How to Change Tire Size with the SCT tuner?

[kert0307]

It worked changing the speedo on my 2000 sport SOHC by changing the tire rev/mile setting...

 

[rwenzing]

They specifically asked for my tire size, and told me I could change it when I switch to winter tires. It has to be possible. when I switch my wheels back to stock, my speedo and trans shifts are off. Maybe its not the speedo they change but the transmission shifts...humm. THey did something that relates to driveabilty. Maybe the Revs per whatever etc.

Yes, the shift characteristics and shift points can be changed in your 99 Ex PCM. The speedometer calibration cannot.

The 99 Ex PCM is an "end user" of the corrected VSS signal supplied by the 4WABS Module - just like the speedometer, cruise control, etc. Changes to the revs/mile parameter for speedometer correction are made in the 4WABS Module, not the PCM. This is true of both V8 and V6 models.

You can verify this by consulting the factory shop manual.

 

[CDW6212R]

If that is the case(Bob I do, and I don't want to believe you), the electronic correction device is the other choice. I have one that I plan to use for my 98 Mountaineer. The used to be about $125 new, I have one on my 91 Mark VII with 3.73's. Search around and shop for the best price, some places are way too high.

Look for speedometer recalibration I believe. Good luck,

 

[rwenzing]

Here's a source for an inline speedometer calibration box for $100 shipped:
http://www.bluegauges.com/prod_yellow_box.htm

 

[CDW6212R]

Cool, that's about half the size of the black units I have. The dip switches are inside of mine, under the cover. They are easy to use, if you have the special chart with the dip switches showing the ratios for the myriad choices. It only takes four wires, red, black, and the speedometer wire in/out. I paid $50 for my used one, but that sounds fair for being new, and there it is. Regards,

 

[rob17san]

i put the 265/70-16 tire next to my 275/45-20 and they appear to be the exact same height. am I wrong? What are the differences?

 

[Nick26]

i put the 265/70-16 tire next to my 275/45-20 and they appear to be the exact same height. am I wrong? What are the differences?

they probably are the same or very close, look up how to read metric tire sizes... the first number is the width of the tire in mm and the second number is the ratio of the sidewall height to the width, the last number is the rim height in inches.

 

[rwenzing]

You can also look up tire diameter and revs/mile on your tire manufacturer's site or you can use an internet calculator like http://www.miata.net/garage/tirecalcold.html

 

[rwenzing]

If you ever need to know a tire diameter and you don't have the internet available, you can use this conversion factor to simplify the calculation: 1270

Multiply the first two numbers together, divide by 1270, and then add the third number to get the tire diameter in inches.


265/70R16:

265 x 70 = 18550

18550 / 1270 = 14.6

14.6 + 16 = 30.6


275/45R20:

(275 x 45 / 1270) + 20 = 29.7


[The above will work on any tire that is sized by three numbers in the commonly used metric/english format like 265/70R16. The format may be called P-Metric, E-Metric, LT-Metric, etc.

Obviously, the 1270 conversion factor cannot be used for “inch size” tires which have the approximate diameter printed right on the sidewall. Example: 31x10.50R15)]

 

[CDW6212R]

Off road tires will have much deeper treads, adding possibly a half an inch to diameter.

 

[rwenzing]

Off road tires will have much deeper treads, adding possibly a half an inch to diameter.

Interestingly enough, many inch-sized all-terrain or mud tires are actually smaller than the embossed size seems to imply. "31-inch" tires are often as much as 1/2" under the nominal diameter although some are close to a true 31" and a few are over. Other inch sizes seem to follow a similar pattern. As an example, BFG A/T's run 0.3" under and BFG M/T's run 0.2" under across the board. Bridgestone Revo A/T "31's" have a spec diameter of 30.5".

Manufacturers' specs for P-metric and LT-metric tires also show that they may vary slightly up or down from the diameter calculated from the tire size but the generic calculation will get you close enough for most purposes.

When it comes to setting up the speedometer calibration for non-stock tire sizes, the best source is the actual measured tire revs/mile value published in the particular tire manufacturer's spec sheet.

