I have a 18x7.5+38 wheels..How wide can i go with tires
 

Need your help guys..

You could probably rock a 245, but for optimal performance, I wouldn't run anything other than a 225/40-18.

Yeah, I have 18x7.5 +48 and chose to go with 225/40/18. You can go with tires as wide as 245/255 but I still wanted a square setup so I can rotate as necessary and reduce understeer some say may come with a staggered setup (although I'm pretty sure you won't really feel a slight stagger). But really one of the main reasons I went with 225/40/18 is because there are a lot of tires made in this size and you have many options from many different tire manufacturers at really good prices - I recently picked up some Goodyear Eagle F1 Asymmetric 2 summer tires for $90/ea after rebate.

As an aside point unrelated to tire width, you can also go with lower profile 35 tires; 40's will be ~12mm larger than stock and 35's will be ~11mm smaller than stock, your choice. Ride quality will suffer some with 35s (not to mention potholes have a higher chance of doing some damage). Both the 40 and 35 are well within the 3% within stock rule of thumb. But again, wider lower profile tires will be more expensive.