How to calculate and install
When you change your tyres from stock or your gearing then your Speedo will be wrong and should be changed to stop you getting booked. It is also important as the computer uses the pulses for a variety of things and not just the Speedo. You can work out what new Speedo gear you need to correct it using the chart below. The stock diff ratio for an XJ 4.0 Cherokee is 3.55 for petrol auto, 3.73 for diesel manual and 4.0 TJ Wrangler is 3.07 for the manual in Australia. There are diff ratios for the 4 cylinders and other combinations released in the USA. If in doubt, look for a tag attached to one of the diff cover bolts or roll the Jeep for one complete tyre rotation while counting the amount of times the drive shaft turns. e.g. Three and half turns of the drive shaft for one tyre rotation equals 3.55 diff ratio. The part numbers and install instructions are the same for both the 231 and 242 transfercases found in the Wrangler and XJ Cherokee.
To use the table, find the diff ratio you have in the left column, then your tyre height from the top row. The Speedo gear tooth number needed is where the two intersect. Check the available gears from the part numbers on the table lower down. Tyre sizes in brackets are BFG AT´s true heights that those sizes in the chart is based on ( add 0.1" for BFG MT tread ). Other sizes are from others given me and the most common brand used in those cases.
I have put the table together using the formula: Old wheel diameter x old Speedo gear teeth, divide by new wheel diameter = teeth on new gear. If you have changed your gear ratio and it was accurate before the change then you can use this: # of teeth on old gear x new gear ratio divided by old gear ratio = # of teeth on new gear. If you know how much it was out before the change then: # of old gear teeth x Speedo reading divided by actual vehicle speed = # of new teeth. Just remember that to speed up the Speedo you need less teeth ( e.g.. bigger tyres ) and to slow it down you need more ( e.g.. going from 3.55 to 4.11 diff ratio ). I have found that each tooth changes it by about 3%. Where the calculations came up needing a half a tooth, then I always made sure it reads 1.5% faster rather than slower to stop the risk of speeding fines. The common tyre/diff combinations have been checked by myself and others using GPS when possible. This will be as close as one tooth difference will allow and there still will be a bit of variance according to tyre wear and brands ( above based on BFG AT heights where size is in brackets ), but it will still be within the one tooth difference ( 3% ). Also, I found the taller the tyre the worse the difference between stated height and true height with even a 1" difference having been seen! If your stock XJ tyres were 225x70x15 ( 27.4" tall ) with an auto then you will have a 3.55 diff ratio and a 35 tooth gear. If you have 225x75x15 ( 28.3" tall ) XJ tyres and an auto then it will be 34 teeth ( both read fast when stock ). TJ´s come with 225x75x15 with 3.07 gears and a 29 tooth gear I have read. If you want to work out the inaccuracy then drive at 100 kph for five kms according to the posts and it should take exactly 3 minutes. In the States you should pass a mile marker every minute exactly at 60 mph. Unfortunately you can´t rely on the odometer as they are often different to the speed so having one right does not mean the other is exact.
Speedo Gear Part Numbers
The dealers call it a ´ pinion Speedo drive ´ and the part numbers vary according to the year of your XJ. For those that have the electronic Speedo there are two shaft lengths. A long shaft ( 4.3" ) from 1991-1993 and a short shaft ( 2.2" ) from 1993-2001 for XJ and to 2006 TJ. If you have a cable rather than wires coming out of the transfer case, another long shaft ( 4.3" ) is needed for the 1987-1990 years. Above is a chart so you can order directly from your dealership like here in the USA. Also Also Terry from Radford found this site that even sells good second-hand ones. It states there also that the speedo gears are the same listed for Dodges and the early long ones the same as used in Dana 300 transfercases. Thanks to Don Hornbrook from Arab Alabama for updating the 1991-1993 part number list too. If you cant get the exact tooth number you want and don´t want to be up to 3% out, White91Limited from JU bought another speedo gear with the right tooth count for a different year, and then used a heat gun to remove the wrong shank, and then pressed in the correct shank from his original speedo gear to get the combination.
