i.e. gear ratios inside them? |
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| Author: | bmacpiper [ Sun Sep 21, 2008 1:36 pm ] |
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Hijack away, I think I got my answer. Once the engine and gearbox are out, I'll look at the gearbox and see if the numbers on it tell me anything about drive ratio. I would expect the Cooper S to have the 3.444:1 with close ratio, but I suppose there were variations over the years. I know the '66 has this setup, and really kicks you in the pants when you step on the gas. Fun stuff for around town! |
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