Besides the obvious approximate 1 inch difference

What are the off road and on road differences

Any opinions welcomed

Thanks

I have 30x9.50's and they fit fine with only a two inch body lift and my mileage is still great!

A 30x9.5 is a lot easier to fit inside your fenders and a lot less meat to turn.  On the other hand, sometimes that extra inch is all it takes to help a rig over something.

i went from 33x12.5 to 31x10.5 recently.  gas milage is around low 20's but that is with onroad and offroad driving.  i have a 2" body lift with homemade bumpers.  the inner front fenders are taken out however.  previous owner did that. i'm extremely happy with the way they fit.   maybe someone will jump in that had both. 

I run a 30 on mine.  want to up grade to 32 next time.  but stock gearing for now 30 inch is about the max. say goodbye to 5th unless u pump up the motor and have a lot of flat ground though

I found a set of bfg A\Ts 30X9.5X15 for $350 so I have to decide on the cheaper smaller tire or the 31's at around $500 to $600

Go with the 30's. I ran 31's for 3 years and it was ALL she wanted to turn. Like yzf said say goodbye to 5th. The 30'2 will also fit better in the fenders. They will fit even better with a 2" coil lift, hint, hint. 

I had 31" road kings for a few years and with stock gears it really was rare to get into 5th gear. I run 30" BFG muds right now and I love them. I"m running 462's now but they work great.

Either tire will require wider rims, at least 7" if you follow the manufacturer's rim width spec. A larger, heavier tire reduces acceleration. Because these diameters are about 9-13% larger than stock, your speedo will read low by that much (but it is adjustable). If you have an early 5th gear ratio, it will become pretty useless.  The later (88.5+) 5th gears were about 9% lower and will work better, but neither will be good.  A 4:1 transfer case offers about a 12% reduction in high range, so you can get back your engine RPM and highway speed in 5th gear.  31's need a bit more lift/clearance, around 3".  The 9.5" tire width will fit within your fender flares, and NOT require wheels with reduced backspacing (unsure about the 10.5" tires).  In Calif, I can get cited for tires sticking out beyond fender flares (but it rarely happens).  Is most of your driving ON or OFF -road?  How often will you really need that extra half-inch of clearance? 

Edit: Oops! I answered assuming Samurai.  The Sidekick has better tire clearance, so you should be OK with what you got.  The tires diameters are so close, only about 4% difference, that I'd keep the 31's unless you really need to eke out the last little bit of freeway performance. The narrower 30's will have less rolling resistance and make for a better road vehicle. The reduced backspacing you already have on your 8" rims is likely to be adequate for your 31's, but check at full lock/stuff just to be sure.  Good wheel tech site at  http://www.rsracing.com/tech-wheel.html

Edit2: From what I can tell, the 4.24:1 Trackick T-case gears do not change the high range. 30's are 8% and 31s are 13% larger than stock, so one route might be to change R&P to the 5.83 gears.  That way, your hi-range gearing could remain about the same as OEM and your speedo would be closer to correct.

At the moment most of my driving is on-road with the occasional Off Road run

I have headers and exhaust to 2" and 5th gear is still used for freeway driving.  Mostly flat to medium grade inclines.  4th gear or lower on high overpasses.  Currently I am running a 31X10.5 on a 15X8" rim with 3.75" of backspacing.

Do you think the backspacing or rim is going to cause an issue

I am leaning toward the 30's at the moment

Go with the 30's. I ran 31's for 3 years and it was ALL she wanted to turn. Like yzf said say goodbye to 5th. The 30'2 will also fit better in the fenders. They will fit even better with a 2" coil lift, hint, hint. 

Jeff I was wondering will your spacers work ok with 30's and a 2" body lift or will the angle be too great for my cv joints? and how much??

Noah

I missed out on the 30's and now I may have a line on some 32X11.5's but I think 31 X10.5 is as much as I would like to go with stock gearing

I run 30 9.5's on my 8v tracker, they seem to be OK on the trails, and still have some power on the road.  Lower gears would be nice.

Running 3" suspension lift and 30x9.50 goodyear wrangler mtr's on 15x7, 3.75 BS.  No rub, no problems. 
I wanted to go 31x10.50 at first since I've seen people do that with my same lift, but I've also heard about having gearing issues and etc...I decided to stick with 30x9.50 to avoid rubbing and also avoid spending more $$$ on gears.  Freeway with the 30's, my speedo (not calibrated for larger tires) reads:
55mph |removethispart|@ 3,100RPM
65mph |removethispart|@ 3,700RPM
Pulls a grade 4th gear pedal to the floor averaging 50-55mph.  Also, any heavier tire than my 30" and I would be worried about using my original spare mount on the back hatch. 

I would recommend the slightly smaller tire, especially if you drive on the pavement a lot.

I have run both sizes. Right now I am running 31" BFG M/Ts, its all my little 16v can do to keep them spinning, and like was said before I almost never get to use 5th gear.

I am actually planning on going back to a 30" tire, for a couple of reasons; first is power, these little rigs dont start out with gobs of it and until I have the correct gearing to go bigger i would like to be able to use what I do have, and second would be fuel economy, I took around a 5mpg loss stepping up to 31" from 30". With the 30" tires I was getting good strong mid 20's on the street, now with 31's its right around 20 mpg. Id bet that has a lot to do with never making it into 5th gear tho.