# yz85 speed

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so Im trying to figure top speed for an 85 im building. I'm playing with different sprocket ratios. But when I do the math I'm running into numbers like 103 mph. I know the 6th gear is direct 1 to 1. im looking at a 13 tooth front and a 47 rear. everything i read says red line is 15000 so i put it at 6000 engine rpm just to see and im getting 103 mph. I'm thinking maybe a ratio between the gear on the crank and the gear on the clutch basket?? I cant figure it out and I know its probably something stupid im not looking at. If anyone knows the answer to my predicament, help is appreciated.

Edited by wiley13

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I wouldn't over-think the top speed of your bike. If you care about top speed (which nobody else does) just take it out and top it out in front of a radar gun or next to a car. there are a lot of variables that come into play like wind resistance and whether or not reaching 15,000 rpms is even possible in 6th gear with such a weak engine and increasing winds. (think about being broad side in an 80 mph head-wind and trying to pedal to the top speed of your top gear on your bicycle)

I bet your bike would hit high 60's or low 70s

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Well the problem is we do every type of riding we can get our hands on. And its not me riding it it will be my little sister. She doesnt like her ttr so were putting her on this and seeing how she does. Her clutch work isnt the greatest so were trying to gear it to make that easier. But on the other hand, i dont want to be on a gravel road putting in 3rd while shes screamin 6th trying to keep up. Theres math to do it, i do it with trucks all the time. I just dont know wats messin me up on this bike. And by the way, those of us who CAN scoot across the desert at 60 and dont want to be revvin it out the whole time do care about top speed.

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I found the following info for a 2005 yz85.

I'm guessing the primary gear ratio is the primary drive gear to clutch basket ratio, and the secondary gear ratio is the sprocket ratio

Did you consider the primary?

What number did you use for wheel circumference?

• Primary Reduction Ratio: 65/18 (3.611)
• Secondary Reduction Ratio: 47/14 (3.357)
• Gear Ratio – 1st Gear: 27/11 (2.454)
• Gear Ratio – 2nd Gear: 32/17 (1.882)
• Gear Ratio – 3rd Gear: 26/17 (1.529)
• Gear Ratio – 4th Gear: 22/17 (1.294)
• Gear Ratio – 5th Gear: 26/23 (1.130)
• Gear Ratio – 6th Gear: 25/25 (1.000)

EDIT: Just taking a guess of 20" for the rear wheel diameter (14" rim + 6"rubber) I get 49 MPH at 10,000 RPM in 6th.

Edited by rjpjnk

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The primary ratio is what i needed thank you very much. Plugging that in with a tire circumference of aprox 66.21 inches and a 14/47 sprocket combo, 6000 rpms is good for 28.78 mph. To get to 60 mph, the little screamer has to run 12505.68 rpms. Thats quite a bit but i dont picture my sister doing 60 all too often. Should work great. Thanks for the research.