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What speedometer gear is needed for a ribcase in a Bugeye?

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AHS

AHS

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Hi all. I'm putting together a ribcase transmission for a 1959 Bygeye that I assume has the original rear gears (I'll check) which if I remember correctly is 4.56:1. This transmission is 1973 vintage which clearly came from a 3.90 ratio car so it struck me that the speedometer gear is probably wrong. This 1973 gear has 13 teeth.

I did some quick calcs and I came up with about 14 teeth is what I need. All the speedometers for Bugeye through at least 1973 are 1472 revs/mile and I believe there is only one worm gear which has 5 teeth. The Bugeye has 145R13 tires which are about 911.2 revolutions per mile.

So 911.2 revs/mile * 4.56 * 5 teeth / 1472 revs/mile = 14.11 teeth needed. If I pull a gear from a smoothcase, will it have 14 teeth?

Oddly with a 3.90 ratio I calculate that 12 teeth would be required, and I know that this 1973 transmission has never previously been apart, yet it has 13! 13 works out well for the 4.22 ratio.

Thanks,
Andy
 
Yes, its a 4.22 ratio but I would suggest changing to at least a 3.9 while you are at it.

Kurt.
 
I didn't know that there were gear options for the transmission- I think that the compensation came about only in the speedo head, Later cars had the 3" gauge (4" for the earlier cars) and the latest 1500's had a special one?
Not sure
BillM
 
Thanks all. I checked with the guys who put the car together and they said it's 4.22. I was told the 4.56 rear gear (I used to have one) actually comes from a Morris Minor - Bugeyes were all 4.22. I also pulled a speedo gear from a smoothcase and it was 13 teeth, so this all makes sense and 13 is what I need. The only part that doesn't make sense is why they didn't go to 12 teeth for a 3.90.
 
AHS said:
Thanks all. I checked with the guys who put the car together and they said it's 4.22. I was told the 4.56 rear gear (I used to have one) actually comes from a Morris Minor - Bugeyes were all 4.22. I also pulled a speedo gear from a smoothcase and it was 13 teeth, so this all makes sense and 13 is what I need. The only part that doesn't make sense is why they didn't go to 12 teeth for a 3.90.
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Other than a materials difference, none of the pinions are different in any other way; they are all 13 teeth. The parts books bear this out as there is no change or superseding part numbers all through the 1098 and 1275 range. Early smooth case pinions were steel instead of nylon, and the early pinion bush was brass, rather than nylon, so these do have different part numbers. Otherwise, they are all interchangeable. The difference is in the speedo gearing. If you look at the face of a speedo, you will see a 4 digit number which indicates "Turns Per Mile" or TPM. For example, a Bugeye speedo is numbered 1472, where a later small diameter speedo from the late 60's, early 70's will be 1376. I have samples of all of the above. The 3.7 did not appear until August '77, so those would have a different number still. I have one around, but can't lay my hands on it at the moment. An early Morris Minor is 1504.

If you saw how many different part numbers there are for speedos (different markets, both KPH and MPH), you'd see why it wouldn't make sense to use a different pinion gear for the gearbox. In addition, speedometers are much easier to recalibrate, and can be done today through many sources.
 
Thanks Gerard, you are exactly right. The speedo I have marked 1973 does indeed say 1376, I had misread it - those numbers are tiny! And the smoothcase gear I checked was a plastic gear in a brass carrier as you described, though I don't know the year. Was there ever a large steel dash speedometer marked 1376? I'd like to change my 67 to that ratio someday, but I suppose I'll need to find a speedometer too. By 1969 when the Sprites had 3.90 gears they had the padded dash and small instruments, at least here in the USA.

911.2 * 3.90 * 5 / 1376 = 12.9 teeth, so the math does work out pretty closely for the 3.90.

I have worked on a lot of GM transmissions with color coded speedometer gears. I suppose Americans do things one way, and the British another.
Andy
 
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