Ok guys, my truck has 32 inch tires on it, 31.5in tall with the weight of the truck sitting on em.
It tends to drop out of OD to easy, thinking a fine tune of the speedo might correct it. Didn't do that before I replaced the speedo cluster.

Tires, Hankook 285/75R16

Not sure what they took the speedo out if, I checked and it says its set to 967. From what I understand, and correct me if I'm wrong please, I need it set to 886, or real close to that at least.

Thanks, Dan

 

Nobody yet huh.....

Well I did the math instead of using that little chart and I come up with 931, 886 would be a little bit to slow going by that.

Was the first time I drove it in weeks where it didn't feel like it was flying just to run at 55 or 60!

No one would pass or even catch up to me, people driving small cars would always be behind me wanting to pass when towing the trailer. Not with that speedo in it!

The speedo had all 6 changes remaining, must have been out of a F-150 with small wheel/tires. That small of a tire, to use 967 isn't even on the chart.

 

I just looked at the tech article. You may not have gone low enough. A 215/75/15 tire uses a 10.17 cal code. A 265/75/15 tires uses a 9.18 cal code, this is same as 31" tire. You need to be a little below 9.18. Using the chart in the tech article, you need to be somewhere between 886 and 914 for a 31.5" tire. Personally,I would stick with the 886.

 

Thanks for the input alz, I appreciate it.

I did the 32x3.14=100.48/12=8.37 5280/8.333=633x.015=949 and was going to use that. But instead I measured the circumference of the tire itself rather then using pi and it was 103" or 8.5 feet. Changed the over all numbers deriving at the 931 figure and used it after trying the charted figure given for a 32" tire.

So I tried 886 anyway and it was 5mph off, to slow. Set it to 931 and its about 3 mph to slow give or take. At slower speeds its right on, faster I go the larger the difference becomes.

Of course all this is dependent on the wifes car being dead on, she follows me for a base line. It has stock size tires on it as per the door jamb sticker so I would think it would be close enough.

So based on that I get 45 difference between the two 931-886=45/3=15, Looks like leaves me needing to add the 15 to the 931 figure. So 946 should be about dead on at 55 huh? Should have trusted "pi" huh! lol (That is if it works)

I can burn up the allowed changes in this cluster unit at will as it is a loaner, waiting for the bone yard to get me the right one. The needle on this speedo is not rock steady and the difference seams to vary a mph or two, 3 mph off is the best guess average.

I can see why things are all based on 45mph but where rim size comes into play I'm still kinda at a loss. Ford uses 1.33" for tire mash under the weight of the truck, that doesn't change the amount of rubber in tread surface that has to be covered each full rotation. The rim could be any size within that same circle but what da heck do I know! lol

 

I was wondering that also.

I have never worried about changing the constant even with 15 years of ownership.
Just yesterday I checked constant for the first time and I was shocked.
Going to calculate correct # and change mine too.

Not to throw another formula or method out there and possibly confusing someone, but couldn't you mark the length of a tire revolution, in inches on your driveway and take the results and divide into 5,280' (63,360") to arrive at the revolutions per mile?

 

I converted to feet but yea that would work too.

Easer to just measure the tire's height and then multiply that by "pi" (pi=3.14) should get you close enough. I would not deduct for tire mash under the load of the truck though.

 

Gotcha.
I didn't pick up on your numbers, until now, that that is basically what your were talking about.

 

Gotcha.
I didn't pick up on your numbers, until now, that that is basically what your were talking about.

It looks as if you used a different number for the revolutions multiple, though. I think it should be "revs x .0135".

Another formula for revolutions that I encountered is 20168/actual tire diameter = revs per mile

***a 32 inch tire would have 630.25 rev. per mile x .0135 = 8.51 constant

The tires that I use change in diameter more than an inch from them being new until they are shot.
I think that the constant would be a mere "estimate" or "ballpark" figure and truely is not exact.

May be best to figure constant based on half worn tread??

 

<i>May be best to figure constant based on half worn tread??</i>

I picked one with about the most tread on it, yea about half worn I'd say. What da heck might as well have a happy medium!

 

Maybe this will help.

Tire Size Calculator - Discount Tire Direct

 

Time to learn something new. (for me anyway)

So the PSOM is in the speedometer and is not the VSS that is located in the rear axle housing?

All the other vehicles I have had or worked on had the cable driven speedo's so this is new to me.

 

Sorry jarod I hadn't seen your post till now. Been up at the cabin for a few days.

Anyway yea the PSOM is in the speedo cluster. It allows for 6 changes to be made for tire size. After that a ford dealer needs to be involved but what exactly they do I have no idea.

946 was the magic number that worked for me, is within 1mph +/- at any given speed to what my wifes Explorer reads so I will leave it at that.

 

Danr1,

Glad it worked for you.
Just as a friendly note though, I'm not sure that using another vehicle as a base figure is all that accurate. You should go off of mileposts or speedometer check stations on the highway for the best accuracy.

Also, if you are referring to this article, you are using incorrect numbers in your above formula.

This is how your above formula SHOULD read:
32x3.14=100.48/12=8.37 5280/8.37=631x.0135=8.52

Once again, I'm only trying to be accurate, not argumentative.

 

Yea not the best or most accurate check but close enough going by her car.

The numbers, yea I just rounded to 8 1/3 feet. Mine "actual" where more then 100.48 anyway closer to 102.5. So it was a happy medium I could live with, as well as the end result.

My truck has 16 inch rims so "x.015" is used, the "x.0135" figure is for 15 inch rims. Again why that makes much if any difference is beyond me. Less side wall give/flex I guess in the larger rim. But a larger taller sidewall or more ply's make up for it in a hd truck tire anyway.

I will compare it to the mile markers on the highway when I use it, just to see how it "fits"
I'll even pull over at one even with the bumper right at the on ramp, reset the trip meter to zero, and then drive down the shoulder to the next marker. See if it says 1 mile even!

I will check it against other means before making any fine adjustment in the final permanent speedo cluster once I get it and have it installed.

 

Thanks for the info. This creates a particular problem for me now as I have three different tire sizes I plan to run. Four if you count the drag slicks.

With the PSOM being in the speedo that means I would have to change out my speedo when I change tires. Unless those aftermarket speedo correctors work without changing the PSOM's 6 changes. Anyone know how these work?