Nate C Posted June 13, 2015 Share Posted June 13, 2015 I know this subject has been beaten to death, from reading posts I'm pretty sure I can do what I intend but would like some thoughts/opinions of how it will change the performance and feel of the truck. So here is what I have: originally 87 2.5L with four speed manual shift which means I have 3.55 gears yes? I have since dropped a rebuilt 4.0L in it and put a BA 10/5 with T-case in from a doner. I also have a 1" coil spacer in the front and shackles in the back. I currently run 30" BF AT's on the original wheels. Here is what I want to do: I'd like to get 15x8" wheels and install 31" tires. Right now I'm happy with how it accelerates and I can put it in 5th gear around 50 mph without any bogging. I don't do any wheeling it's just a street / look pretty truck. The questions: Should I get a wheel spacer for this new setup? How will acceleration and 5th gear be effected? what adapter should i get to compensate for the speedo? I've seen the chart Eagle posted, but I'm not really sure what I'm looking at. Can my gears and old tranny handle the 31's I know the motor can, and I should have enough clearance in the wheel wells with the 1" lift. I don't think I'll ever go bigger than 31" I've wanted to do this for 6 years since the rebuild but other things have been sucking up my money and time. If anyone can think anything else that could be an issue please let me know. Link to comment Share on other sites More sharing options...
Eagle Posted June 13, 2015 Share Posted June 13, 2015 originally 87 2.5L with four speed manual shift which means I have 3.55 gears yes? Correct -- assuming it's all original. Here is what I want to do: I'd like to get 15x8" wheels and install 31" tires. Right now I'm happy with how it accelerates and I can put it in 5th gear around 50 mph without any bogging. I don't do any wheeling it's just a street / look pretty truck. The questions: Should I get a wheel spacer for this new setup? No. There are no aftermarket 15x8 wheels that have as much backspacing as factory Jeep wheels. Unless you get a set of factory Jeep 15x8s, the wheels will take care of any spacing issues. What you will have to look out for is the shoulder of the front tire hitting the lower back corner of the wheel arch when turning. How will acceleration and 5th gear be effected? Acceleration will be worse, and 5th gear will be nearly useless. I ran my '88 on 31s for several years with 3.73 gears. In terms of speed-to-RPMs, this combination is exactly equal to stock tires with an automatic and 3.55 gears. Very nice for street, but 4.10s would have been better for off-road. I am currently running 31s on my '88 XJ with the stock 3.07 gears. I've been driving manual transmissions for well over 50 years so I'm not likely to burn out the clutch, but fifth gear is only useful on highways with a speed limit of 65 or higher. With 3.55s, you'll be somewhere in between my two examples. what adapter should i get to compensate for the speedo? A new speedometer gear. I've seen the chart Eagle posted, but I'm not really sure what I'm looking at. What don't you understand? Find the chart for the tire size. Find the pair of columns with your gear ratio, and each row tells you the RPMs your engine will turn in 4th and 5th gear for the road speeds listed on the left. For your situation, 31" tires and 3.55 gears, at 65 MPH the engine would turn 1956 RPM in 5th gear, and 2608 RPM in 4th gear. That's not a horrible setup.In fact, a stock 4.0L 5-speed on 225/75-15 tires would be turning 1821 RPM in fifth gear and 2428 in fourth gear at 65 MPH, so your proposed setup is actually a slight improvement over a stock five-speed. Can my gears and old tranny handle the 31's I know the motor can, and I should have enough clearance in the wheel wells with the 1" lift. Yes Link to comment Share on other sites More sharing options...
Jeep Driver Posted June 13, 2015 Share Posted June 13, 2015 Any 8" stock wheel will fit with 31s with no issue......no spacers........no lift.........notta. Aftermarket +30 will take you to stock position........best you'll find will likely be +20 (4.78" of backspacing vs 5.25) and that'll be fine. The difference between 31s and 30s is negligible. My Cooper AT3 31.s actually measure closer to 29" mounted and inflated. As for the speedo gear......I wouldn't bother. That is, of course, unless you were going to have your speedo calibrated.......after 25 years, it's off anyway. Link to comment Share on other sites More sharing options...
HOrnbrod Posted June 13, 2015 Share Posted June 13, 2015 Agree with all the above except the speedo gear. At present, your MJ should have a 36T gear installed that came with the the original ~27" tires. With 31" tires and 3.55 gears, you will need a 33T gear for the speedo to indicate correctly. Each tooth represents roughly a 3%-4% difference in indicated speed, so keeping the original speedo gear will cause your speedo to be off by at least 9%. That was too much for me when I went to 31's so I changed the gear to the correct 33T gear. Link to comment Share on other sites More sharing options...
Megadan Posted June 13, 2015 Share Posted June 13, 2015 I will say this FWIW. 235/75-15's (29") on 3.07 gears sits right at 1700rpm at 60mph and is a complete dog trying to accelerate. Once I get to around 65, which is just shy of 2000rpm it starts to pick up. 31" tires with 3.55 gears will have you sitting at about 1800rpm at 60mph, which as said before will be a bit doggy, but sadly would be a mild improvement over where I'm at now, which I can cure with a downshift if I need to. I actually want 3.55's for my 29" tires.As stated 3.73's would be ideal for a daily driver with 31" tires, and 4.10's would work well but would be a bit better for off road. Link to comment Share on other sites More sharing options...
