But seriously, you can regear your axles for the larger tires to regain the stock feel. But since you only have 31s, IMO, I wouldn't worry about it. They are only 1" or 2" bigger than stock tires. The cost wouldn't justify the gain to me. And also your speedometer is usually off when you put on bigger tires. It reads slower than you are actually going. But, again, since you only have 31s, it won't be bad at all and it might actually be more accurate than before. Use a GPS and see what it says and compare it to your speedometer. If it is off and you want to fix it, you can buy a new speedometer gear.
I switched to E-3 plugs. Smoother running, quicker throttle responce and increased torque (Dyno Measured).
Added one at a time - Banks Cold Air Kit, Banks Torque tubes, Banks exhaust, Ported and polished head, eliminate the fan with right electric fan, Royal Purple fluids, ECU tune and crancase breather plumbed to the header collector, not the throttle body or manifold. Each item was a small gain, all together the gain was tremendous. If you can resist the temptation to lead foot then you'll see the mpg increasing. PS, TB spacers don't work and actually have a negative effect.
The only way to improve/increase the torque delivered to your tires is to install lower ratio gears into your axles. It sounds like your axles may be geared to 3.07 which is a very high ratio intended only for maximum possible fuel economy, not good performance. Regearing both axles, about $1200, to 3.73 or perhaps 4.10 will produce a major boost to the amount of torque the engine will be able to produce.
And with 31" tires, you are traveling slightly further and faster than your speedometer and odometer is indicating. The speedometer and odometer is easily calibrated in 5 minutes with a new snap-in speedometer gear using the procedure at http://www.4x4xplor.com/speedo.html
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1. How can I give my 98 Wrangler some more torque? the acceleration is brutally slow (I know jeeps weren't made for speed but I cant deal with the slowness much longer).
2. I put new tires on my Jeep and I was told that the speedometer would be wrong now (I upgraded to 31s). Is this true and if it is, does it read it faster or slower, and how can I make it right?
With 31's a 410 gear ratio swap will be much benefit, my Rubi has factory 410/411's and 31's stock, and it's plenty powerful and enjoyable to drive, and will accelerate uphill in 5th. I get 17.5 mpg on the highway, and about 16 in town if you want to know or are concerned about gas mileage.
My 2005 4.0L TJ with the 4 speed auto and 3.73 gears feels very good with 31" tires. I can go up hill without the tranny downshifting. Although, the 3 speed auto is a different beast. I'm also coming from a 2.5L(my last jeep)......so the 4.0L is an animal in my mind.
If you don't have a Dana 44 in the rear, you could swap in some 4.10 TJ axles from a 2.5L Manual. That would be much more cost effective versus regearing. It would also be a simple bolt in swap.