forscan tire size wiki

[charwest]

Given how many people are changing tires, and the ease of just putting "mm circumference" in forscan to fix speedo error, I was surprised about how every thread I read seemed to suggest there was a lot of trial and error in getting the circumference right to actually match GPS speeds.

I envision this as a spreadsheet where people can update with their data, so that future people mounting the same tires wont have to do all the legwork.



If you reply to this thread, I can add the data to the google spreadsheet so it will show up here without hunting down the thread.

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[BcP28]

Helpful tip... if you consider the "Tire Squash Factor" when you calculate tire circumference, you'll be much closer to the true tire diameter that the vehicle needs in order to show the correct speed in the cluster. Accounting for the effective radius with a nominally compressed sidewall will be closer than the theoretical unloaded diameter will, as a place to start anyways.

A common squash factor is 0.967. So you take the diameter provided by whatever calculator you're using (dare I say it, but there's a calculator built into the TacomaWorld forum that's pretty good too), and you multiply that by 0.967 and that should give you a much better starting point for forscan.

Example: 255/75/17 = 2558mm * 0.967 = 2474mm

 

[charwest]

Helpful tip... if you consider the "Tire Squash Factor" when you calculate tire circumference, you'll be much closer to the true tire diameter that the vehicle needs in order to show the correct speed in the cluster. Accounting for the effective radius with a nominally compressed sidewall will be closer than the theoretical unloaded diameter will, as a place to start anyways.

A common squash factor is 0.967. So you take the diameter provided by whatever calculator you're using (dare I say it, but there's a calculator built into the TacomaWorld forum that's pretty good too), and you multiply that by 0.967 and that should give you a much better starting point for forscan.

Example: 255/75/17 = 2558mm * 0.967 = 2474mm

and WHERE were you three days ago?!

four unnecessary trips to the highway and a lot of bugging my wife to read me GPS live numbers when you knew the end result down to the mm without even trying.

damn. ill go through a couple of the other numbers other forum members came up with. if that formula works there’s no reason for a wiki!

thanks. pretty awesome tip. seems like it would be worth a pin in the tire forum to me.


edit: at least in this thread (https://www.ranger5g.com/forum/threads/forscan-speedo-calibration-for-larger-tires.4200/) two of the posters would be really close with your formula (or the basically similar formula where you substract 3.2 0r 3.3%), and this formula works within a few mm for the stock fx4 tire, but the other two posters which gave enough info for me to do the calc would be off by 20 or 30mm. assuming their GPS data really confirms the right number, maybe there is something to some empiric data after all, and hence a wiki?

 

[Adventure Ranger]

Toyo Open Country AT2 AW P265/70R17
Calculated size = 2520mm Forscan = 2400mm
Confirmed with I-phone GPS and Garmin GPS
FX4 Crew Cab Lariate w/2" BDS front lift on truck

 

[BcP28]

and WHERE were you three days ago?!

four unnecessary trips to the highway and a lot of bugging my wife to read me GPS live numbers when you knew the end result down to the mm without even trying.

damn. ill go through a couple of the other numbers other forum members came up with. if that formula works there’s no reason for a wiki!

thanks. pretty awesome tip. seems like it would be worth a pin in the tire forum to me.


edit: at least in this thread (https://www.ranger5g.com/forum/threads/forscan-speedo-calibration-for-larger-tires.4200/) two of the posters would be really close with your formula (or the basically similar formula where you substract 3.2 0r 3.3%), and this formula works within a few mm for the stock fx4 tire, but the other two posters which gave enough info for me to do the calc would be off by 20 or 30mm. assuming their GPS data really confirms the right number, maybe there is something to some empiric data after all, and hence a wiki?

Haha my bad! But yeah, that squash factor should get you close, but you may have some fine tuning depending on the tire, since different constructions, load ranges, tread compounds, etc are going to drive differences in nominal tire radius, so group feedback is still helpful for common tires.

 

[charwest]

Haha my bad! But yeah, that squash factor should get you close, but you may have some fine tuning depending on the tire, since different constructions, load ranges, tread compounds, etc are going to drive differences in nominal tire radius, so group feedback is still helpful for common tires.

well, you predicted my size on the dot. so in my book you're still a magician.

but if people post up their data here i can keep compiling it into a master sheet for future reference

 

[Jrel209]

Is there a walkthrough on how to perform these changes on forscan? Not sure where everyone is finding these numbers/options at. I found the main forscan thread but the first page doesnt have the tire size options, and theres way too many pages to go through lol

 

[Jrel209]

Nm figured it out… it is really accurate using the mm x 0.976 method. Thank you for posting this info! This groups awesome haha. I fixed my tire size and turned off the dumb auto stop/start.

 

[CHIEF]

After completing the tyre circumference change in forscan how do you reset so you don't get the drive train error icon?

 

[thatwhiteranger]

Is there a walkthrough on how to perform these changes on forscan? Not sure where everyone is finding these numbers/options at. I found the main forscan thread but the first page doesnt have the tire size options, and theres way too many pages to go through lol

Which module did you have to go into for changing the tire size? Thanks!

 

[Jrel209]

Which module did you have to go into for changing the tire size? Thanks!

Damn sorry i dont remember and i dont have a labtop to use to check atm

 

[kieefer]

Which module did you have to go into for changing the tire size? Thanks!

That’s in BdyCM module.

 

[thatwhiteranger]

That’s in BdyCM module.

Thanks man! Got it all configured properly

 

[Espo78]

This "Squash factor" had me completely baffled until I realized the mm you were referencing was the circumference of the tire not the diameter. Could you update the spreadsheet to make that easier to understand?

 

[Ed Low]

Hi,

Can you explain how the squash factor is calculated. I can't find any reference to it when searching. My tyres are Cooper AT3 LT 275/65/18 so the circumference is 2558mm. I've input the value of 2474 using FORscan however at 50km the speedo under reads by 3km and at 100km it's 6-7km under.

I don't want to just try random numbers to get it to the correct reading if there is a formula for calculating this squash factor.

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