2003 XR100 gear ratios

I have tried to search on this several times but I really haven't come up with anything. I am mini road racing my XR100 and have been playing with sprockets but I have a spread sheet that will calculate speeds and ratios based on the transmission. I don't really think it would do too much for me but it is a tool to help me with gearing. I have been staying with the 46 tooth rear sprocket and adjusting the fronts to best match what track I am at. I wanted to use this tool to help me decide what size rear aluminum sprocket I want to buy next time without spending $200 on different sprocket sizes.

The last couple of tracks I ran a 16/46 sprocket with a bridgestone BT45 tire at 110/90-16 on the front and rear. I ran a 17 tooth front but it couldn't pull 5th gear well. The sprockets are both steel with a 420 chain conversion. I had a 120/70-17 rain tire on when I tried the 17 tooth front sprocket so that may have made some difference.

I just got a set of Sun rims in 2.75 front and rear and bought a set of 125 slicks last weekend to run on them. I think the rear is a 120/595-17 and the front is a 95/580-17 based on the bridgestone site. The math follows in theory...

16in = 406.4mm + 110/90 = 99mm tall = 505.4mm total

17in = 431.8mm + 95/580 = 55.1mm tall = 486.9mm total

17in = 431.8mm + 120/595 = 71.4mm tall = 503.2mm total

17in = 431.8mm + 120/70 = 84mm tall = 515.8mm total

So I couldn't pull the 17tooth front sprocket with a 46 tooth steel rear sprocket and a 120/70 tire. But I am curious of I can pull a 17/48 aluminum sprocket with a 120/595 slick that is 12.6mm shorter than the rain and has the reduced weight of the slick and the aluminum sprocket helping it out.

I have 13-17 front sprockets with a couple of chains that I swap out when I need to go shorter. I am trying to find the longest gearing that the bike will pull on a flat straight away and still have the ability to change the front for more grunt on the tighter courses and when the tracks have elevation. How much does the aluminum sprocket help out? I look at the other sites and I see a 36 rear aluminum. I wonder if the 36 can be pulled. A 13/36 is almost the same as the 17/46. I wonder if reducing that much reciprocating mass will allow the bike to run. I could try going between a 12 and 13 front sprocket and having a 36 tooth rear sprocket and see if I can race on those gearings.

Thanks for the help, I can't find the gear ratios anywhere. If anyone can offer up any information please do.

Thanks again,


I should have said that I have a 120 wiseco piston and with a hot cams cam and an ATC200x carb. It has a few more ponies than stock. I have a friend that has the super head and the inner rotor ignition but I haven't sprung for that kit yet.

Thanks for the help though,


Dont get the inner-rotor, there kick start killers.

well.. I tend to agree there too. I have raced it at Jennings Gp, Nashville and Barber during the weekends that I raced the big bike there. I am really just trying to decide which rear sprocket I want to buy by this whole topic or if I want to go ahead and get 2. Mabe the 36 and the 47 or 48 rear aluminum sprockets. That way I have a bunch of room to play.

Thanks for the responses.


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