If you run staggered tyres and a standalone ECU with traction control, there is a field in your configuration that almost certainly matters more than you think it does. It is the tyre size.
Not for speedometer calibration. For whether your traction control knows the difference between wheelspin and a rear tyre that is simply a different size to the front one.
The problem, in one number
My car runs 265/30/19 at the front and 295/30/19 at the rear. Work the rolling diameters out:
- Front: 482.6 mm rim + (2 × 79.5 mm sidewall) = 641.6 mm
- Rear: 482.6 mm rim + (2 × 88.5 mm sidewall) = 659.6 mm
The rear is 2.8 percent larger, so for any given road speed it turns 2.8 percent slower than the front.
A traction control strategy that compares driven wheel speed against undriven wheel speed and calls the difference slip would therefore see a permanent 2.8 percent slip. At a steady 50 mph on a flat road, doing nothing. Every slip target you set would be sitting on top of an offset that has nothing to do with grip.
What the ECU does about it
Status: Verified
On the Emtron you give it two things per axle: the number of teeth on the ABS reluctor, and the tyre size.
In my case that is 48 teeth on the Toyota hubs at both ends, and the two tyre sizes above. The ECU converts each tyre size string into a rolling diameter, combines it with the tooth count, and derives a scalar that normalises one axle against the other.
The tooth count cancels here because both ends are 48. On a car with different reluctors front and rear it would not, which is exactly why the ECU asks for both rather than just the tyre size.
You can watch it work. At a steady cruise on a flat road, in the same log, at the same moment:
- Raw wheel speed channels: 2.8 percent apart. Exactly the geometry, uncorrected.
- Drive Slip: 0 percent. Corrected, and reading the truth.
That pair is worth knowing about, because if you go looking at raw wheel speed channels and find them 2.8 percent apart, nothing is wrong. Those are sensor channels. The correction is applied downstream, inside the slip calculation, and the corrected channel is the one that tells you whether it is working.
The failure mode nobody warns you about
The correction is derived from a tyre size you typed in. It has no way of knowing what is actually bolted to the car.
This car has been through 18 inch wheels on Yokohama A052, and is now on 19 inch Rays G025 with a staggered Continental SportContact 7 setup. If the configuration still held the old sizes, every wheel speed on the car would be wrong, road speed would be wrong, and the slip calculation would be wrong.
And none of it would look broken. The ECU would be doing correct arithmetic on stale inputs, confidently, forever.
So: change your wheels or tyres, change the configuration. It belongs on the same checklist as the alignment.
What geometry cannot reach
Status: Working conclusion
The scalar is calculated from the free rolling diameter, meaning the tyre’s size when nothing is pushing on it. What the wheel actually rolls on is the loaded radius, which is smaller, because the sidewall deflects under the weight of the car.
That deflection is not the same front to rear. My rear sidewall is 88.5 mm against 79.5 mm at the front, and it carries more load. So the real ratio between the two axles is not quite the ratio the ECU calculated.
Worse, it moves. Under hard acceleration weight transfers rearward, the rear sidewall squashes further, the rear rolling radius shrinks, and the rear wheels read faster for the same road speed. The system sees slip that is partly just the tyre changing shape.
Which means the residual error is largest exactly when traction control is working hardest, and it errs in the direction of over-reading slip.
Add the slower drifts on top: cold pressure against hot, a fresh tyre against a half worn one, a summer session against a winter one. All of them change rolling radius. None of them change the number in the configuration.
I have not quantified any of that on this car. It is reasoning from geometry rather than measurement, and it stays a working conclusion until there is a number behind it.
The check that takes two minutes
Flat road, steady 50, constant throttle, no load transfer. Log all four wheel speeds and your corrected slip channel.
If corrected slip reads zero, your static correction is right. If it reads a consistent offset, either your configured tyre sizes do not match the car, or your tyres have worn far enough to matter.
Do it after every tyre change, and once mid-season. It costs nothing and it is the only way to know whether the number your traction control is acting on means anything.
Why this matters more than it sounds. Everything the traction control does downstream, the slip target, the feedforward torque model, the PID that corrects what the model missed, is built on the assumption that slip is being measured correctly. Get this wrong and every one of those tables is being tuned against a number that has an offset baked into it. The strategy that sits on top of these wheel speeds is covered in Traction Control on 1,114 whp.
Full specification and the wheel and tyre setup are on the Supra project page.
If you run staggered tyres and have measured your loaded radius rather than assuming the free one, I would like to know how far apart they were. That is the number this article is missing.
