I've tried looking for this information on the forum and I haven't found a specific thread that lists all the different speedometers that appeared in a GT. This could be a very useful thread to cover the different W numbers, what years of the GT they could be found, and KPH or MPH. Talk about the calculation involved to understand error, etc. This could be a good thread to sticky in this forum.
Speedo details I know for sure about...
My 1973 GT, produced in January of '73, has a W1020 MPH speedo and a 4 speed manual. I don't know if the transmission type mattered in the last year of production. I also have a W897 MPH speedo from Gordo (thanks BTW). I've seen W1062 MPH, W1094 MPH, and W660 km/h speedos. I don't know what years you could find these in a GT and if they were transmission specific. I've seen other potential W numbers listed online but I can't confirm what vehicle you'd find them in.
To calculate the W number, I believe the following equation works? I pulled it out of Dieter's speedo calculator. -This has been verified, see thread.
W No. = unit x rear axle gear ratio x gearbox screw wheel / tire circumference / speedo drive teeth.
Note: this equation can also be done as (unit x rear axle gear ratio x gearbox screw wheel) / (tire circumference x speedo drive teeth). Both ways will get you the same answer, following the order of operations.
Unit number - 1000 for km/h or 1609 for MPH. This value is specific to how many meters per hour. 1609 more accurately would be 1609.344 meters in 1 mile.
Rear axle gear ratio - stock GT's will be 3.44, a rare Kadett ratio is 3.18 and a common performance ratios are 3.67, 3.89, and 4.11.
Gearbox screw wheel - 7 for the GT and Manta, the Kadett has a 5 I believe.
Speedo drive teeth - this is the plastic gear located in the transmission which drives the speedo cable. There are 3 options, always done by color. Red have 18 teeth, yellow have 19 teeth, and blue have 20 teeth.
Tire circumference - you could add tire slippage to this value, or use the actual tire circumference. For choosing a speedo and speedo drive gear, it's probably best to not factor slippage into the calculation because slippage is not constant. Wet versus dry driving conditions alone would be different. Tire slippage can also vary a lot by the exact tire being used, too hard to know how much slippage there would be to help setup a speedo. For this calculation, tire circumference needs to be in meters.
% of Error = ((calculated - actual) / calculated) x 100
I don't know how easy or hard it would be to modify a speedo to better match the calculated W number. It might be easier to create a different speedo drive gear option that has more or less teeth than the standard 3 options for this, no one has done it that I know of however. I'm adding this to my gear calculator which focuses on tires, transmission gears, final gears, and vehicle speed. I'll upload it to this thread when I'm done.