How does one speedo gear work with all gears??

I know the speedo gears for the T5 say they're most accurate for the 3.55's, and decrease in accuracy up to 4.10's. I'd guess they can't accurately and economically make a gear that's 100% accurate for 3.73's or 4.10's, so they just sell the 3.55's. Maybe it's the same way with the AOD.

I've heard about a thing that allows you to run any rear end/tire combo and change for it, but it's like $200 if I remember right. I think JRichker or HISSIN50 told me about it a couple years ago.

The 23 tooth gear will be close but the problem is the drive gear inside your tranny. The 90-95 t5's have an 8 tooth drive gear. To really get it right you will need a 6 tooth drive gear and a 20 tooth speedo gear. This is all assuming that you have stock height tires.

One speedo driven gear can't be good for all those different gears. What they probably mean is that it is good for those gears if you also have the appropriate speedo drive gear to go with the combination -- 6-tooth for 4.10s, 7-tooth for 3.73s, etc.

Even if you buy the "correct" gear, your speedo still might be off a little since it's rare that any gear will get it 100% accurate.

here's your answer...

The stock range of speedo gears work correctly with a limited range of tire sizes.

See http://www.corral.net/tech/gearcalc.html for more help

Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy with within 1%-3%. The drawback is that it isn't cheap.
Please excuse the typo (their's, not mine) in the following picture - it should read Custom ratio adapters, not Custom radio adapters.

See http://www.gaugeguys.com/ratioadapt.htm for more info.