Calibrating The Speedometer

First off, my apologies if this subject comes up quite frequently here. I read the FAQ's, and did a search of "speedometer" and "calibrate" before posting this.

I have a stock 1996 XJ with a 4.0L engine, auto transmission, N231 transfer case, and 3.55 gears. I'm planning to lift it, put 31" tires on it, and regear it to 4.56. Does my XJ require the $150 electronic device to recalibrate the speedometer, or the cheap nylon gears that I see advertised? If the gears, any close approximation to the number of teeth for my application? Thanks.

Plastic gears and see the chart on Go-Jeeps site!

Here's the link. http://www.go.jeep-xj.info/HowtoSpeedoGears.htm. You can take the part number and order one online at someplace like http://www.chryslerpartsdirect.com/ for around $10 plus shipping.

X3 on Go-Jeep's speedo gear chart (about $15 from my local stealership).

But, I wonder about your plans to run 31s and 4.56s. Engine RPMs would be awfully high with that combo. 4.56s are about perfect for me, and I run 33s. My RPMs are a tad higher than stock, but it works out well, due to the added weight of the larger tires.

ColoCherokees said:

X3 on Go-Jeep's speedo gear chart (about $15 from my local stealership).

But, I wonder about your plans to run 31s and 4.56s. Engine RPMs would be awfully high with that combo. 4.56s are about perfect for me, and I run 33s. My RPMs are a tad higher than stock, but it works out well, due to the added weight of the larger tires.

Click to expand...

+1; 31s and 4.10s give higher than stock RPM on my jeep. I'm not sure I'd want to take 31s and 4.56s on the highway.

But, I wonder about your plans to run 31s and 4.56s. Engine RPMs would be awfully high with that combo. 4.56s are about perfect for me, and I run 33s. My RPMs are a tad higher than stock, but it works out well, due to the added weight of the larger tires.

Click to expand...

Everyone in the following thread seems to think that 4.10's are a waste of time, and a chart posted in the link shows that I'll be under 2500 RPM at 65 MPH.

http://www.naxja.org/forum/showthread.php?t=46901

I currently have 3.55 gears with stock tires. According to my calculations, adding 31's will bring me down to 3.09. Regearing to 4.10 would bring me exactly back to 3.55, so that's why I was wanting to go with 4.56's -- to get a little lower than stock.

M4Madness said:

Everyone in the following thread seems to think that 4.10's are a waste of time, and a chart posted in the link shows that I'll be under 2500 RPM at 65 MPH.

http://www.naxja.org/forum/showthread.php?t=46901

I currently have 3.55 gears with stock tires. According to my calculations, adding 31's will bring me down to 3.09. Regearing to 4.10 would bring me exactly back to 3.55, so that's why I was wanting to go with 4.56's -- to get a little lower than stock.

Click to expand...

My Big 0 235/75/15s (stock size) tires measured right at 28". My BFG T/As measure about 30.5". I'm running 4.1x gears (3.55 before). The engine speed with my 31s (with 4.1x gears) compared to the old tires (with 3.55 gears) is for the same ground speed is:

(28/3.55)/(30.5/4.10) = 106%

I.e., the new tires/gears increase RPMs 6% over the old tires/gears. (Changing speedo gears by a couple teeth fixed my speedo). With 4.56 gears, you get:

(28/3.55)/(30.5/4.56) = 118%

IOW, your RPMs should increase by approx 18% over what you have now. For me, that was too much. YMMV

fizassist said:

My Big 0 235/75/15s (stock size) tires measured right at 28". My BFG T/As measure about 30.5". I'm running 4.1x gears (3.55 before). The engine speed with my 31s (with 4.1x gears) compared to the old tires (with 3.55 gears) is for the same ground speed is:

(28/3.55)/(30.5/4.10) = 106%

I.e., the new tires/gears increase RPMs 6% over the old tires/gears. (Changing speedo gears by a couple teeth fixed my speedo). With 4.56 gears, you get:

(28/3.55)/(30.5/4.56) = 118%

IOW, your RPMs should increase by approx 18% over what you have now. For me, that was too much. YMMV

Click to expand...

Not to rain on your math parade but......while your gearing calculations appear to be accurate, I see no equation for gearing compensation for the added weight and drag from the parts in your signature? I've said this till my tongue aches, the math gets you back to stock. YOUR JEEP IS NO LONGER STOCK.
I made the 3.07 to 3.73 mistake 10 years ago on my first XJ when installing 30's and a 3" lift. Felt like stock and sucked on the hills, which I have a lot of here in Utah. 4:10 would be a marginal improvement, until you jump up a few more inches in lift, add 32's-33's, bumpers and more gear. It all robs power, and burns more fuel.
Your Mileage Will Vary....it will likely suck.

Do ALL the math, buy gears once.

fizassist said:

My BFG T/As measure about 30.5". I'm running 4.1x gears (3.55 before). The engine speed with my 31s (with 4.1x gears) compared to the old tires (with 3.55 gears) is for the same ground speed is:

(28/3.55)/(30.5/4.10) = 106%

I.e., the new tires/gears increase RPMs 6% over the old tires/gears.

Click to expand...

What are your current RPM's at 65 MPH? I'm currently running slightly less than 2100. Reading the above-linked thread shows that I want that to be closer to 2400 for both better fuel economy and more power. 4.56's are supposed to put me in that RPM range.

M4Madness said:

What are your current RPM's at 65 MPH? I'm currently running slightly less than 2100. Reading the above-linked thread shows that I want that to be closer to 2400 for both better fuel economy and more power. 4.56's are supposed to put me in that RPM range.

Click to expand...

I run about 2200 at 65 MPH, but I routinely drive at 75-80 (limit is 75) on the interstate. That puts me at about 2700, which is a good place. 4.56s would be a smidge high for me.

lawsoncl said:

You can take the part number and order one online at someplace like http://www.chryslerpartsdirect.com/ for around $10 plus shipping.

Click to expand...

I wish it would have been that easy. I'm starting to round everything up, since my lift kit shoud be here any day. I called the local Jeep dealer first, who told me that there's a large backorder on the 32-tooth gears. I then searched some of the companies that sell Jeep parts online to no avail.

I then called Chrysler Parts Direct from the link above hoping to get one from them for the $18 quoted on their website, but the guy told me the same thing -- "national backorder in the hundreds". I asked him if he knew of any other places to try, and he was nice enough to do a dealer search of my home state of Indiana. Only two Jeep dealers in the entire state had any -- both had one each. The bad news is that the closest one to me was at least 150 miles round trip.

I called them and they agreed to ship it to me to save me the trip. It was high-priced at $29 and almost unreasonable at $40 once they added shipping and handling. I figured it was still a bargain though, considering I would have been out about 10 gallons of gas and over 3 hours of my time to pick it up in person.