Tire size /odometer error

[F150]

Look at the thread Ratheon posted named "this thread". Steve83 tells how to do it.

[Steve83]

<font color=green>That thread is getting really old, so here is the info again slightly reorganized:</font color=green>

The "PSOM" is the (electronic) Programmable Speedometer/Odometer Module, which is on '92-97 only F-Series &amp; Broncos. Earlier ones use a cable-driven (mechanical) speedo which is calibrated by changing the plastic gears in the transfer case tailhousing (or transmission tailhousing on 2WD). The E-series and Aerostars also used the PSOM for short periods, but I don't know the ranges. Resetting it is EASY: the PSOM uses the "conversion constant" to calculate the vehicle speed from the sensor frequency. You can see yours by holding the Reset button while turning the key forward to Run, then release Reset. The needle will do a full sweep test, and the odometer display will show "E ## #", meaning English units, the microprocessor revision (mine is 08), and the conversion constant lockout count (maximum is 6). Canadian ones might show "o" in the first position meaning "Overseas" (Metric) units. Pressing Reset again shows "###CAL", which is the conversion constant (without a decimal). 31x10.50's should be 9.18 .

If you want to change the constant, find the single-wire connector under the glovebox and ground the LightBlue/Yellow wire before turning the key on (you still have to hold the Reset, as before). Everything is the same as before, except "CAL" will flash, indicating that the constant can now be changed. Press the Select button to decrease the constant by 0.01, each time. When the constant reaches 5.00, it goes back to 11.00 . Larger tires or fewer teeth on the tone ring require a lower constant - R&amp;P changes have no effect. When you have the constant you want, press the Reset button to lock it in. <font color=red>THIS WILL DECREASE THE LOCKOUT COUNT BY 1 - WHEN IT REACHES 0, NO FURTHER CHANGES CAN BE MADE.</font color=red> To abort the changes and maintain the lockout count, turn the key off before pressing Reset. When finished, turn the key off and disconnect the LB/Y wire from ground. The "PSOM Programming Enable" circuit (the LB/Y wire under the dash) is only used when you want to change the constant and use one of the lockouts. You can check the constant without grounding that wire.

Here is a partial list of constants and revs/mile for certain tire sizes:
P215/75 R12XL/AS........10.17........753
P235/75 R15XL/AS..........9.72........720
P235/75 R15XL/AT..........9.67........716
P275/60HR17XL/AS........9.32........690.5
P265/75 R15XL/AT..........9.18........680
31X10.50R15C /AT..........9.18........680

When the lockout count reaches 0, it doesn't self-destruct; you just can't change it again. It keeps using that constant forever. If you want to change again, you have to buy a new PSOM. But, remember: the only reason to change it is for a tire size change. After 6, you ought to have the size you like!

I think the reason a) it's not publicized, and b) there IS a lockout count, is so that people don't set it so the odometer registers fewer miles until they take it in for warranty work - they'd use up the 6 changes in 3 trips, and the techs would see the lockout going down, and void the rest of the mileage warranties. But that's just a guess.

The E4OD is the only one that uses the PSOM signal (I think), and it's connected to the OUTPUT, so it would be reading the converted speed. But it's fed directly from the PSOM to the EEC even on vehicles with mechanical automatics and manual transmissions, and I don't know why. My truck ran fine after the engine swap, but before I added the ABS rear end and PSOM, so I don't think it's used on trucks without the E4OD.

Here are some useful formulas:
*I've found the published formula for deriving conversion constants:*

ConvConst = RevsperMile x ToneRingTeeth / 8000

(The '87-current 8.8" rear end has a 108-tooth tone ring.)
What's that? You say you don't know how many revolutions your tires make per mile?????

RevsperMile = 20168.11439/TireHeight

The height of metric tires (for instance P265/75R15 - 265mm wide, aspect ratio 75, rim height 15") is calculated by:

Height = 2 x Width x (AspRatio/100) x .0394 plus RimHeight

Since metric tire width (LT<font color=blue>265</font color=blue>/75R15) and flotation tire height (<font color=blue>31</font color=blue>x10.50R15) are nominal and not the actual dimensions, using these formulas will result in an approximate value for RevsperMile. The actual numbers for each particular size, model, &amp; make of tire are published and available at <a target="_blank" href=http://www.tirerack.com/tires/index.jsp>TireRack</a>.

