wazup_shorty

Quick question for anyone who might know, I got My hands on some 245/40R19 potenzas, quite expensive tires that I do not want to pass on, I curently have 245/35R19 on the vehicule, I was planing on installing the higher potenzas on the rear, the tire sidewall difference I calculated would give me 3.6% speed difference from the front tire, My question is: will this give me a ABS Light for iccoerent signal since the speed difference might be to much?

thanks

Im planing on installing them soon so I will test it to clear this out

godesshunter

Personally I would be more concerned with the TCS wanting to engage when its not needed.

wazup_shorty

true, im expecting issues from the DSC or ABS if any, I have access to a tire mount and balancing machine so I will try it out and see the results.

godesshunter

The margin you are changing is very small. The system is designed to compensate for rolling diameter changes during turning and running the spare tire. I doubt you will have a problem. Doesn't mean it wont look goofy though. We aren't driving muscle cars here.

ASH8

PCM (ECU) is re-programmable for rim/tyre size for all Series 2's, for either 18 or 19 inch rims, but as far as I know Tyre profile ( ie 35R19 to 40R19 ) is not ECU adjustable.

Sorry can't confirm your ABS or DSC effects, personally it would be minimal and within range, after all rubber does wear off tread so technically so does a tyre profile change.

oltmann

I think that wheel size is configurable by the dealer tools because in Japan the speed limiter must be set to the speed rating of the OE tire. Different wheel, different tire, different speed limit. So it is more of a vehicle option than a numerical parameter.

The ecu might use wheel size for some other purpose, but I doubt it, and it is probably programmable by the dealer tools just to make things 100% legal in certain markets. I'd like to know if I'm wrong!

However it works, I would tend to agree that there is probably some tolerance built in and 3% is fine.

There does seem to be a tire size parameter, and the PCM sends it to the DSC/ABS control module. However, I don't think WDS gives a way to set this, but I think Mazda does allow it for some other models.

seanp

I don't quite understand why the ECU would need to be re-programmed for a "stock" rim change between 18 & 19, and how does one go about it??

225/40/R19 - is ~660mm diameter, ~2073mm rolling circumference.
225/45/R18 - is ~662mm diameter, ~2080mm rolling circumference.

So only a 0.34% difference and easily within the parameters the ECU / DSC / ABS is designed to handle (the difference between worn and unworn tyres could be up to 16mm in diameter!)

When comparing - you need to consider the tyre WIDTH as well as the aspect ratio too. Latter is a %of the former!

It is generally accepted that anything over 1.5% difference in rolling circumference can cause interference with the DSC (though granted I do not know where this is "officially" documented by Mazda)...

So, given these 225/40R19 mixed with 245/35R19 (~704mm diameter, ~2212mm rolling circumference)- a 6.35% difference!
I would personally stay well away and expect all manner of flashing DSC lights if fitted.

The tyres are the main thing keeping you from a hard learned car vs immovable object interface lesson

ASH8

Not repeating what I have already said..

In Series 2 the PCM IS re-programmable by a Mazda Dealer, repeat a Mazda Dealer.
Why, well obviously for Speedo accuracy among other things between R3 and non R3 with different rim and tyre size, and design rule regulations.

Here from 2006 Speedo error (the difference between actual car speed and speedo read out) the dash speedo cannot read lower than actual car speed, but it can read higher.
See "United Nations Economic Commission for Europe (UNECE) Regulation 39" which applies to most countries.
http://www.enotes.com/topic/Speedometer

From Mazda USA...

If the wheel and tire sizes are changed, the set value for the tire rotation diameter will be different, causing a discrepancy whereby the speedometer needle exceeds the allowable range. If the wheel and tire sizes are changed, perform a PCM configuration to change the tire size setting. (See PCM CONFIGURATION [13B-MSP].)

Wheel And Tire Size Change
Connect the M-MDS to the DLC-2.

After the vehicle is identified, select the following items from the initial screen of the M-MDS.

When using the IDS (laptop PC)

Select the “Module Programming“.

When using the PDS (Pocket PC)

Select the “Programming“.

Select the “Module Programming“.

Then, select items from the screen menu in the following order.

Select “Programmable Parameters“.

Select “Tire Size / Axle Ratio“.

Select an item name, and then select option.

Items
Tire Size (“225/40R19”/”225/45R18”/”225/50R17”)

oltmann

Great info! Thanks for posting that.

I would guess that the parameter could, in principal, be configured to accommodate any size, but the WDS is only programmed to use those preset values.

There are actually a ton of things that can be configured over CAN, without requiring full reprogramming. For example, you can disable DTC setting, and any CAN interface could do it, like the device Paimon is developing, as opposed to a full reprogramming interface. Hopefully someone will figure it out someday.

seanp

Thanks Ash.

Nerd

Hi

I changed the rear tires with the same width/profile and rim size. Used 20% tread Potenza RE050A on the front, new Potenza RE002 on the back. The difference in size was enough to trigger the DSC on all corners (especially left-handers) taken at speed. This resulted in a disconcerting pulsing "knock" from the rear. It stopped when I turned off the DSC, and became less pronounced over the next few months. Mazda NZ did not know why, but a smart educated mate of mine said that the DSC might be seeing the slight variation in wheel speed as evidence of a skid . We swapped front to back to see what would happen and the problem went away immediately. I will now rotate tires and buy sets of four. I have written this post for those who may have shared my pain and not know what the problem was.