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How does overdrive Annulus effect speedometer

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F Schreiber

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I figure my speedo reads higher than it should. I have a BN7 and the Eng, trans and OD unit was swapped out at one time or another. 3000 engine, and I believe i have a 100-6 trans and OD. AEC3174 trans, and 28/1447 OD unit.
So I have an 28/1447 overdrive instead of a 22/3009 and I have the 3.909 rear end. Moss catalog shows different annulus (item 143) and speedometer pinion assemblies ( 168, 169) between those two units. BN2,4,6 is different than the BN7, BT7, BJ7, BJ8. I am figure those parts are what they use to compensate for the different ratio rear ends.

If so can i swap out those items 143, 168, 169 to get my speedo to read correctly and can I swap those parts without swapping out the whole overdrive. Are they interchangeable???? Will And will any of these 100-6 OD and 3000 OD units fit on any of the 100_6 or 3000 transmissions. I know the output will be different if output is different but are they physically interchangeable.
 
The different annuluses (annuli?) are what's used to achieve engine speed reduction (or output speed increase) between the 22% and 28% ODs. The angle drives are different between the BN2 and later cars (note the price delta). Interesting question, though; I've always assumed the angle drives were 1:1 but, now that I think of it, probably not (speedo cable probably can't handle 3000RPM, so the adapters probably reduce to 2:1 or even more; either way, they are likely the same reduction for all models).

What you need is a ratio adapter or a different speedometer gearset. There are several places that sell ratio adapters, they will want you to measure how many cable turns it takes to go a certain distance. I got a gearset change when I went to a 3.54 rear-end in my BJ8 from the standard 3.9, but your situation is an odd duck and a ratio adapter is probably the easiest way to go.
 
+1 on the ratio adapter. If you are happy with the current setup and don't plan any additional gearing changes the ratio adapter is a one-time expense. The last time I checked they were typically around $150 plus perhaps the cost of a new speedometer cable (which will be modified by the shop to accommodate the ratio converter).
 
And now that I look at the moss drawing, I see the annulus has teeth that mesh with the carrier assy, and those parts are different for different ratios. So I can't change the annulus without getting a different ratio. So looks like with 28% ratio OD, getting the ratio adapter is only way out, I would never that thought of that. Thank you. If you know who sells something compatable with a healey let me know. I will start checking around. Plus my tires are the 185/70 -15 that might have changed things a bit.
 
There are other ways but I assume you want your dash to look traditional. To name a few, there are cell phone apps, stand alone GPS units, GPS portable speedometers that lack an odometer, GPS in-dash speedometers that include an odometer and trip meter, and there are bicycle computers that can be adapted. All will work, some are less expensive than others. However, the ratio adapter is a clean way to keep your original speedometer.
 
If you know how many turns your speedometer cable makes per mile, an instrument shop can change the speedometer to match. If you can roll the car forward 52.8 feet while counting the turns of the speedometer cable, and multiply by 100, you will get a good approximation.
 
Palo Alto Speedometer in Palo Alto (duh) did the gear swap for me (and returned the originals). That corrects the odometer and tripmeter, the speed indicator is a bit of alchemy (which they neglected to do the first time, but a CHP officer was kind enough to point out the error in my velocity).
 
I just use an app on my cellphone. A glance at it every once in a while is all I need__the speed of a Healey doesn't drift upwards with near the unintentional ease of some of our other cars (that I DO need to near-constantly monitor...).
 
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