This has only been addressed a few hundred times, so once more should be about right.
Why on earth would you compare your rig on 38s to your rig on its stock tires? Doing so will get you an equivalent gear ratio, but the gear ratio will be the ONLY thing that's equivalent. Jeep chose the stock gears to work well with everything the way they made it. Consider how a rig running 38s differs from when it was stock.
First you're going to need to put a big lift on it to fit those meats. That means your rig is sitting way up there in the wind, and dragging all its running gear through the wind now too. This rig on 38s is likely to have a few things added to its exterior that will also cause more drag - like a 38 inch spare, roof rack or exo, big bumper and winch, extra antenna(s), etc. All that stuff makes pushing that XJ through the air much more difficult than it was stock. But we're just getting started. (Mind you, these points are in no particular order.)
Next let's consider how much this rig on 38s weighs. Have you ever looked at what a 38 inch tire weighs? Especially a bias mud tire? You're talking hundreds of pounds of additional unsprung rotational weight. Add the weight of a 38 inch spare. Add the additonal weight of axles beefy enough to handle 38s. Add in the extra weight of heavier bumpers, winch, HiLift, tools, spare parts, etc. All of a sudden this rig on 38s weighs on helluva lot more than it did when it was on the design board being assigned an axle gear ratio.
How about the rolling resistance of a 38 inch tire? Especially a bias mud tire? You think it might take a tad more oomph to roll an XJ on 38s than it does to roll one on 225/75R15s?
Let's consider reasonably anticipated use for a moment. A stock XJ can be expected to serve duty running interstates at 80 or 90 miles an hour. I hope anyone running 38s on an XJ has the good sense to keep their top speed a good deal lower than that, regardless of what they've done to their brakes and running gear. At the other end of the spectrum, a stock XJ crawls around pretty well at slow speeds on its dinky little tires, but an XJ on 38s will be struggling to roll those 38s over every individual tread block with an "equivalent" gear ratio. What effect do you suppose lugging around with too tall a gear will have on either your clutch or your auto tranny? In a word: HEAT. And in case you haven't heard before now, heat kills.
I could go on, but I trust I've made the point I sought to make. It is pointless to try to figure appropriate gear ratios for large tire sizes by figuring an "equivalent ratio" to stock. Instead try asking those who've been there and done that. Ask them what gears they are running, on what size tires, on what type of terrain and under what sort of usage, and whether they'd make the same choice again, and why.
okay?
Agree that you can't calculate your gearign in relation to stock settings. I'm at 5.86 with 39.5 Boggers and 37 Army MT/Rs for on road use right now. The gering is a bit low for on road driving but just about right off road with the Boggers.