Nate C Posted June 13, 2015 Author Share Posted June 13, 2015 Fantastic info as always! Eagle thanks for explaining the chart / comparing to stock 4.0L. I will track down a 33T speedo gear too. Ok - now I just have to choose some tires and wheels. Any thoughts? basically all street, I like the aggressive look of BF AT's but am ready for something different. Link to comment Share on other sites More sharing options...
mikejenn2 Posted June 13, 2015 Share Posted June 13, 2015 I put these on my truck 3 days ago an really enjoy them. http://m.discounttire.com/dtcs/findTireDetail.do?ra=searchTiresBySize.do&fl=&tc=KPBRA2&c=0&rcz=55339&pc=19514&ar=75&rf=true&rd=15&rc=MNMINT&cs=235 I'm sure you can get them in 31's I didn't want to go bigger yet. Link to comment Share on other sites More sharing options...
Minuit Posted June 13, 2015 Share Posted June 13, 2015 I like the Cooper Discoverer A/T3s on my truck right now. I have them in 235/75/15 but they also make them in almost any size you could want. Link to comment Share on other sites More sharing options...
Jeep Driver Posted June 13, 2015 Share Posted June 13, 2015 I like the Cooper Discoverer A/T3s on my truck right now. I have them in 235/75/15 but they also make them in almost any size you could want. Mine are loud, so loud that I'd chuck'em in the trash today if I could afford to replace them. They are not even 50% worn. (XJ) They will be replaced with Nitto. I have Nitto on my MJ now and they are a REALLLLL nice tire. Smooth and fairly quiet. Link to comment Share on other sites More sharing options...
Megadan Posted June 13, 2015 Share Posted June 13, 2015 I like the Cooper Discoverer A/T3s on my truck right now. I have them in 235/75/15 but they also make them in almost any size you could want. Mine are loud, so loud that I'd chuck'em in the trash today if I could afford to replace them. They are not even 50% worn. (XJ) They will be replaced with Nitto. I have Nitto on my MJ now and they are a REALLLLL nice tire. Smooth and fairly quiet. I have the same tires, and they are very quiet. At least compared to the horrid POS tires that the PO put on the thing (Primewells). I like them a lot. Link to comment Share on other sites More sharing options...
Minuit Posted June 13, 2015 Share Posted June 13, 2015 I've never noticed mine being loud, either. :dunno: Link to comment Share on other sites More sharing options...
Jeep Driver Posted June 13, 2015 Share Posted June 13, 2015 Mine are cupped. The dealer is aware of it and offed to prorate to a highway tire............I passed. Link to comment Share on other sites More sharing options...
shelbyluvv Posted June 14, 2015 Share Posted June 14, 2015 I like the Cooper ATPs. A tad cheaper than the AT3s and have a slightly more aggressive shoulder. Link to comment Share on other sites More sharing options...
Dadinator Posted June 14, 2015 Share Posted June 14, 2015 Love my at3s. got em on the mj and the wifes tj. 31s on both Link to comment Share on other sites More sharing options...
FrankTheDog Posted June 14, 2015 Share Posted June 14, 2015 Pick a style you like of 1997-2006 wrangler TJ 15x8 wheels and look for a set on craigslist. There are several different styles. Perfect backspacing and width for a 31" tire. Search for: Ravines, Gamblers, Canyons, Grizzly's and Moabs which are 16x8. Link to comment Share on other sites More sharing options...
shelbyluvv Posted June 14, 2015 Share Posted June 14, 2015 I thought Grizzly Wheels were 15x7s? Link to comment Share on other sites More sharing options...
Siopposition Posted June 14, 2015 Share Posted June 14, 2015 Agree with the nittos. Discount tire didn't have my bf Goodrich rugged terrains in stock when I blew one. And they warrantied out two nitto trail grapplers, I like them more than the bfgs....IMO discount tire will always get my business for tires due to their amazing customer service standards. They have always been awesome Link to comment Share on other sites More sharing options...
FrankTheDog Posted June 15, 2015 Share Posted June 15, 2015 I wasn't sure on the Grizzlys. Never ran across them. Link to comment Share on other sites More sharing options...
Jackrabbit41 Posted June 19, 2015 Share Posted June 19, 2015 You'd have no problem running 31s. Although I can only go into fifth around 65-70 with 3.08s Link to comment Share on other sites More sharing options...
Crash Posted June 19, 2015 Share Posted June 19, 2015 Since you want bigger tires and new wheels anyways, try to find a set of TJ MOAB wheels. They are 16x8 and bolt in. 265/75 will be the size you want to run, maybe 265/70 if you find a deal. Link to comment Share on other sites More sharing options...
shelbyluvv Posted June 19, 2015 Share Posted June 19, 2015 265s will rub like crazy without spacers. I still had to trim my flares and bumper caps to run them. I have a 4.5" lift, 1.5 spacers, moabs and 265s on my truck. Link to comment Share on other sites More sharing options...
Incommando Posted June 20, 2015 Share Posted June 20, 2015 Check a site like tirerack.com for the true specs of the tires that you want. The same size can vary by nearly an inch from make to make. Link to comment Share on other sites More sharing options...
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