Speedometer compatibility

Aloha guys.

Long story short, I'm planning to purchase GPz750(A) gauges for my KZ/Gpz750. Question I have is how much of a difference does this makes? What I mean is that our KZ came with 19 front. GPz had 18. Went to Partzilla to compare the speedo gear housing and they both had same part number on both gears. So my guess is that speedo gear(?) inside the meter is different.

I'm buying this because I need something for now and also the cost of the gauges. Both of my tach and speedo is out because my original speedo was out of whack and my attempt to repair it was unsuccessful (it mean it fell apart).Unlike the other KZs, my gauges are in one plastic housing so if speedo is out, I need to remove the housing to put in new/old gauges. I will get correct or at least round KZ style gauges later down the line, but for now, I'm just trying to make the bike work. Also, I have hard time getting both as set as most of the KZ up until 82 or so came with mechanical tach drive. Mine is electrical. I know I can get the tach without any problem, my problem is with the speedo.

Anyway, anybody know how much of a difference does this makes? Once the bike is completed, bike will go to my son and I don't want him to get ticket because of this. If it shows little faster, no problem as actual speed should be slower. But if it does show slower, then bike is going little faster then what it actually shows, so I want to know how much of a difference. Is there like a calculator somewhere?

Speed's do not have internal gearing. They are driven by magnetic field. 
It's more likely the face is printed slightly different with slightly larger gaps between the digits. 
19 to 18 inch wheel circumference is a 5% difference. 

Tire size is also a factor. I've done the math on a number of models & I think you'll be fine.
The ZX750A front wheel circumference works out to about 6.75', so it rotates about 782 revolutions per mile (5280/6.75)
& spins the speedo ~2260 revs per mile (782 x 2.89 speedo drive ratio).
Kawasaki speedos are marked 2240 revs per mile, so the difference is about 1% (2240/2260 or vice versa).
The 82 750-R1 spins the speedo about 2234 revs per mile. not much difference (2234/2260).

The 2240 is kind of a Japanese standard. Aftermarket speedos are often listed as 2240@60mph.
At 60 mph you'll cover 1 mile per minute, which matches the 2240 revs per mile that's marked on Kawasaki & other Japanese speedos.
The math on a number of stock setups doesn't equal the 2240 exactly, but it's close enough.
 

One upon a time, I dove into this issue by looking at Kawasaki speedometer drive gears. As I recall, I found exactly two gear ratios to cover the entire KZ line and ZX and ZG models. I suspect this is the real reason Kawasaki speedometers have a reputation for being optimistic, that is, reading faster than the bike is actually traveling. All of the ones I have owned have read between 5 and 10 percent faster than true speed, except one. My ex rode a 1997 Concours that was almost spot on. As Martin says, I think you will be hard pressed to notice a difference in your case.
If the new combination proves to be too far off for your comfort, it is likely that you can move the indicator needle to give you a more satisfying reading. There is a tutorial over at ZRXOA if you are really interested. 

Good thing about the 2240 is that any variance from that number usually keeps the indicated speed within an acceptable range.
Every speedo drive unit I've checked has a 9T pinion but there are at least 4 different speedometer drive gears: 23T 24T 25T 26T. Most KZs are 26T.  Plus the Front wheel size varies, from 16" to 19", and tire size varies as well. Some models w 18" Fr wheel use a 26T speedo drive gear, while others such as a 2006 Concours have a 25T.  I think Kawasaki was able to mix n match the various options to keep the speedos within a certain range of accuracy.

Not sure about speedo's but kawasaki seem to be the odd one out when it comes to drive ratios on rev counters. Kawa use 5:1 whereas everyone else uses 7:1.
Found this by chance when I wanted a tiny 2 inch rev counter for a specific build. Fitted it and at 1k tickover the guage read 1400.
As it was the only guage I could find that fitted the design I was after so I had to make an inline reduction gearbox. 
 

Thanks for all the info guys. I didn't know much about the speedo and this was a great tutorials for me. I did do some search but now I know what I need to look for.

It's a good news that it won't be off by much, and it reads faster than it actually is. As I mentioned, it's for my boy so I didn't want him to get ticket by going faster than the limit (although somewhere down the line, I know he will but not for now).

When I used to ride this baby, I got caught several ties but cops for some reason, they always let me go without ticket. I know that is a rare occasion nowdays, maybe never..................

I didn't know much either but wondered about the 2240 revs = 1 mile stamped onto the back side of several speedos that I had. 
I think I started w the speedo drive gear:  since it turns w the wheel & has more teeth than the pinion, the pinion has to spin more times per wheel revolution:  26/9 = 2.89 speedo drive ratio rounded.

Then I figured you'd need to know the circumference of the tire. Easy way is to measure the radius of the Fr wheel from the garage floor to the center of the axle: C = 2(Pi)R.  Mine measured a little under 13", so I used that number for convenience. hee.  The numbers on a tire can be used to figure out the circumference. sizes are usually listed in the parts diagrams: this is what I did in reply above. 

Since the back of the speedo said revs per mile, you'd need to know how many times the wheel spins per mile, so I divided 5280 by C which was 6.81 in my case: 5280/6.81 = 775 wheel revolutions per mile. The pinion spins 2.89 times per wheel revolution so:  775 x 2.89 = 2240.  Using the actual radius (~12-7/8"), the number is about 2263.  From the size on the tire, it works out to 2234.