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  #1  
Old 18-10-2008
Gary Peters Gary Peters is offline
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Top speed formula

Gday everyone, Can anyone tell me the formula to work out the top speed, knowing the final drive ratio,tyre circumference, top gear ratio of 1:1. Thanks, I need to know what a lower profile tyre will do to my top speed.
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Old 18-10-2008
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Thumbs up I have a spreadsheet

I found a spreadsheet on the net but no longer have the link to where I got it from. If you PM me your email address, I will send you the file.
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Old 18-10-2008
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Methinks what you want is % difference.
http://www.blackdog.net/games/math/percent.html
Measure the circumference of both tyres/wheels with a tape measure.
Or mark a point on each tyre and roll it along around once like a distance wheel.
Then convert those two measurements to their percentage expressed difference.

Then for example; say you have 5% more wheel rotations; for the same engine revs.
Then you would be going 5% faster for the same amount of wheel/tyre revolutions.

Say your top gear is usable now from 80K to 140K:
Work out how that 60k range would change by starting at the middle point (in speed) of each gear and widening it up by 5% total.
You could also change the number of teeth on your rear sprocket by 5% for a nil change overall; if you wanted to.

http://en.wikipedia.org is useful for this sort of stuff.
And google calculator for google's calculator.

ℬγℯ.

Edit: From what you said below Gary:
2150 ⇛ 2070 lose the zeros.
= 215 ⇛ 207 then halve it.
= 107.5 ⇛ 103.5 The differance = 4
Which is very close to a change of 4 % change in speed. (per cent = per hundred, as "centum" means hundred from Latin)
So doing 107 kph before is doing 103 kph now.
4 % is 2 teeth on a 50 tooth sprocket.
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Old 19-10-2008
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baileyana baileyana is offline
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to double speed, of your vehcile you need
4 times the power to overcome
8 times the air resistance

ie if 100 hp = 100kph, 200 kph needs 400 hp
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Old 19-10-2008
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http://www.gearingcommander.com/

Dont break your brain use this, it has all the data pre loaded for a DRZ and many other bikes. Takes some time to load be patient.
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Old 19-10-2008
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Gidday Gary, can your bike hit the rev limiter in top at the moment? If so, fitting a lower profile tyre will slightly lower your top speed. If it doesn't hit the rev limiter, sometime the lower gearing a lower profile tyre affords you, can actually give you a bit extra torque and give you a higher top speed.

There a some cars that go faster in 4th gear than they do in 5th, cause there's not enough torque in 5th to overcome the huge wind and friction resistances.

Also, be aware that a lower profile tyre will slow down your steering. Are you running road or dirt tyres?
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Old 19-10-2008
Gary Peters Gary Peters is offline
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Thanks everyone for your help. The problem I have is that I bought new chain & sprockets to stay with the same ratio that I've had for a while and then got a new Kenda tyre and discovered that it was much lower profile,2070mm circumference compared to the old worn out tyre at 2150mm.
I'm going on a fairly long ride this weekend and don't want to discover that it took away something silly like 20 klm/h, as it's geared pretty low already.Haven't had a chance to check out the sugested web sites as yet but will do in a minute. Thanks again all
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Old 22-10-2008
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KPH= (RPM x Loaded Tyre Radius in mm x 2) div by (overall gear ratio x 5305). New Ratio needed= (New Tyre Diameter div by Old Tyre Diameter)X Original gear ratio.
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