KZ1000 LTD - Speedo 20% off, what's going on?

I bought a set of gauges for my KZ (it came without) and when I put on the speedo I realized it was about 20% of (shows 90 mph when I did 75 mph, measured with a GPS). I tried another speedo with the exact same results.

I have the original front wheel and tire, what else could be wrong? I am not sure my speedo is for my exact model year. I thought the gauges for all KZ 1000s trough 79 were the same, is that true?

Thanks for any help, it kinda sucks having to do the math all the time

Kawasaki used 18"diameter front wheels on the 1978 Z1R's. Those bikes have different gearing in the hub speedo drive.

Other than that one model, all the other early KZ1000's, KZ900, KZ650's use the same speed drive and came with 19" front wheels.

What diameter front wheel do you have?

My frontwheel is a 19".

However the tire is a 3.25-19 Avon Speed master. I don't know what the stock tire size is, but assuming stock is 100-100/19 ( I am just guessing here), my tire circumference is about 5.5% less which would cause a 5.5% "faster" speedo. Source: www.gearingcommander.com/

Can someone tell me what the original front tire size is?