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Gear Selection and Speedo Gear Correction

Choosing Gears and Correcting the Speedometer


Background
One of the oft-overlooked expenses when lifting your Jeep (or other 4wd) is the change in gearing once you swap to those larger tires. By changing the tire size, the engine seems to lose power, and the speedometer will be off. Once you select a proper gear ratio for the axles, correcting the speedometer is a pretty easy fix.

I did a little research into this when I was originally planning my lift, since I didn't want to hurt my performance. I had a target tire size I wanted to run, so I made sure the lift matched it and would fit those tires. From there, I needed to choose a gear ratio for the axles so they were better matched to the powerband of the engine. I've heard many differing opinions regarding the best gearing for tire swap, but haven't seen many of those opinions actually based on fact. I am currently running a set of 32x11.50s but plan to install 33x10.50s in the near future. In my case, my Jeep has an AW-4 automatic and came with 3.55 gears. For that Jeep, 27" tires were stock so the speedometer would be WAY off if I didn't correct it. To muddle things up a little farther however, I also regeared the axles to 4.56, which by itself would also throw off my speedometer. So with the two major changes included, I needed to recalibrate the speedometer..

Not only are new tires often much heavier, the increased diameter changes the effective gearing of the vehicle, often making it difficult to maintain speed in the higher gears. This can burn up your clutch or overheat the automatic transmission since it requires more slippage and frequent downshifts to maintain sped. One of the most common questions I get involves the loss of power involved when you add larger tires. Not only are they often much heavier, but the increased diameter changes the effective gearing of the vehicle, often making it difficult to maintain speed in the higher gears. This can burn up your clutch or overheat the automatic transmission, in addition to requiring more frequent downshifts to maintain speed.

The Part with the Math
Solving the target gear ratio is as simple as knowing a few things:
           New Tire Diameter =   X  
Old Tire Diameter Old Gear Ratio
X is the target new gear ratio.

Another way to read that is (New Tire Diameter) * (Old Gear Ratio) / (Old tire Diameter) = Target Gear Ratio
In my case, that would be 32 * 3.55 / 27 = 4.20

Or....you could use the handy calculator below. These calculators come from Mark Medina's www.4lo.com, and represent some of the common calculations available regarding gears and tire changes.

Use this calculator to help determine a new gear ratio that brings your engine back into its powerband. Enter the values on the left and press Compute. 

  Find new Gear Ratio with new tire size 
New Tire Diameter: 
Old Tire Diameter:  
Current Gear Ratio:  New Gear Ratio: 
 

One thing to note about the above results - gear higher than the indicated number to allow for rolling resistance and weight of the tire. My personal change from stock 27" tires to 32" with 3.55 gears indicates I should use 4.20 gearing. While there's no 4.20, there's 4.10 and 4.56 available.

So according to the math, 4.10 is pretty close to the 4.20 answer I found. HOWEVER - The larger tires and wheels will increase the rotational mass, requiring me to "get into it" more to accelerate and hold speed. Larger and wider tires also have a much larger contact patch on the ground, adding more resistance that must be overcome as well. Thirdly, I'm driving a Jeep, not a Ferrari so there's a little thing called 'drag' involved. The engine is going to have to work harder to overcome all of those things, so with 4.10s that would mean decreased gas mileage. Looking at the available gear ratios, 4.56 might be the next best bet for me.

WARNING!!! Don't forget to regear both front and rear differentials before attempting to use four-wheel drive...

So I've selected a Gear - Now What??
I drove around for awhile with the stock 3.55s and 32" tires. I went so far as to estimate the correct speedometer gear to correct my speedometer, but the lack of power in higher gears drove me crazy. The transmission would hold 75mph but gentle hills would cause a downshift, unlocking the torque converter and consuming more gas. It was time to get the axle regeared. For those of you who still may not get the concept, it's like riding a 10-speed bike but always starting out in 5th gear...you can do it but it takes awhile to get up to speed. It's the same thing with a 3.55 geared XJ with 32s - it takes some effort to start, and every time you hit an uphill you want to downshift more in order to make it.
R/P Ratio Tire Size This table came from
http://www.ilovejeeps.com
  30" 31" 32" 33" 35" 36" 37" 38"
3.73
34 33 32 31 29 28 28 27
4.11
40 38 37 36 34 33 32 31
4.56
43 42 40 39 37 36 35 34
4.88
46 44 43 41 39 38 37 36

Correcting the Speedometer Gear
After regearing the rear to 4.56 I had power to the ground again, but my speedometer was now inaccurate again. Looking at yon handy chart, I deduced that I needed a 40-tooth speedo gear for the 32s and 4.56s. It took a whopping four minutes to make the switch, and most of that was spent trying to find the right wrenches. Had I just looked underneath first, I could have saved a lot of that time and simply grabbed my sockets because there's that much room for the ratchet.

Use a 1/2" wrench to remove the speedometer gear holddown bolt. That's the bolt that holds a 'Y' shaped bracket to the t-case housing. Undo that bolt and remove the bracket, then gently pull the the housing out and remove the old gear. There is an o-ring around the outside of the housing, so it doesn't just slide out.

Note:There may be a difference in the bolt thread over the years. My '96 speedometer holddown bolt was a 5/16"-18 but when I asked about this on NAXJA I learned that some of the Cherokee owners had metric bolts in their speedometer housings. Those appear to be a very similar M8x1.25x16 thread but measure if at all possible if you need to replace the bolt for any reason.

Removing the hold-down bolt
Speedometer gear housing with the hold-down bolt partially removed
The speedometer gear housing with a 40-tooth gear installed
The speedometer gear housing and 40-tooth gear

The old gear simply pulls straight out of the housing; this step took me some time because it had been in there for 170,000 miles and was very set in its ways. Push the new one in and note that there are three sets of numbers cast into the body of the speedometer housing. This allows the differently sized gears to contact the speedometer gear in the transfer case.

Note: The housing must be turned correctly so that the gear meshes properly with the gear on the transfer case output.

I did a measured 60 miles on I-25 immediately afterward during a drive to Denver, and the odometer dropped 0.4 miles one way. On the return trip, it showed 59.8 miles on the 60-mile stretch. I can live with an average 59.7 miles shown for 60 miles travelled, I could probably adjust my air pressure to make up for it but that's just excessive.

Installing the speedometer gear and housing
Installing the new, corrected speedometer gear into the housing

Tire Speed Ratings
This really has nothing to do with the gearing question at hand, but I have been asked about the dangers of going fast with large tires. Also, since I love meaningless trivia I figured I needed to add this somewhere on the site.
Speed symbol Maximum speed
Q 99mph / 160kph
S 112mph / 180kph
T 118mph / 190kph
U 124mph / 200kph
H 130mph / 210kph
V (without service description) Above 130mph / 210kph
V (with service description) 149mph / 240kph
Z Above 149mph / 240kph
Z W (sub-category of Z) 168mph / 270kph
Z Y (sub-category of Z) 186mph / 300kph
Z Above 186mph / 300kph

Source: Kelly Tires' Website

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created: December 6, 2004
All content is copyright 2001-2005, and unless otherwise noted content comes solely from the mind and keyboard of Jim "Yucca-Man" Langdon
Any changes or modifications to your vehicle are at your own discretion; I take no responsibility for your lack of responsibility