Have 92 sidekick with OME and am running  ???235X75X15 and want to run 30X9.50X15 tires. My question is do  I have to run a lower ratio ring and pinion or can I run what I have now.

You can run what you have now.

Do you have an auto or a stick?

Auto's are a little more forgiving with the gear ratios.

You could just try putting to tires of and see how it drives.

You can run what you have now.
But you will really love 5.83's!

BUT.

I wouldn't do 5.83's until I was also cracking the diffs for another reason, like installing lockers, and going to a steel 3rd member carrier in front, with an Anvil.

Its a stick and sometimes in the mountains I have to rev up the RPMs before climbing a steep hill/

Its a stick and sometimes in the mountains I have to rev up the RPMs before climbing a steep hill/

I live in the mountains, and all three of my Suzuki's are sticks.

I typically run above 4,000 rpm on steeper hills.  Do not be afraid to run it at 5,000 rpm!  On steep stuff, I shift above 5,500.

A quick calculation says 275/75/15 is roughly 31" so 30" tyres should be an improvement. It does depend on rim width and manufacturer though.

BFG MT tyre chart says:
235/75/15 on a 6.5" rims is 29.1" diameter
30x9.5x15 on a 7.5" rim is 29.8" diameter

Using these numbers in the absense of any more specific info you will notice the difference especially if your existing tyres are part worn and your rim width remains the same. My stock tyres were around 27" diameter which means the 30s are around 10% higher geared. 5:83s are around 14% lower so you would end up with gearing around 6% lower. I once drove my truck with 255/60s on which are around 25.5" and are about 6% lower and the difference was amazing it really flew along. If you are driving around the mountains the 5:83s would be really good. Personally I found that they were much better on the highway too as it took much less throttle top maintain speed and revved much better too. Driving styles are a little different between the two sides of the pond though, here we are used to a few rpm. In the US you seem to prefer running much lower RPM (except Wild obviously  ). The difference will be around 200rpm up from 3000 stock more changing from 235s. Don't forget it will also seem faster still as the speedo will read higher! though the RPM/MPH readings will remain the same (+6% on rpm and +6% on speedo too)

Below is the 255/60 next to a 33x12.5

I have the 33 13.5 LTB Swampers, with
the 5.83 gears I'm still showing 5 MPH
faster on the speed than I'm moving.

5.83s were designed for 35 inch tires,
and with 35s should be very close to
stock gear ratio.

I have the 33 13.5 LTB Swampers, with
the 5.83 gears I'm still showing 5 MPH
faster on the speed than I'm moving.

Well, that's odd.  My speedo and odo are DEAD ON with the 5.83's and Goodyear Wrangler MT/R 31 X 10.50's!

Your best bet?  Check your speedo with a GPS.  Then you will KNOW.

How about the Moab Police Department
roadside RADAR  billboard ??

I think that's close enough  

40 MPH indicated, sign said 35 MPH

With my 32 - 11.5 MTR's and 5.83's, my speedo is dead on with my GPS.  At all speeds.

wildgoody are you auto? going from 5.13 to 5.83 is not enough for 35" tires!

How about the Moab Police Department roadside RADAR  billboard ??

Nope - those things can be way off!
Hey - let's GPS you while we're in Moab!

How about the Moab Police Department
roadside RADAR  billboard ??

I think that's close enough  

40 MPH indicated, sign said 35 MPH

I've NEVER seen one of those tell the right speed. Always SLOW, go figure. "Sure, you can drive 5 MPH faster, go right ahead..."