Metric tyre to Imperial Chart
The conversion chart above is to help you to use the Speedo gear chart. If you have a tyre size that I have not included then it is easy to work out your own. Multiply the width by the profile in percent which will give you one side wall height, so multiply that by two, then divide that by 25.4 to get it into inches and then add the wheel diameter, e.g.. 225/75/15 tyre would be; 225 X 75% = 168.75 X 2 = 337.5 mm, divide by 25.4 = 13.28" plus wheel diameter 15" = 28.28". I have found that the metric sizes are very close to real height where as the imperial can be 1/4" to 1/2" out.
Speedometer Error Calculator
If you just want to know how fast you are actually going compared to indicated then you can use this simple calculator above. Enter your old tyre diameter ( e.g.. 28 ) in the first box and then the new tyre diameter in the second ( e.g.. 31 ) and then enter the speed at which you would like to know the true speed and then click ´Calculate´ and the real speed will be shown. Do all the common sign posted speeds to get the overall picture as it is a percentage difference not just 10 mph at all speeds for example.
Changing the gear
The Speedo gear housing is located on the left side of the transfercase on the side of the extension housing the rear output shaft comes out of. It is simply a matter of removing one bolt ( 1/2" socket ) that is pointed to with the screw driver which releases the metal fork holding it as shown in the next picture. Also take note of the numbers on the side of the housing directly above the bottom fork groove. The gear that you are taking out will be within that range. More on it later.
With the fork off, you pull it straight out. The gear itself is now removed by just grabbing a hold of the toothed part and pulling it free. You then just push the new gear in its place making sure it is all the way in ( you will feel it clip in place ) and none of the metal shaft is showing. Before replacing it you need to be aware of the position it must sit in to suit the new tooth count. There are three positions the Speedo housing can fit for a range of tooth gear numbers. This is because the more teeth, the bigger the gear and vice a versa as you can see that below with the green one having 39 and the black one 32 teeth. So that they all still mesh with the drive gear inside, the gear sits off centre in the housing and rotating it moves the Speedo gear ( driven gear ) closer or further away. The three positions are 26-31, 32-38 and 39-41 teeth. These ranges are moulded into the side of the housing and I have highlighted them in silver with the right photo above showing the 26-31 tooth range.
Now when replacing the housing with the new gear, make sure that the range of numbers on the underside of the housing is within the new tooth number, which in this case says 32-38. Line that range up with the bottom fork prong and the slot it sits into. You may have to rotate it a bit to line up with the fork and its bolt hole. That´s all there is to it.
Gearing for best MPG
Most people think that less revs will equal better MPG. That is not always the case as less revs can also mean more strain on the engine which causes more fuel use. According to a engine design text‚ maximum fuel economy occurs with open throttle and at a piston speed of 1200 – 1500 ft/min. So if you know the stroke of your engine‚ you can calculate the rpm for best fuel economy. Usually comes out somewhere between 2000 – 3000 rpm. Dino provided me with the following numbers. The 4.0L has a stroke of 3.411" and a 4.5-4.8L stroker has a stroke of 3.895" so the numbers would be as follows: 1200-1500ft/min piston speed = 2110-2640rpm on a 4.0, 1848-2310rpm on a stroker. So use the table above to pick the right gears for your tyre size. Above gives the average cruising speed of 65 mph or 105 kph. The more weight you carry the more revs you need to stop the extra fuel use through engine strain. The extra revs is also better for the auto and even engine temperature with less strain and the water pump running faster to circulate the coolant.
You can also use the gear chart to work out what axle ratio is needed to return you to stock gearing by finding your stock gear tooth number under the new tyre size. Below is also a guide to what revs you will have with different combinations. This is about what you will have in overdrive or forth in a auto, ( 0.75 ). Pre ´91 will be slightly slower at 0.70, Peugeot ( BA-5/10 ) had a 0.72 and faster for a AX-15 manual at 0.79 or a AX-5 with 0.85.
RPM´s at 65 MPH/105 KPH w/ 3.55:1 axle gears running 28´s: 2,076 RPM
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