Drivetrain Speedo Gear? 87 Gt, T-5z, 3.55, 255/40/17

I've got an 87 5.0 with 3.55 gears. I recently installed a rebuilt T-5z, and plan to install some 255/40/17 rubber on 17x8.5's. What speedo gear will I need?

As an aside, I assume (since the LRS website states "Requires a 1/8" spacer when used on all 79-86 cars or 87-93 4cyl cars.") that these will fit my GT without rubbing...?

Exile said:

3:55 gears take a black 20 tooth speedometer gear.
http://www.latemodelrestoration.com/item/LRS-17271B-K/83-95-Mustang-20-Tooth-Speedo-Gear-Kit-Manual

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Is this regardless of the drive gear? And, doesn't overall tire height fit into it? Based on the link below, it seems like 255/40 might be just outside a stock height tire (255/45s would be perfect, for comparison)
http://www.discounttiredirect.com/d...1987&mk=FORD&selectedModel=MUSTANG&vid=006900

Mustang5L5 said:

Considering you mention you rebuilt the T-5Z...which speedo drive gear did you use? Black 8-tooth, or yellow 7-tooth?

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I didn't personally rebuilt the tranny...and I didn't check before it was installed. What, aside from the obvious, is the difference between the two drive gears? And which driven gear would be appropriate depending on the drive gears?