so i have a 1.8L 4door kick 1998

and currently have 5.12 diff gears and 30" tires atm
and im hitting about 110km/h at about 3750 rpm

but im planning on running 34" ltb
and i wanted to know what kind of diff gearing i should do
because i wanted to do a front locker this winter
but i would like to run on highway at lower rpm's.
any idea's? 
would the 4.62 gears work? or is that to low for 34" tires?

If you are going up in tire size, that will lower your RPMs with your current gears.  I wouldn't swap to higher gears with 4" larger tires, that would drop the RPMs too much, IMO.

Are you sure your numbers are right?  110KMH is about 68MPH and in my calculator it says you should only be turning about 3100RPM at 68MPH with 30" tires.  That is figuring you have a 5 speed tranny and the tires actually measure 30".  If you have the auto, you are probably turning a little higher RPM than what I figured, but not 600RPM more.  And the tires probably do not measure 30", but I still don't see there being that much of an RPM difference.

If your numbers are correct, then going to a 4" taller tire will drop the RPMs down to 3232 at 110KMH, if you keep the 5.12s.  If you go to 4.62s, the RPMs will drop to 2917 at 110KMH.

225/75r16 = 29.28 sorry off an inch

but thats the rpm i'm getting and atm im doing allot of highway driving so do you think 34" tires would give me a good rpm range in between 2000-3000rpm? with the 5.12

225/75r16 = 29.28 sorry off an inch

but thats the rpm i'm getting and atm im doing allot of highway driving so do you think 34" tires would give me a good rpm range in between 2000-3000rpm? with the 5.12

I was questioning the RPM, more than what the tires measure.
IMO, anything down around 3K RPM at cruising speed will be too low, unless you are always driving on flat roads.  I think the engine will have a hard time holding speed, if you have to go up any hill at all.  Of course you can always downshift.
Why are you wanting to drop the RPMs at cruising speed?