So I've got the rear end off the car at the moment so it's a prime time to correct the speedo. Using the forumla posted to the sticky on the main thread, I've come up with the following combinations for P255/50ZR16 tires that I have on the rear:
Obviously the 18/45 is the closest, but does that combination actually work with the gears? Also, if I want to go back to the stock tire height (245's), I believe I calculated that using a 15/38 gear it's the closest for a 15 tooth drive, 17/43 is the closest for a 17, and 18 is off the chart so wouldn't be a good choice. Would it be best then to use a 15/38 and round up or a 17/43? Or should I go with 18/45 since that's the closest to the calculated?

 

keep in mind there are two different speedometer housings. one is for 34-39 tooth gears and the other for 40-45 tooth gears. avoid the plastic housings all together. if you have one and upgrade to an aluminum housing you will need to get the appropriate ALL plastic driven gears. the plastic housing use a plastic driven gear on a metal shaft and will not interchange.

as for what gears to use whatever it s closest but you may need to swap (and source) housings/gears. i used teal automotive (tealautomotiveinc.com) to source all my parts.

 

So I've got the electric speedo which looks like it uses a plastic housing for the driven gear, with a metal shaft and a plastic driven gear. I tried looking for the numbers of gears acceptable on it, should it be the same set as the sets you mentioned? It was 2.73 originally, and it has a 17 tooth drive gear, so it must be the 34-39 tooth set right? In that case I'd have to get a 15 tooth drive and 37/38 driven if I want to use the same housing?

 

What do the tires actually MEASURE? (Pull a tape measure from the center of the wheel to the ground, double it, THAT'S your "real world" diameter)

If I was the betting kind (I'm not), I'd bet MONEY that all your "calculation" crap about tire diameter, compared to TAPE MEASURE REALITY, is farther off than the worst error of any of those gears. Go ahead, I dare you, go ahead and post it.

I'd also bet that you can let 3 psi out of the tires and put in 3 extra psi, and ALSO get more error than any of those gears.

Furthermore, I'd bet that if you take the 9/32" tread depth new, and subtract the 2/32" tread depth worn, and double THAT to find out the change in diameter just over the lifetime of the tires, THAT will also have more error than the worst of any of those gears.

"Measure with micrometer, mark with chalk, cut with axe"; except in reverse.

 

While I agree that it'd be best to have the actual tire diameter while it's on the ground, I have the rear axle off the car right now and don't have any measurements from before I took it off (fixing the speedo was more of an afterthought). Since the sticky says to use the unloaded, new tire diameter, I presume they did the math and got it close enough. It doesn't matter if it's a little off, I just don't want to have it reading 90 when I'm actually going 75 anymore. I'll probably measure once I get the car back together just to see how far off it makes it.

The main purpose of this post was to make sure that the gear combos would actually mesh together properly. I forgot about the housing, so thanks for reminding morgsie. I guess I'll have to use a 15 tooth drive and 37 tooth driven so I don't have to find another housing. It actually came with a 37 tooth driven gear (red) so I'll only have to get a drive gear, at least for now. Hopefully I can find somewhere that has it before this weekend, otherwise might have to either hit the junkyard or just forgo fixing the speedo right now.

Thanks for the replies everyone!