<font color=green>TireHeights, RevsPerMile, &amp; ConversionConstants from muddy:</font color=green>
33....611....8.25
35....576....7.77
38....530....7.16
40....504....6.80
42....480....6.47
44....458....6.17

Interesting to see how they get closer together as the tires get bigger. So I guess FORD anticipated people running 52" tall tires on these vehicles since the constant goes down to 5.
<font color=green>**********</font color=green>

[havack]

Your linear concept is right on but the formula is not quit right there Kemosabi. [img]images/icons/wink.gif[/img] A 10% increase in height is right, but what counts is what is the percentage difference in rolling length. A 30 inch tire will be about 94 inches and a 33 will be about 103. This nets about 9% decrease in a speedometer's reading. For the PSOM users you will need to decrease the Conversion Constant by 9%.

With the way you did it, it will give you a working number but to be exact you need to go by length not height. Most people figure out the revs-per-mile and work with that.

As stated: Actual tire height is not the same as stated tire height. one 30 inch tire might be 29.8 and you buy a 33 that is actually 32.1 inches. You might think you need to accomodate for a 9% increse but actually you need to use about a 7%.

[F150]

Still 10%. A 10% increase in raduis is the same as a 10% increase in rolling distance. It is linear.

You are figuring it wrong a 30 inch tire is 9% shorter than a 33 inch tire. But a 33 inch tire is 10 percent taller than a 30 inch tire. And therefore 10 percent increase in rolling distance and 10 percent difference in the odometer.

All linear.

IF you were going from a 33 to a 30 it would be 9 percent.
Also you would have to assume that your odometer is perfectly matched to the actual radius of the wheel(measured from hub to ground). Then take the actual radius of the original tire and the actual radius of the new tire (again measured from center of hub to ground).

The constant from muddy regarding 33" tires isn't valid. He considered only the nominal diameter of the tire. In reality, all tires compress when the weight of the vehicle bears down on them. To get an accurate rev/mile figure, try marking a point on your tire and a point on the ground. Roll forward until that mark has made exactly 10 (ten) revolutions. See how far that distance is (and be as precise as possible). Divide that number by 10 to get the distance/rev of the tire. Then figure out how many revolutions there will be in a mile (1 Mile = 5280 ft = 63360 inches) or if in metric, 39.4 inches = 1 meter, 1609 meters = 1 Mile, the number you need to find is in revolutions per Mile.

Good Luck, I used this method and my speedometer is dead on accurate.

93 Bronco 33x12.50/15's

[muddybronco]

yes, please disregard my constants, they were done to get a ballpark figure and are useless! for instance, i'm using 8.15 (i think) for 35's and it's dead accurate, whereas i state 7.77 is the constant for a 35.

Unless somebody knows of such a thing already in existence, I am going to write a JavaScript web-based calculator and put it on Havack's site. I've never used it before but I have a couple examples here to work from and it appears similar to C++/C, so no worries [img]images/icons/smile.gif[/img]

Does squat really matter? The tire still has to rotate the full outer perimeter of the tire so I dont see how it could throw the numbers off. However, what DOES matter is the actual manufacturer diameters, which are almost always less than their nominal sizes.

Oh yeah, along with the calculator I want to list common Mfgr sizes and constants that work with them, so list those here if you know them off-hand.

[reptillikus]

Well, seeing as i have an 88, i never looked into the constants, but i do have <a target="_blank" href=http://javascript.internet.com/calculators/high-tech-calculator.html>this</a>. Its a link to javascript.com, for an online calculator. It allows you to preview the calcluator, and it includes all the code for you to put it in a html file. Super easy!

[muddybronco]

OK, I'm done coding [img]images/icons/smile.gif[/img] I went and checked and I'm using 8.25, but now I remember the construction zone speedo readouts say I'm going 66 when my speedo says ~67, so the calculator output is pretty close to actual values. Let me know if it works for you or not... we really need somebody with 44's to test the high end of it for us, then I'll be satisfied [img]images/icons/wink.gif[/img]

http://www.arkansasmud.com/psomcalc.html

Yep, sure will. I have found that to be true even after I changed gearing. Measured with GPS, and the needle at 60, I am actually going 58 or so. I don't know if it was that way with stock tires and stock gearing. Maybe it was.

OK, I just put 33's on yesterday so I decided to reset my speedo to be correct. I used your calc and the chart to see how they matched up. useing your calc with 32.80" tires "manufacture spec." I should set my speedo to 8.6

So I go out to see what it's set at now and compare that with the chart and your calc as a test. Well my as far as I know stock set up is 8.6 for the stock 30" tires I had on there. SO was my truck set wrong or is there a problem with the calc.? I'll give you this, my truck seems to run better on the big tires.