Correcting spedometer for tire size

[rammertide07]

Ya'll guys seem to be a little more knowledgeable on the electronic side of the SD's vs some other guys I know, so maybe ya'll can help out.

I have 315 (33") tires on my truck and I have wanted to correct the spedometer ever since I have owned it. I can go 50 miles but my odometer will say 46 miles. Ford dealer here says they can't flash the pcm for the tire size. So is there any way, other than buying a programmer, to correct the spedometer? Thanks for any input

[Longshot270]

The only way around programming is to adjust the axle gears or use a GPS.

[907DAVE]

Superlift TrueSpeed or Auto Enginuity can re-program your ABS module so your speedometer displays the correct speed.

[Longshot270]

Originally Posted by 907dave

Superlift TrueSpeed or Auto Enginuity can re-program your ABS module so your speedometer displays the correct speed.

I stand (well sit) corrected!

[rammertide07]

Don't know anyone with AE.... can a MODIS do it?

anyone ever heard of doing this???:

"First, you'll need to know the actual height of the tires, not the
nominal height of 35" that's printed on the sidewall. To get that,
make a mark on the edge of the tire at the ground, and mark the ground
directly under the mark. Now, roll the truck far enough for the tire
to have made three revolutions. When your mark on the tire is at the
very bottom, near the ground, mark the ground again, and measure the
distance between the two marks on the ground. Divide that distance by
three, since there were three revolutions of the tire, then divide
that number, (which is the circumference of your tire,) by 3.14. The
answer you have is the true height of your tire. Write that number
down, because you'll need it for the next step. Divide 30282 by your
actual height. If your true height is 34", the math goes something
like this: 30282/34=891. That would be your speedometer calibration
code."

"To set the calibration, ground the single wire connector under the
glove box it says PSOM (Programmable Speedometer Odometer 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 door 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."

[907DAVE]

I could be wrong but a PSOM is an OBS thing. The SD's dont have them.

[rammertide07]

Originally Posted by 907dave

I could be wrong but a PSOM is an OBS thing. The SD's dont have them.

How great is that lol. Tru Speed is pricey. Looks like one of them things that takes $30 to build and they want to make 10x's what they got in it.

[rammertide07]

My goal is to get my odometer right, for servicing and fuel milage reasons. Anybody know how many gallons can be held in the tank and packed up the neck?

And according to the gps vs the odometer... for every 46 miles on the odomater, I have actually gone 50 miles.

Thanks guys

[Longshot270]

If it is just for your personal use then you can adapt a little formula for finding the correct tire size on the programmers. You take the GPS mileage and divide that by the odometer mileage (resulting in a number around 1.08). Multiply that number back to your odometer to find the true number of miles you've driven in that time frame...or does this also sound funky?

[rammertide07]

Originally Posted by Longshot270

If it is just for your personal use then you can adapt a little formula for finding the correct tire size on the programmers. You take the GPS mileage and divide that by the odometer mileage (resulting in a number around 1.08). Multiply that number back to your odometer to find the true number of miles you've driven in that time frame...or does this also sound funky?

Well the spedometer is right up untill 35-40mph....then it starts to get off. So sometimes its right...sometimes its wrong. Will A MODIS not correct for the different tire size?