I think actually, the correct tire circumference is used in that equation, no?
for 37s it is in the 2770 range
Speedometer Forscan Fix for 37 Inch Tires
- OP 🔹 omcvay
- Start date
for 37s it is in the 2770 range
You could do it that way:
(new ratio/factory ratio)*nominal circumference
It might still be off by a bit (manufacturing differences, tread wear, tire pressure, bed load, etc.). I figured since you had already done the speedometer vs GPS test, it would be easier to go straight to the new settings using the two calculations above.
Separately: I was wrong. Looks like I changed these using plain English and I did use a default axle ratio. I have a diesel so I had to bump down to a 3.31 axle ratio. It’s interesting yours is causing a problem at 2700. Apparently, my tire circumference is set to 2780 (34.84” diameter). I haven’t had any issues. Anyway, good luck.
- Oct 26, 2020
- 2,090
- 4,386
- 2021 Tremor 6.7L Lariat Ultimate / Lariat Black
- 2023 Corvette C8 Stingray R
Are you planning to drive more than 100 MPH in an 8K lb truck?
Are you planning to drive more than 100 MPH in an 8K lb truck?
Not who you were talking to but about 2 months ago I had an emergency situation trying to drive to the hospital and I was pretty angry I couldn't go more than 154 KPH. Truck seemed pretty stable to me at that speed in a straight line.
I was lucky you did this for Wildpeaks, i straight copied your values. Thank you! My speedo is now spot on wildpeaks 37s.
A few words of caution for folks who will attempt this.
Do not attempt to run “PCM Module initialization/relearn” with the ignition on. It will throw errors and stall in the middle of procedure. It might freak you out because it doesn’t say why it can’t continue. Surprisingly no one seems to have mentioned this. It makes sense intuitively that the engine should be off, but i made this mistake.
The “TCM Module initialization/relearn vehicle data” procedure is OK to start with he engine on, it will prompt you to shut it off and then back on. No errors.
Also, if you are changing axle ratios, MAKE SURE your new axle ratio matches in both BDY CM module and PCM control module. If the values don’t match, (mine was set to no values in BCM) your Check Engine Light (CEL) will remain on even after you do all your BCM and TCM module relearns have been applied.
I’m mentioning this because i did not see it mentioned above and it caused me quiet a bit of nervousness and trial and error, repeat runs to get it right until i figured it all out.
Ok guys, i want to warn you and everyone else - do not update your axle ratio. No where on the forum(s) has it been confirmed as a safe mod. Especially do not update it in PCM, but i would leave BCM alone too. Soon after i updated my axle ratio i started noticing shifting issues, especially in lower gears. The shifts would happen at higher rpms than normal, the tranny did not feel as smooth as before. I reverted back to factory settings for both axle ratios and tire circumference and everything is back to normal.
I don’t care that my speedo is off by 2-3 miles but i do care when the transmission starts feeling off. This forum can be great with good advise but sometimes you’ve got to be careful and not follow someone’s untested changes that might have worked for them (or they simply did not notice how their tranny started to behave differently)
I don’t care that my speedo is off by 2-3 miles but i do care when the transmission starts feeling off. This forum can be great with good advise but sometimes you’ve got to be careful and not follow someone’s untested changes that might have worked for them (or they simply did not notice how their tranny started to behave differently)
- Oct 26, 2020
- 2,090
- 4,386
- 2021 Tremor 6.7L Lariat Ultimate / Lariat Black
- 2023 Corvette C8 Stingray R
I also left mine alone, as I did not want to mess with it for only 2-3 MPH at highway speeds.
Exactly. Nowhere is it confirmed that changing one’s axle ratio has no effect on powertrain and how transmission shifts. On a contrary, i did see posts on other Ford forums that when people updated axle ratios, their gear shifting went into limp mode, other problems, etc. Is it worth the risk? Not for me. Plus if something goes go wrong with the transmission Ford will have a good reason to void the warranty given one is playing with it at the core.
