I went from the original 245/75 16 tires to 285/75 16 when I put on my RE 7000-3 (That's the 4.5 inch lift with adjustable upper front and rear control arms and adjustable front track bar, but it comes with the 3.5" springs instead of the 4.5" springs) and the speedometer was off after that tire change of course. When I did a similar tire/gearing change on my 93 YJ it was just a matter of changing the drive gear for the speedo. On the 04 Rubicon the speedometer is a 3 wire Hall effect electronic speedometer via the sensor, and you have to be able to convert the signal electronically. There are a variety of options available, I explored a few of them and decided the Yellowbox converter was what I was going to use. The vendor for this is a forum member (Skinny pedal) and communicated with me promptly with my inquiries, gave me a Jeep Forum discount, and shipped to me USPS priority instead of this usual shipping option--USPS is the cheapest way to get most items shipped out to the Islands.
I wanted to do a clean install, and Josh recommended putting it behind the glovebox, and cutting into the sensor wire just before it gets to the computer, which happens to be just behind the glovebox on the firewall side, behind the battery. I'm going to run you thru my install, following the ample instructions and initial advice from the vendor.
This is just the way I did it, it isn't necessarily the "approved solution," and I'm not claiming any special expertise. You have to make your own choices and take your own risks. I am not representing the vendor here, nor advertising the product. Accept that this is just a photo writeup of my experience.
I started out by pulling out the glovebox for a peek behind it to see what was in the way. If you slide the nylon keeper strap out of its slot, the glovebox will drop down and you can lift it off its hinges.
I went over to the engine side, disconnected and removed my battery, then took off the 3 screws holding the computer to the firewall. The sensor wire comes thru the loom into the middle connector going into the computer, position 27. It is a white with orange stripe wire at this point.
This is where I drilled thru the firewall to run the Yellowbox wire loom. I recommend you use a step drill, it won't grab and send the bit suddenly to the hilt thru expensive things you didn't want holes in, like the A/C evaporator in the heater/AC box......
You'll notice in the photo above, there is a yellow dot, well to the right of where I drilled thru. I want to share my mistakes as well, and the yellow dot is really
where I should have drilled my hole. There is nothing behind there, and is a better location. Where I drilled, there is 1" clearance and then the heater box. I carefully selected an open/clear spot by looking thru the opening where the glove box was, then I went over to the engine side and drilled in precisely the wrong place.......I did make a small pilot hole first, then poked a small piece of stiff wire in there and measured the clearance, I had an inch, so just went ahead and worked with that. Key item to notice though, is drill your hole where I made the yellow dot, "after the fact."
There are 4 wires to the Yellowbox, the red is power into the box, it will accept 5V like at the level of the sensor loom on top of the transfer case, but it was recommended that the Jeep computer liked it better if you use a 12V switched
power source. The black wire is the ground, and the other 2 wires are for the sensor signal, white and gray. The white one is the signal into
the Yellowbox from your speed sensor, and the gray wire is the altered signal out
of the yellowbox.
I painted the exposed metal edges of the hole I drilled, inserted a grommet, then pulled the loom thru from the passenger cabin side using another piece of wire as a "fish tape."
After putting the computer back in place, I cut into the sensor wire just before the connector plug going into the computer. You should solder your connections, I used heat shrink tubing to cover the connections and then re-wrapped the loom with electrical tape.
You'll notice that you only see 2 wires coming out of the loom, I pulled the red and black out of the loom since those connections are going to be made inside the passenger compartment, behind the glovebox.
Here is the connector for the Yellowbox, behind the glovebox. I hadn't pulled the black/red wires from the loom yet. You can see the fuse box, which is a handy place to get your 12V switched power from, and there was already a ground wire from the aftermkt stereo unit installed in a convenient location for me to use as well top right of the fuse box (and it even said
ground on it!! Wow.)
I took the fuse box off the firewall, and used fuse position 10 which is power to the instrument panel. This is a 12V switched source. The red wire coming diagonally across the photo from the right, power in for the Yellowbox, is soldered into exposed wire cut into the blue with white stripe wire going to the instrument panel. I wrapped this with electrical tape afterwards, and reinstalled the fusebox.
After joining the ground (black) wire from the yellowbox into the ground point, I reinstalled the battery to demonstrate switched power.
If you want to drive your Jeep and still have the speedometer work to verify your percentage correction, you can install the jumper as shown, this just bridges between the gray and white wires and makes the system essentially as it was originally.
You can then install the Yellowbox and start making your corrections based on the instructions. It tells you which switches to throw for percent correction, and whether or not your speedo reads faster or slower than actual ground speed. I haven't set any correction into it at this point.
Make sure you tape up your loom again, you can see the new loom running down and back into the firewall.
I'm up to the limit on photos/post so will add more, edit, etc shortly.