Pablo's gs500 page
General tire info for the GS #1
R1 taillight install on 89-00 gs500
Clear Taillight Lens for 01-03 Models
My Local Canyons
What's it worth?
Progressive fork spring install-the lazy way
Rear shock swap information
More rear shock info-SV650 install
Suspension tuning 101
Installing Suburban-Machinery SV650 handlebars
Install an SV650 headlight
Converting standard 1/4 throttle to 1/5 throttle
Installing LED's in stock taillight
Uber Fender Eliminator
R1 Taillight on 89-00 GS500
Speedo/Tach light replacement
Better handlebar grips
Installing Buell turn signals - Rear
Installing Buell turn signals - Front
Flush mount rear (stock) turn signals
Installing a 2001 Ducati Monster fairing
Fix a broken oil filter cap bolt
General battery info
A Wal-Mart sealed battery?
Chain Maintenance
Installing SV650 Chainguard
Spark plug information
Installing a GSXR front end
Installing a GSXR swingarm
Installing GSXR rear passenger peg brackets
Installing an OEM Bandit 400 rear hugger
Installing a Bandit 400 rear wheel
Installing a GSXR rear wheel-simple swap
Polishing your wheels
"The Mystical Art of Tire Reading" and other tire info
General tire info for the GS #1
General tire info for the GS #2
General tire info for the GS #3 - mixing brands/types
gsJack's opinion on tires for the GS
Gearing and sprockets
Slip-on mufflers
Slip-on exhaust : using stock SV650 muffler
Installing an R6 tail
Installing GSX-R600 stock rearsets
Installing CBR600 stock rearsets
Replacing stock front pegs with CBR pegs
Replacing stock front pegs with Katana pegs
Why re-jet a stock US bike
More on re-jetting your carbs
Jetting for 2001 models

Everything you ever wanted to know about tires for the GS
(or at least what I could gather)...

Many say you should stick with stock sizes since the bike was meant to ride certain size tires and anything else upsets the handling. it is suggested that a 130/70 is sufficient on the rear for this bike.  Many tires run by various people are not approved by manufacturer for our narrow rear wheel. You run different size tires at your own risk.
That said, there are plenty that run 140/70, 150/70 and 150/60 tires on rear and love them. The people I have talked to do not have any handling complaints. Once you get into the 150 series you open yourself up to lots of really good racing/super sport rubber. Once I need to replace mine I'll likely go with either Metzeler Sportec M-1 150/60/17, which coincidentally ends up with a width of ~143mm and an overall diameter only 5mm bigger than stock, or Dunlop D205's. These are for sticky rubber not for having a really wide looking tire. A 150/70 will do that better and will *just* fit inside the brake stay bar. For more info on (easily) installing a GSXR rear brake stay for more clearance see the bandit rear wheel install page.
Nothing in particular to note about the front except that changing the size will likely alter your speedo reading (like it was ever accurate).  One rider installed a 120/70 front and mentioned it barely cleared hardware.
This from David R. Johnson, GS500 racer and contributor to Gstwin.comFor racing, the front tire size is the same as stock (110-70-17). For the rear, most people I know use 150-60-17s (stock is 130-70-17). The fitment guide will tell you the rim is too narrow for this tire. It is, but it does not result in bead failure, even under extreme racing conditions. It does modify the profile enough so that it impossible to get all the way to the edge of the tire. I can live with it. I once tried the other racing tire choice (Metzler 140-80-17) and hated it. It is taller tire than stock and a non-radial. The bike steered like a cow, flopped unpredictably into corners and felt very top heavy. When I switched to the 150 radials, I almost drove off the inside of the track because it turned so fast. Race rubber is available from Michelin (Pilot), Metzler (ME Z3), and Pirelli (Dragons) in sizes 110-70-17 and 150-60-17. I have ridden the Pilots and ME Z3s back to back, and could not feel any difference. One might assume that the Dragons are similar to the ME Z3s, since the Pirelli and Metzler are the same company, but the Dragons cost more.  
More to follow...

Stock 130/70/17 tire

Michelin Pilot Sport 150/60/17 rear tire

Stock-compare to edge of rim

150/60-compare to edge of rim

Special thanks to gsjack (resident tire expert) and many others from as well as those on forum.