Im in the market for new semi slicks and Im trying to decide on the tyre width front and rear. I dont want to adopt the 'as much as you can squeeze in the wheel well' approach. Instead, I want to select a tyre width that is appropriate for the setup I have, say 160rwkw,1200kg SW20, 40/60 weight distribution etc. From what I can see for 17" wheels, each 10mm increase in width results in a 1kg increase in tyre weight roughly. I use the car mostly on the circuit with occasional hill climbs.
 
Does anybody know of a way to determine the correct width and the stagger between front and rear??
 
Once I have pushed the limits of the current setup I'll up the power, tyre width etc to suit.

Your rim widths front and rear are going to be the major factor for your tyre sizing.

215 is plenty for the front and lighter than the 225 most run. In alot of cases means more space to the suspension as well.
 
For the rear i am a firm believer in 255, and for the future this is where you would want to be with the extra power.
 
In my opinion that amount of tyre is worth the weight penalty.

I think more on the front if possible but 215 is a good start. If you are running a bigger brake system, 235 will help take advantage of it( more grip under heavy braking and more mid corner grip).
For the rear, anything from 245 up will be fine. The Porsche teams run a tyre with a higher profile to create a 'baggy' tyre. From what I can gather, this helps settle the car under heavy load out of corners. It does work for these teams and may work for our cars but its a bit of a unknown. 
Just remember that the balance can change with different tyres so the suspension/bump/rebound/ geometry settings also need tuning to maximise the new tyre.
Basically, if the car is not set up right the tyres may not heat up enough. It may cause locking of the tyre under brakes wrecking the tyre, it can overheat the tyre too and then there is the effect the balance of the car. After talking with a few drivers from the MR2 Production GT days, they ran 6mm toe and 3 degrees camber on the rear and front to get the tyres(240mm/16 all round) up to temp.
I'm sure you suspension mech is up to speed on all this and will be able to set the car up to suit what ever tyres you end up using.
Whatever tyre you use, make sure you follow a program of bedding in the tyres and warm the tyres sympathetically so you do not destroy the life of the tyre.  
But before you fit the new ones, you must manage a 1.05.5 at Baskerville.
 
 
 

thanks Trav and Steve. Im trying to approach this scientifically hence the post. there is such little information specific to the SW20 so its hard to know. they only thing I can conclude for now is that my local track is very technical with short straights therefore a wider tyre is more beneficial.
 
steve I wont be breaking any personal best on these tyres as I was checking the suspension yesterday and noticed the inside edge on the front tyres have no rubber left at all! I need to get a new set quick as I want to do more laps soon.

Lol, well I did notice the tyre wear! The 12hr and production championship cars ran the same size all round which was 240mm. This is what Ric Shaw recommended I use and did infact sell a set for the set up. I sold these off for one reason or another. I think 235 front and 245/255 back would be ok.