Speedometer Calibration???

[4x4 factory]

Just changed my gears in my truck from 3.55 to 4.30 ,read article on recalibrating speedo with code setting but I have tire change35" and gears,is there a way to figure this out, went to superlift site about there Truspeed but looks like with ABS may not work with my year truck. has to be a way,and the othe places superchips and hypertech,same way looks like if speeed sensor and ABS being one sensor not able to do it. Maybe I'm wrong ,but mathimatically should be able to calibrate tire and gear if you changed both. Wnat is stock tire height on these trucks 31?? 97 F350 4x4

[73strokin]

I picked up a Truspeed for my truck because I got a good deal on it used. I have yet to wire it in, but from what I understand it is infinitely adjustable. There is a small screw on it that you can turn and it will adjust your speedometer needle accordingly. Just drive next to somebody going a certain speed and adjust to him, or use a dyno. I was told that this is wired in right after the VSS, so it should correct any and all things based off of that sensor. I've never heard of that ABS problem you mention. Our trucks are rear ABS only, from what I understand. And the stock tire on these were 235/85/16s. I know there are formulas to recalculate your new drive ratio, but if you get the Truspeed, I don't think you need to go through all that trouble.

You can also recalibrate the PSOM module, but I don't know if it will affect other functions like the Truspeed or just adjust the speedometer needle. And you can also only recalibrate this a few times before having to replace it. And if you replace it, your mileage will be wiped clean, which I didn't want. And I think somebody might have messed with mine already, so I don't know how many times are left to recalibrate it. So I bought the Truspeed instead.

[4x4 factory]

If you read the dirrections on the Truspeed it says that on systems that have only one sensor for ABS and VSS that you have to enter a percentage for your calibration, but it doesn't tell you how to derive that percentage, my guess it that if you speedo is 10% off that is you % and says that using a GPS is what the recomened to calibrate with ,but it has -% and +% so I could call, But also says to hook up to wire in back of speedometer, sounds like a job to me,I have been sitting here calculating figures for the PSOM and think I have come up with a code, seems simple to do kinda and if I write down old code and just reinstall if it doesn't plus 200+ dollars saved.But the code I came up with is like 1047 ( want to measure the bigger tires accurately first.But what I did on here with the caculators is put in a smaller tire size to get a lower gear ratio to match what mine is now. Sounds right to me I don't know.

[tjbeggs]

from here:
http://pages.prodigy.net/stevebaz/_i...az/index3.html

Recalibrate Your Speedometer After Changing Tire Size

There is no tire size/code chart because tire size vs. actual tire dimensions are different between manufacturers. Even between the same tire size and same maker there will be a difference in actual size between the different tread designs.

The Ford calibration equation is:
20186 / tire height in inches = some number, then: some number /.6666 = program code

Example:
Start with 20186, divide by actual tire size in inches. Divide again by .6666

You want to use a rolling measurement to get your actual tire height (If you try to measure the tire in the driveway, use conversion charts, or anything else you will end up with an inaccurate reading again). Use your rear tire since the sensor is located there. Mark the ground and your tire and roll forward 3 times marking the ground each time. (I ran over a spot of oil at a local gas station and measured between my tread mark prints), then divide that number by three to get the average length of one revolution (this will protect yourself against a bad reading using just one revolution). You can then divide the length of one revolution by 3.14 to get actual tire height. Be careful in your measurements, a small amount will change the code. EX: 31.69" tire height is code 956, and 31.21" tire height is code 970.

Now, to program it into your truck:


Warning: You can only change the calibration 6 times without replacing the module.

To set the calibration, ground the single wire connector under the glove box it says PSOM (Programable Speedometer Opdometer Module). ”Yes there is only one and it has a spade type plug on the end". I used a wire with clips on both ends to ground the plug to a screw under the dash or try a test light and use the clip end to the PSOM and the other jam in a dooe hinge. Now look at your speedometer on the face of your dash while sitting in the drivers seat and there should be a "reset" button and a "select" button. While holding the reset button in on the trip meter, turn the ignition to 'on' while the wire is still grounded. Let go of the reset button. The speedometer display will sweep once and will show a code of some kind then push in again the "reset" button and the existing code will be displayed with RECAL?, mine said 976 RECAL? Now you enter your new code you came up with by dividing the rolling measurement by PI or 3.14 by pushing the "select" button until it gets to the new calibration code number. You may have to push “select” a whole bunch of times until you get to your code. If you turn off the key at this point the original code will not be changed or any of the 6 lives will not be used. To store your new code press “Reset”. Turn off the key. Unplug the ground wire. To check the new calibration, use the mile markers on the interstate. Run an indicated 60 mph and it should take exactly 60 seconds to go 1 mile.

[4x4 factory]

Yep thats what I have in hand, did the change but still winds up high before shifting question is does this tell computer whats going on seems better, but if you accerlate to fast can wait and shift as high as 3500, something just doesn't seem right, might be my calculating, came up with 1097 code which is 1099 is the max , but better if you are driving normal from what I had it was shifting at 2500 easy driving. Maybe I don't know if this will change shift points if something has to be done.

[tjbeggs]

i think you had better recheck your numbers

[4x4 factory]

Yep I did,#s were right just over thought the process, read in another furum or something how it works,the ring gear that gives you the pulse has not changed it still is turning the same rev, per mile meaning tire on ground moves one mile if stock tires are still on it,if you gop to a larger tire you have moved farther than one mile for same revs, right bigger tire!! Not the gears in the differintial, so by trying to compensate for lower gear I didn't need to. More rev for engine at a differant speeds but not pulse ring, so I put in the figures for tire only which came out to a 890 and drives great shift points are back and everything and I still have 4 changes left in speedo, I am though about 4% off on tire size to big GPS says 50 and speedo says 52 but think I will leave it alone, would take alot to figure out the exact tire size did measure rolling distance but fractions and decimals are alot when it comes the the formula you use. But saved me money and like I said earlier the ones that make them except superlift won't do it anyway. And superlift to set it is a % so I could dial it in so to speak to make it perfect but even milage we be very close it was like 10% before and it was the other way 50 speedo 55 GPS At least won't be speeding.

[tjbeggs]

yeah I would leave it as well. I guess I wasn't really thinking either when you asked the question. your right that a gear change won't affect it but a tire change will. you should be good to go now.

[c00nhunterjoe]

for anyone else that runs into changing the speedo calibration you can get the tire rpm's off of the tire manufacturer's websites. it is usually in the tire charts with recomended rim width and height and such

[tjbeggs]

I wouldn't recommend using Manufacturer numbers though. those measurements are for the tire mounted and aired up but now weight on them. Air pressure and load (weight) will make a difference in the measurements.

[4x4 factory]

Thats very true,rolling method is the only way to measure, distance divided by 3.14.