Ok here the deal my stock 4.30 diff roars while driving. Wanna swap it BUT!!! I was thinking about doing a 5.12 diffs front and rear I have a 1995 geo tracker lsi 1.6 16v 3 speed automatic transmission.  How will it run with 5.12s and 30 inch tires. Can't make tires bigger no more lift Thanks

Should be a good combo, now get off the computer and do it already. No one can tell you if you will like the end result so do it, drive it, change it if you don't like it!..... EZ!

perfect combo for on and off road in my opinion.

The decision point for you would seem to be if you use your truck for a daily driver or not.  If it isn't then go with the 5.12s and enjoy the extra power the taller gears will give you.  On the other hand if you log a lot of highway miles you might not want the extra RPMs (and extra fuel cost - especially if the prices spike again) all the time and might be happier with a set of 4.62s or even 4.88s.  Both will get you back closer to your original mechanical advantage.  30" tires are only a 3 inch jump over stock diameter tires, providing they are actually a full 30 inch diameter tire and they are only slightly heavier.  Decide what you use your truck the most for and go for it.

I also run stellies will that have a big effect

These trucks came with 5.5" steelies.  Another inch or so of width will add a couple pounds or so.  I've ran 235 series tires on my trucks for years.  I put a set of BFG AT KOs with 3-ply sidewalls on my red one.  I could feel the difference in the ride (partially due to stiffer side walls).  If your truck is pulling your 30s on steelies now with 4.30 gears... it will just pull them better with taller gears.  You will just be regaining some of the original mechanical advantage you lost when you went with the taller tires.  It will probably be a day and night difference when you make the change over how it is running now.

I'd go 4.88 if it's a mixed use DD ride and 5.12 if it was used more than 50% off road.

as said, 4.30 only if you cruze the highway a lot.

I see 4.88 listed as a gear ratio quite often, but who makes a 4.88 for
a Sidekick? never seen them offered, not that I would want one but?

2nd Gens Trackers with auto trannys came with the 4.88 gears (except for the '99 year 2-door models that came equipped with with the 1.6 Liter engine (instead of a 2 liter) and they still had the 5.12 gears).  The manual tranny trucks came with the 4.62s and the 2 liter engine.

What is considered 2nd gen ? Do they have abs? And if they do do I have to wire them I don't have abs.

The 99-up Tracker/ Vitara/ Grand Vitaras are 2nd Gen.

The 5-speed / 4.30 gears are the rarer ones.
Almost every one you look at will be an auto with the 4.88

The ABS was only an option on the tracker and standard on the Vits.
It's out on the hub anyway.

If it was my choice I'd do the 5.12's unless you just want the challenge finding some ultra rare gears. what happens when you smoke one of those 4.88's?  ???the 5.12's are everywhere and cheap. also they used them by Suzuki in most sidekicks and some trackers for a reason IMO. ask a few who have them stock and added 30" tires its still not low enough.

i have a 5spd 5.12 kick with 31's. it runs the highway fine (still accels up to speed and hold it the same as any other day) im compairing this to 235/75r15's which are my street tires. takes some downshifting to get up the long hills (getting low in 4th going into 3rd, where as the 235's i could keep up the hill in 4th, but its getting low) dont expect it to be a monster on the highway if you commute with it, but they can still motor along at speed.

My thing is isn't my 3 speed auto trans lower then the 5 speed manuals. Won't that have a massive effect on how much it pulls

I run 30 tires with a set of 4.625s and it works well as a DD/offroad Samurai.  Stock t-case gears.

I run 4k at 65MPH...

5.12s might be better offroad but the engine would run at higher RPMs on the road.