I'm trying to understand what I have and this chart doesn't match. I bought a very used 2008 base model Wrangler, with a 6 speed manual transmission and unknown mods. Using a Flashpaq, I reprogrammed the ECM for the gear changes I THINK it has, and the actual size tires (31" tall). Now the speedo is correct but my rpm's at 70 MPH in 6th gear are WILDELY higher than this chart. At 70 MPH in 6th gear my engine is turning about 3,100 rpm (by factory in dash tachometer). Using this chart, I'm running 5.13:1 gears, where when programming the ECM for 3.73:1 gears the speedometer reads correctly. What gives?

 

That chart is for the 3.6l, you'll need the one for the 3.8l. I don't have it off hand otherwise I'd share it here.

 

Blackmanpants99 would argue those gear charts are all just made up and we're spreading false information cuz we're idiots, but yes, get the appropriate chart for the 3.8L manual and it should be very close to accurate.

 

That is a nice chart, is there one available for the 3.8l/6 speed manual?

 

https://bit.ly/3fZCLC0

 

perhaps. I know from experience that the rpm is pretty close on those. speaking from dealing with 32s, 33s, 35s, and 37s with 3.21, 4.10, and 4.88. I imagine that there's a good argument that the categories of 'power' are very subjective. Though, I'd say you could look at the HP/torque curves versus rpm and get a close estimate of if the 'power' is correct.

Personally, I used the calculators at 4LO.com and compare stock to what I want/am running to decide if it's acceptable for what I'm looking for.

Also important to point out that you should be using ACTUAL tire diameter rather than advertised... not all 37s are created equal for example. (mine are actually 35.5")

 

Well now. Using the RIGHT chart (thank you), actual measurements of the tire diameter, calculating the gears by spinning a tire, using a Flashpaq, and using a GPS speed ap, my speedometer and odometer are now reading correctly. For some reason my base Wrangler has been modified with 4.88:1 gears, front and rear. Still using open diffs, so I guess it's time for lockers or at least limited slips. Does it ever end?

 

It does not ever end, but that's an awesome find that you've got 4.88's, they'll work well with 35's.

 

No, it doesn't ever end. Even when you get way down the build line, there is ALWAYS something else. You get through axles, steering, suspension, random interior things, armor, wheels/tires, and every accoutrements you can think of, and then you start considering things that you never thought you would, like upgraded radiators!

That would kinda stink to have to add traction control to axles that have been regeared, as you're basically having to pay the labor to regear it again. I guess you could just add a locker to the rear and only be in it for half the labor at least. I think that LSD is a good option for a lot of folks that are regarding anyhow, but I almost think to break that open now to add something you should just go full on selectable locker to be worthwhile.

 

Do you need lockers? Or is it more of a 'want because other people have them'? As noted above, going with full-case lockers/limiteds has you re-doing the same labor from the gear install. Maybe it makes sense for your usage and build, maybe it doesn't. But there are options in several different (lower) price categories, so it's worth spending a little time on research.

Probably not a lot of snow and ice in Albuquerque, right? If you decide that you do want to add traction, you might look at lunchbox lockers. Relatively inexpensive, (compared to the full-case options), and no gear setup, (unlike the full-case options) so many folks will self-install them in the garage rather than paying labor. Worth a look to see if they meet your needs, anyway. (there was some caution about auto-lockers in the rear of a manual trans, but I don't recall the gist of it...)

If you think Limited Slips would work, go for the gear-driven variety rather than clutch based. The Detroit Truetrac is the long time favorite for these, but the Powertrax GripPro is a new contender that may be a bit cheaper. For these, you'll still have the install labor, but you're probably looking at half the cost of a pair of selectable lockers, with no wires/hoses/switches/etc. From a performance perspective, I think they are a great option 'for the majority' of us, but they certainly don't have the same bragging rights as full lockers when you get in the rocks with tires in the air. (I have air lockers now, but for my use, my old truetracs are still my favorite traction devices...)

Anyway, put some thought into the options, the costs, and the performance differences.