speedo error ... a mystery ...

[Godfrey DiGiorgi <ramarren@domain.elided>]

I have a small puzzle that I'm trying to work out with regard to 
speedometer gearing and error...

FrankenSpider is a '79 model chassis with '82 powertrain. I've got 
Yokohama 195/65 tires on the '82 cast alloy wheels. When I first put it 
together, I fitted the '82 differential and '82 speedo gearing to the 
transmission. The speedometer measured to be quite accurate, showing 
about 22 mph per 1000 RPM.

I fitted a '74 GTV rear axle, supposedly the same drive ratio as the 
original '79 Spider gearing. The speedometer is the original '79 unit 
still. At first I hadn't changed the speedometer drive gearing and I 
know the speedo was reading about 10% high... So I ordered a set of 
speedo drive gears from APE that were marked when I got them for '79 
Spider. They were fitted this weekend when I had the driveshaft 
overhauled. They were definitely different gears from what was in the 
transmission before. Now the speedo seems to be reading 58 mph at 2500 
rpm in top gear ... or about 23.5 mph per 1000 RPM. Huh? That's moved 
the speedo reading in the wrong direction; according to what I 
understood, the gearing should be about 19.5-20 mph per 1000RPM in top.

First off, what should the overall gearing be?

Second, what set of speedo drive gears in a Spider transmission would 
make the speedometer perform this way? I thought the Spider and GTV 
both came with the same final drive ratio until the Spider was fitted 
with the taller final drive in 1982?

thanks for your assistance. Please copy email to me directly if you 
respond as I often cannot get to the digest right now.

Godfrey
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