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I have a 1982 KZ 650. It came with 130/90-16" rear tire. Chain is 630. Stock sprockets were 33 / 13, which gives a sprocket ratio of 2.54. Your 38 / 17 sprockets give a 2.23 ratio, so you are already running fewer RPM than I am - at any given speed.

Now I tried a rediculous arrangement of a 26 / 13, which gives a 2.00 sprocket ratio. Got the rear sprocket from Rebel gears. The bike started off OK for normal riding, but don't plan on getting in a hurry with it. On the highway it did well if you were cruising at around 65 MPH or less, or had a strong tailwind, or some downhill runs. But if there was much of a headwind, or you increased the speed, the engine just did not have the torque, and you notice that you have the throttle open quite wide just to maintain a given speed. Under light ideal conditions the bike got some great gas mileage, but the harder it was pushed, the worse it got, until it was terrible.

I am now running a 140 rear tire and 33 / 14 sprockets, which gives a little lower than a 2.36 ratio, and I consider it to be ideal. I recommend that you are already at an optimum ratio. Allowing the engine to rev gives it more torque multiplication through the gears and transmission, which results in lowering the pull on the engine. It runs much more freely; has better acceleration; and can push through a headwind or uphill much more easily.

I agree WABBMW. I know I'm close to the optimum ratio. Quite possibly at it, like you mentioned. I would still like to give a one tooth smaller rear sprocket a shot.

If I can pick up a used sprocket on the cheap, the labor is not a problem for me. I don't want to buy a new, one off item, and find out it was a mistake.....

I wonder if the kz650's and kz750's have the same mounting pattern on there rear sprocket's ?? If not, I wonder if the sprocket hubs can be swapped out from one to the other ??

650/750/750 twin/900/1000 use the same sprocket pattern, so does ZX12
Not sure what others will fit.

Save yourself some time and money, go to JT Sprockets (Every time I type that I think of the Jetsons) and look up the sprocket for your bike. Take note of the dimensions listed for the rear sprocket, take those dimensions and compare them to other bikes on the same website.

To save time, just look up the rear sprocket for a 1980 KZ650 and search using the first part of the part number.

I just happen to know that it is JTR488 because it also fits a 750 twin. A search using that part number will show you a list of every bike that uses the same bolt pattern and 530 pitch.

I've already done that, and no bike using that bolt pattern has less than 38 teeth.

I have a ZX12 LSR bike, and the ZX12 comes stock with 45 tooth rear, but I need sprockets with a lot fewer teeth to make the bike go faster (it has enough hp). And KZ650 rear sprockets are a crap load cheaper than ZX12 rear sprockets, even though they are the same, so I shopped eBay for cheap 650 sprockets.
Thanks to JT sprockets allowing me to see what will work.

Fantastic info steell !! I will do as you've suggested.....

Thanks !!

I remember the Jetson's. I watched that cartoon a lot.....Many, many, years agoooooo..........LOL

What the !! The 630 pitch sprockets for the kz750's go down to a 32t One of these should be able to be milled down to the same thickness as the 530 pitch sprockets.....shouldn't they ?? I'm getting a small mill in the near future. I think I've got a project for it already....

Never mind, I just remembered the teeth spacing is different from the 530 to 630 chains...

No eBay APP ID and/or Cert ID defined in Kunena configurationNo eBay APP ID and/or Cert ID defined in Kunena configurationI did find these on ebay.....





How hard could it be to center the sprocket over / under another sprocket, and drill out proper mounting holes if needed ??

Just out of curiosity what is your RPM at 70-75? Why are you trying to lower the RPM? I regularly cruise at 75mph on my KZ750, with one tooth up from stock on the front and the tach shows 5500. Correct me if i'm wrong but i don't think operating the RPM in this range for extended periods of time is bad as long as everything is in working order.

It was running 5500rpm at 65mph, and 6500rpm at 75mph. With the 17t counter, I got it to 5500rpm at 69mph, and 6500rpm at 79mph. I'd be extremely happy if I could get my bike to run at the same rpm and speeds as yours !!

IMHO my bike is working to hard at HWY speeds.

DoctoRot 750E1 with 18" rear tire meet skooterbum4real 750H with 16" rear tire.

As far as drilling new holes. The first 750 twin with Morris Mags that I bought had a black steel solid industrial sprocket on it that someone had drilled to fit. Would have been better if the hole pattern had been on center

LOL !!!