I don't recall running across anyone stating they have had any issues with their car running poorly due to improper tire sizing and their VDC. I've read plenty of people warn not to go too far from stock tire size both in overall diameter and ratio between front and rear, but no one has given technical information giving conclusive limitations. Many have posted with various other vehicles having had problems when they went too far out of spec., what is "TOO FAR?" I know all the generic answers you get from tire supply houses...but who knows any factual information backed by computer tolerances and outside perameter points?

Can someone with technical knowledge of how this system and others such as anti-lock braking and EBD work please comment on the outer acceptable limits? I'd like to know in order to select tire options with the understanding of the implications of doing something beyond that limit. All those tempted to answer the standard, "stay within 2% of stock" please don't bother...I'm looking for something a bit more detailed.

If you are wondering about the sizes I am specifically considering, it is a 285/35/18 and 265/35/18 combo which has the following diameters relative to my stock Performance:
225/45/18 245/45/18
Stock: 26" 26.7"
265/35/18 285/35/18
After: 25.3" 25.9"

Ratio front to back damned close....overall diameter off by 3.1%

Thanks for your comments in advance.

 

The generally accepted rule-of-thumb is to stay within 3% variation of the OEM overall diameter. I'd say that your 3.1% will be perfectly fine and cause no problems (remember, thats just my educated opinion, if your car blows up, don't blame me!).

As for EBD.... it could not care less what tires you have. All it looks at is the amount of weight over the front and the rear wheels and then adjusts brake bias to add more or less to the rear brakes.

VDC also does not care what size tires you have. All VDC cares about is "is one or more tire(s) spinning at a different rate than any of the others and am I sliding?". It takes into consideration the direction you are sliding so that is knows on which side of the car to apply a little brake pressure, to bring the slide under control.

ABS also does not car what size tires you have. All ABS cares about is "is one or more tire(s) NOT spinning in relation to one or more of the other tires?". If that is the case, ABS will "pump" the brakes to that channel(s) only, stopping that wheel(s) from locking-up and allowing you to remain in control of the car.

The only system that really cares what size tire you have will be the speedometer. When you alter tire diameter, you effectively change the final gear ratio of the car. Your speedometer will be off by whatever percent change you have made in the overall diameter.

The wheel/tire calculator at the Miata site is really informative and will tell you how your speedometer will read if you have a different diameter. I like to use this calculator to test tire sizes
http://www.miata.net/garage/tirecalc.html

Hope that helps!

PeteH

 

It helps..but logic seems faulted. If VDC (for example) cares if one or more wheels are spinning at a different rate than the others...then if you go out of spec with the front to rear ratio the computer could interpret this as excessive wheel spin. ie you go with a stock diameter in front but drop the rear diameter 1" and you have already "accelerated" the rear wheel @3.5% from stock relative to it's believed front wheel spin...wouldn't this contribute to the interpretation of excessive rear end wheel spin and be catagorized as potential icy conditions causing intermitant rear brake application? Or cut in power? I don't think these systems "learn" like our ECU does and if this is true this continuous braking would certainly affect handling not to mention overall performance and brake life....or am I all wet?

 

I complained about my VDC coming on late when I put the 255/45s on my 8" rim. The VDC was not malfuctioning per se, but coming on several seconds after it should have when first responding to the tires breaking loose.

The real and honest issue I did have with the 255/45s was that they were a bit too wide for the 8" rim. Sidewalls would fold in hard corners and gave me a floaty ride.

 

close... you are not spinnnig the rear wheel (or the different diameter wheel) at a rate enough to engage the VDC. When you are on a slippery surface, the slipping wheel will accelerate at a much greter rate than the rest of the wheels. Also, the car knows if you are sitting still with a wheel spinning or moving forward with a wheel spinning.

So, if you are at a stop sign and the rear wheels slip when you accelerate, your ECU will sense the slip and momentarily cut power, allowing the rear wheels to get a little traction before putting the power back. But, if you have tires on the back that are just a little off from the OEM size, that tire is not accelerating fast enough to make the VDC/ECU worry about traction.



There's really not any learning going on. Maybe a minor ECU fuel mapping adaptation based on the changed gear ratio... but nothing more.

Like I said in my earlier post.... you should be fine. These are good point to consider when purchasing wheels and tires. But, don't over-analyze things. Lots of people have been putting different size tires on lots of different cars with no problem. I've done it myself.


PeteH

 

Appreciate the responses...not trying to over analize...just want to be educated so as not to make a stupid/expensive mistake! Made too many in my life already...so I'm cautious.

 

No worries... like I said earlier, I think you will be fine with that tire size that is just a smidgen over 3%. Buy them, put them on the car, enjoy them.... nothing's gonna go wrong.


PeteH

 

Will do

 

You might want to PM Cazzzz or Silverchrome (if they are still around) because those two had 255/xx-20 for their front and rear tires. I don't remember what their aspect ratio's were, but I do know they used the same size tire in the front and back, and if they ran anything other than 255/30-20 they would have been more than 4% off the stock size (the 255/30-20 is an almost perfect match with the stock, so if they ran that size then they might not have any good data points for your quest). I have never heard either of them mention any problems with the VDC on their cars.

-D'oh!

 

thankx