If I'm understanding correctly you want to just add a little more rubber, for more protection for the wheels (i.e pot holes or construction) i would go with 235 it's not as wide as the 245 but it will reduce some of the height of the tire and that will keep you from scrubbing. If you can compare them side by side at the same time they should be about even in height but the 235 will be not as wide
When you go up in size ( 30, 35,40) The actual tire itself is becoming taller in height. Take that in consideration and how much clearance you will have in your fender and how the suspension will adjust while you are driving. I'm not saying that you can't do it but if you can compare them you will see that the 40 are taller and will take up more space in your fender.
The best tire size calculator. Use our tire size calculator to find tire size specs and suggested tire sizes. Then check prices for all available tire sizes.
This should help I use this as a guideline when people come in and want to go from 17 in to 20 in. You put your factory tires size in and then what you would like to change them to. It will give you the difference between both
So you have 30's front and rear right now. I am pretty sure you can go 35 but not sure about 40. I know the 19" rime have 35's but as SSG said you need to check your clearances. Several members have changed their tire sizes hopefully someone will chime in and clear it up for you.