Tuning for new wheel diameter? - Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community

phrosty · 04-21-2014, 07:27 PM

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

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

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

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

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

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

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

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.

04-21-2014, 07:27 PM	#1
phrosty Senior Member Join Date: Apr 2013 Drives: 2013 FR-S Location: Seattle Posts: 806 Thanks: 202 Thanked 321 Times in 199 Posts Mentioned: 18 Post(s) Tagged: 1 Thread(s)	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? __________________ 2013 Whiteout 6AT FR-S \| Perrin Inlet Tube + 2.75" CAI \| OpenFlash Header \| P&L Catback \| 4.88 Final Drive \| Dialed in OFT 2.0x Stage 2 E85 \| 18x8 Enkei Raijin + 225/40 Michelin Pilot Super Sport \| 17x7 Stock + 215/45 Michelin X-Ice Xi-2

04-22-2014, 01:00 PM	#5
phrosty Senior Member Join Date: Apr 2013 Drives: 2013 FR-S Location: Seattle Posts: 806 Thanks: 202 Thanked 321 Times in 199 Posts Mentioned: 18 Post(s) Tagged: 1 Thread(s)	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? __________________ 2013 Whiteout 6AT FR-S \| Perrin Inlet Tube + 2.75" CAI \| OpenFlash Header \| P&L Catback \| 4.88 Final Drive \| Dialed in OFT 2.0x Stage 2 E85 \| 18x8 Enkei Raijin + 225/40 Michelin Pilot Super Sport \| 17x7 Stock + 215/45 Michelin X-Ice Xi-2

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04-22-2014, 01:59 PM	#6
Ranatsu Cone basher Join Date: Mar 2012 Drives: 2017 Subaru BRZ Series.Yellow Location: Dodgin cones in San Antonio, TX Posts: 800 Thanks: 51 Thanked 256 Times in 169 Posts Mentioned: 7 Post(s) Tagged: 0 Thread(s)	The ecu doesn't do the calculation. Its done within the transmission.

05-11-2014, 04:13 PM	#10
350matt Senior Member Join Date: Jul 2013 Drives: toyota mr2/ Toyota GT86 Location: UK Posts: 438 Thanks: 3 Thanked 140 Times in 92 Posts Mentioned: 1 Post(s) Tagged: 0 Thread(s)	you can set the gear ratios within ECUtek so ostensibly you can change the speedo reading with this can't you?

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