Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community - FT86CLUB
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Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community - FT86CLUB (https://www.ft86club.com/forums/index.php)
-   Software Tuning (https://www.ft86club.com/forums/forumdisplay.php?f=88)
-   -   Tuning for new wheel diameter? (https://www.ft86club.com/forums/showthread.php?t=63992)

phrosty 04-21-2014 07:27 PM
Tuning for new wheel diameter?
 
Are speed calculations done via the ECU? Is it possible to specify a new wheel radius to fix mph with larger wheels?

Boss Paco 04-22-2014 11:51 AM
My radar does gps based speed and I'm off running 225/40 & 255/35 18s. Just slightly at 70 or faster. Maybe 1 or 2 mph up to around 80. When I go faster, it's not something I keep an eye on.

wparsons 04-22-2014 12:11 PM
Depending on the size of wheel/tire you want, there's probably no need for correction. Like ^^ said it'll still be really close. My 225/45/17's are dead on based on GPS speed on a straight road.

Luckrider 04-22-2014 12:27 PM
This is something I have wondered about myself. The only compensation I have seen is for the calculated gear based on Engine RPM compared to revolutions per mile. I don't believe it effects how the speedometer reads.

Edit: This interests me more for higher HP builds that can benefit from a larger diameter tire.

phrosty 04-22-2014 01:00 PM
Yea, I realize it's only a tiny discrepancy and doesn't bother me much, I was just curious if it was possible. Why have any error if it's possible to get rid of it? :)

Ranatsu 04-22-2014 01:59 PM
The ecu doesn't do the calculation. Its done within the transmission.

wparsons 04-22-2014 02:27 PM
Quote:
Originally Posted by Luckrider (Post 1689657)
This is something I have wondered about myself. The only compensation I have seen is for the calculated gear based on Engine RPM compared to revolutions per mile. I don't believe it effects how the speedometer reads.

Edit: This interests me more for higher HP builds that can benefit from a larger diameter tire.
It will definitely affect the speedo.

Quote:
Originally Posted by Ranatsu (Post 1689916)
The ecu doesn't do the calculation. Its done within the transmission.
No, it's 100% the ECU. There's actually no sensor in the transmission or rear end at all, the ECU uses the wheel speed sensors (ABS/TC/VSC) to calculate the vehicle speed. This is why changing the final drive doesn't affect the speedo at all, only a change in tire diameter will affect it.

Luckrider 04-22-2014 02:32 PM
I knew that the speed was calculated using the ABS sensors but I was/am not sure that the ECU uses the same table as the gear calculator to calculate speed. I might play with it on mine and if that is the case, I will adjust mine to compensate for the 245/40R17s that are on mine right now just for the hell of it.

Ranatsu 04-22-2014 02:46 PM
Quote:
Originally Posted by wparsons (Post 1689988)
It will definitely affect the speedo.



No, it's 100% the ECU. There's actually no sensor in the transmission or rear end at all, the ECU uses the wheel speed sensors (ABS/TC/VSC) to calculate the vehicle speed. This is why changing the final drive doesn't affect the speedo at all, only a change in tire diameter will affect it.
News to me, Subaru has always used a gear within the transmission.

350matt 05-11-2014 04:13 PM
you can set the gear ratios within ECUtek so ostensibly you can change the speedo reading with this

can't you?

Kodename47 05-11-2014 05:17 PM
Quote:
Originally Posted by 350matt (Post 1729662)
you can set the gear ratios within ECUtek so ostensibly you can change the speedo reading with this

can't you?
The gear ratios are just so the ECU knows which gear you are in. Speed is done by wheel speed sensors, although I'm sure there is a multiplier in the ECU somewhere.

brn12345 05-11-2014 07:23 PM
Quote:
Originally Posted by phrosty (Post 1688102)
Are speed calculations done via the ECU? Is it possible to specify a new wheel radius to fix mph with larger wheels?


You would want to use the Toyota Techstream software. It allows you to adjust speedo error by percentage. If you have a Tactrix cable you can install the Techstream software and do the changes yourself.

Grymm86 05-12-2014 01:26 AM
Quote:
Originally Posted by Luckrider (Post 1690007)
I knew that the speed was calculated using the ABS sensors but I was/am not sure that the ECU uses the same table as the gear calculator to calculate speed. I might play with it on mine and if that is the case, I will adjust mine to compensate for the 245/40R17s that are on mine right now just for the hell of it.
No need to recalculate. That tire ratio is near identical to stock.

OICU812 05-12-2014 01:59 AM
Quote:
Originally Posted by Grymm86 (Post 1730455)
No need to recalculate. That tire ratio is near identical to stock.
Correct I'm in 245/40/17 and my speedo is not even 1mph off.


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