 

[CDW6212R]

That's all true, the manufacturers don't follow the rules often times. My current Destination AT's are a hair larger than my last 245/70/16's, closer to the factory 255/70/16 size.

 

[Jakee]

so your saying if I go change my Revs per mile parameter on the SCT device, nothing will change? (2001 sport)

I believe you just take that formula posted above and get the circumference; convert a mile into inches (1 mile = 63,360" ); then dived the the mile by the circumference (30"=94.24777962), you now have the value you need to add on the revs per mile. 63360 / 94.24777962 = 672.27047953238561398800622481975

 

[rwenzing]

so your saying if I go change my Revs per mile parameter on the SCT device, nothing will change? (2001 sport)

No, I said you can't use the SCT flash tuner to change revs/mile on a 99 4-door Explorer.

There are several VSS methods used on different models and years of Ranger and Explorer variants. Some use a signal generator that drives the speedometer directly, some read the OSS sensor on the transmission and some read the ABS sensor on the rear diff housing. Correction of the sensor signals may be done in the PCM, the GEM or the 4WABS Module. The corrected signal from the module may be carried over the Module Communication Network or through a dedicated wire.

For Ford vehicles that do the speedometer correction in the PCM, the SCT Xcal2 can change the revs/mile parameter.

If the correction is done in the 4WABS Module or GEM, then revs/mile cannot be changed by the Xcal2 because it cannot access those modules.

I THINK that 2001~2003 Sport and Sport Trac do the correction in the PCM like Rangers from those same years. If so, then the Xcal2 can change the revs/mile in the PCM. I can't say for sure because I don't have schematics for every year and model.

 

[rob17san]

I mounted the Bridgestone Dueler A/t's in 265/70-16. They seem very much the same height and going by Rwenzing's math, and the general statement of his that an A/T tire is 0.3" shorter, then they should be nearly the same. Is that small of a difference something that would throw off my shifts or speedo? It seems to be perfect as of my test driving today. The truck actually gets up and goes faster since the wheels are MUCH lighter then my 20"s.

 

[rwenzing]

going by Rwenzing's ..... general statement of his that an A/T tire is 0.3" shorter.....

Actually, that's not what I said.

BFG's spec chart says that inch size BFG A/T KO's show a 0.3" diameter difference from what the sidewall marking seems to imply. The average BFG A/T KO 31 is 30.7" actual diameter, a 33 is 32.7", and so on. You didn't install an inch size tire and you didn't install BFG.

Again, the specs vary among different tires that have the same nominal size on the sidewall. The best source for true diameter and true revs/mile for any tire is the manufacturer's spec chart. Many manufacturers publish this information on their websites.

I mounted the Bridgestone Dueler A/t's in 265/70-16.... Is that small of a difference something that would throw off my shifts or speedo? It seems to be perfect as of my test driving today. The truck actually gets up and goes faster since the wheels are MUCH lighter then my 20"s.

The calculated diameter of the 265/70R16's is less than 3% larger than the that of the 275/45R20's. That means that the speedo will read slower by about that same percentage. At 50 MPH, a 3% change only amounts to 1.5 MPH, At 70, it's about 2 MPH. Personally, I could live with that.

 

[rob17san]

Actually, that's not what I said.

BFG's spec chart says that inch size BFG A/T KO's show a 0.3" diameter difference from what the sidewall marking seems to imply. The average BFG A/T KO 31 is 30.7" actual diameter, a 33 is 32.7", and so on. You didn't install an inch size tire and you didn't install BFG.

Again, the specs vary among different tires that have the same nominal size on the sidewall. The best source for true diameter and true revs/mile for any tire is the manufacturer's spec chart. Many manufacturers publish this information on their websites.



The calculated diameter of the 265/70R16's is less than 3% larger than the that of the 275/45R20's. That means that the speedo will read slower by about that same percentage. At 50 MPH, a 3% change only amounts to 1.5 MPH, At 70, it's about 2 MPH. Personally, I could live with that.

Ok thanks. Math is not my thing. i cant even look at what you wrote. It bothers my mind for some reason. Anyway, I can live with that amount of error in speed estimating. I am just more worried about the shifting being off, if it would be.