Been reading through a lot of this stuff tonight. I should not have messed with my tire size diameter. I have the wrench on the dash now and cannot get it to go away. Thankfully I did not mess with any axle ratios yet and after reading this I will absolutley not mess with those. I have changed my tire sizes back to the original 2689 value but still can't get the wrench to go away, even after resetting the PCM and TCM
Axle Ratio changes do not work on the 17+ Super Duty. Certain trucks (no rhyme or reason) can take larger numbers. Tire Circumference is the only way to correct speedo with ForScan. There is not (1) magic number for every tire. Every truck is different. I posted up a calculator that should get you close then you can micro adjust from there.
ForScan Resources
ZooDad
ForScan Resources
ZooDad
Reviving this. About to install 37s in the next couple weeks and would like to update via forscan so my Speedo is correct.
I have read 10+ threads regarding this process. Seems as if most folks are using different values (example 2780 vs. 2768) to get the same result. What is the correct value?
Second, seems like most folks get a CEL after the change. I understand that a PCM/TCM relearn may clear this. Have folks had no issues for a long time with this method?
Lastly, some people say don’t mess with the values as it causes more harm than good. TIA!
I got this from ZooDad the other day. I wanted to dial in my 35’s (2664 was what I got in his calculator) so I followed his instructions and it worked perfect. Start with the number your said above and go from there.
No problem. The tire size you have calculated should be very close. I recommend the following
Plug in your 2664 number
You will set a P160A code, thats OK.
Go drive your truck at a variety of speeds. Compare speedo vs GPS.
Adjust your 2664 number up or down to "fine tune". (You are dealing with mm so go in blocks of 12 per 1 MPH.
Find your sweet spot number.
Then using ForScan perform a relearn of the TCM, PCM and AWD (if available)
It may take more than once to get it to stick.
I'm very happy with my 5 star tuning device with performace tune. Prior to installing the tune it allows you to adjust the speedo via entering the tire size
@Ford Funatic was able to help me get it ordered and was fantastic to deal with. It may not have added much true HP but the overall feel of everyday driving is more responsive as well as correcting the speedo was worth it in my opiniong. I do have the 7.3L gasser.
@Ford Funatic was able to help me get it ordered and was fantastic to deal with. It may not have added much true HP but the overall feel of everyday driving is more responsive as well as correcting the speedo was worth it in my opiniong. I do have the 7.3L gasser.
Super helpful!
That’s a cool feature. The 5 star tune seems cool, but probably won’t go that route. Thanks for the response!
- Feb 13, 2021
- 281
- 576
- 2020 Ford F-250 Tremor 7.3 Gas ProCharged
- 2004 Mustang SVT Cobra Terminator
Thank you for the shout out and Merry Christmas!
QUOTE="Mas13pharmd, post: 343357, member: 6854"]
I'm very happy with my 5 star tuning device with performace tune. Prior to installing the tune it allows you to adjust the speedo via entering the tire size
@Ford Funatic was able to help me get it ordered and was fantastic to deal with. It may not have added much true HP but the overall feel of everyday driving is more responsive as well as correcting the speedo was worth it in my opiniong. I do have the 7.3L gasser.
[/QUOTE]
QUOTE="Mas13pharmd, post: 343357, member: 6854"]
I'm very happy with my 5 star tuning device with performace tune. Prior to installing the tune it allows you to adjust the speedo via entering the tire size
@Ford Funatic was able to help me get it ordered and was fantastic to deal with. It may not have added much true HP but the overall feel of everyday driving is more responsive as well as correcting the speedo was worth it in my opiniong. I do have the 7.3L gasser.
[/QUOTE]
Trying to get a little more feedback from folks who have successfully adjusted specs in forscan for 37” tires. Anybody else corrected there speedo without a CEL? If so, what value did you use for the circumference? For the number of folks on here with 37s, seems like a small number that comment on this
It’s been awhile since I’ve done it, but if you go over a certain value on forscan in mm it will have a CEL that cannot be cleared with that value entered, I would have to get back into forscan to see what number I landed on but I was able to find a value that was slightly under the max # and very close vs GPS speed.
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