RPM/Speed vs Sprocket Size

[Alan_nc]

I know this has been covered but can't find what I'm looking for.

02       140/70/17 rear tire,  39 tooth rear sprocket

In 6th gear   60 MPH = 4600 RPM (as best I can tell on the tach)
In 6th gear   65 MPH = 5000 RPM 

Can you tell me what tooth front sprocket I'm running?

I feel like the bike bogs off the line unless I slip the clutch.  Runs great once your past 10 MPH.

[radodrill]

I feel like the bike bogs off the line unless I slip the clutch.  Runs great once your past 10 MPH.

The question is also what jet sizes you're running in the carbs.  With the stock jets my bike didn't launch to well or pull very hard when really going on the throttle; after a rejet with 5 sizes larger on the main it feels like a whole new animal and is a much more spirited ride.

[gsJack]

Both my 97 and 02 GSs indicated about 4800 rpm @ 60mph stock but I use the 4730 from the 1st published road test when calculating.  For a standard size 140/70 tire the rpm would be 4634, I'd guess you have a stock 16T front sprocket:

4730x24.2/24.7=4634

My 02 has a Roadrider rear tire on it now and they are specked at 25.2" dia about the same as a standard size 150/70.  I like the standard gearing on the 02 GS and have a 15T front sprocket on now to bring the rpm up to a tad over stock which I like better than a little under:

4730x24.2/25.2=4542
4542x16/15=4844

You might like a 15T front with your oversize rear tire too.

http://www.gs500.net/gallery/data/500/GS500gearing-rpms.jpg
http://www.gs500.net/gallery/data/500/CalcTireDims.jpg

On my first GS, the 97, it took about 4000 rpm with clutch slippage until the bike caught up to the engine for a quick smooth launch.  The 02 took about 3000 rpm when stock.  Feels so normal after a short time I never gave much thought to it.  Both my 97 and 02 had/have stock carbs all the way.

[Alan_nc]

Thanks.  Guess it must be a 16 tooth.  I like the suggestion of a 15 tooth to try to improve launch.  They seem to be going for $12 to 15 on Ebay (including shipping) so it wouldn't be much of a cost to give it a try.

[Joolstacho]

Lots of little variables here I think. Speedo is driven by the front wheel so front tyre size change or wear will affect the speed reading. Cables, heads aren't perfect either.
On my first decent run (about 500k's) on my '89 with standard new sprockets, 70% worn front tyre, new 140/70 rear, I seemed to be revving higher, (clutch is fine) but the needles waver so it's hard to pin down exact revs and speeds. I guess a gps check would help with the speed accuracy. I found the gearing a touch 'long' in city crawl traffic, but no problem launching at all.