Not sure "If" this information has been mentioned before (probably has), but, for those who have changed their Wheel and Tire SIZE (say from 18" to RX-8 R3 19"), you can have your ECU (PCM) Updated to show correct Miles Per Hour or Kilometers Per Hour on your Speedometer.

This can ONLY be done (AFAIK) at your Mazda Dealer who has and uses the Mazda Factory M-MDS Software/Hardware.

This is the Procedure if you have access to Mazda's M-MDS.

1. Connect the M-MDS to the DLC-2 (OBD-II Dongle).

2. After the Vehicle is Identified, select the following items from the initial screen of the M-MDS.
A. Select 'Module Programming' or 'Programming'.

3. Then, Select items from the screen menu in the Following ORDER.
A. Select "Programming Parameters".
B. Select "Tire Size/ Axel Ratio".

4. Select an item name, and then select options.
ITEMS..
* Tire Size ("225/40R19" "225/45R18" "225/50R17")

Sorry, IT LOOKS LIKE THIS COULD ONLY BE FOR SERIES 2 (2009~) RX-8's,
The Owners who change from Standard 18" to the R3's 19" Rims and Tires.
UNLESS someone has the Factory Series 1 WS Manual..Read Below.

 

So if you take the above information shown IN BLACK to your Mazda Dealer they should be-able to re-calibrate your SPEEDO.

This is Mazda's CAUTION...

If the wheel and tire size 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 size are changed, perform a PCM configuration to change the tire size settings.

BTW..If you have ALSO or Separately Changed your Differential "Crown Wheel and Pinion" for a Diff Ratio Change, you should also
perform the above PCM (ECU) re-calibration.

Sorry, I do not know what a Mazda Dealer would charge for this Service..I would hope it would be only a few dollars??

Disclaimer..
The Above PCM (ECU) Re-calibration is taken from the Genuine 2009 Mazda RX-8 Workshop Manual,
I am assuming these software alteration settings can and would be OK on a Series 1 2003-2008 RX-8 PCM.

 

Lol so I can really go up to Mazda and all id need is my rim size, 18x8.5 and my tire size, 245/40-18? And they will recalibrate my ECU to the correct mph?

 

What is so funny?

What was the standard rim size on your car 17 or 18", if you have not changed the diameter (I thought all US RX-8's had 18"), then there is no need is there..

If you had 19", I would say yes, but read my disclaimer..

 

Well OEM stock tire size is 18x8 I thought... So going to a 18x8.5 rim and the slightly bigger tire size of 245/40-18 would throw the ECU off a little no? And I guess its just funny because ive asked this question like 2 years ago and wondered why Mazda has not done this before now...

 

even if you stick with 18s and change the tire size wouldnt that change the overall diameter of the wheel and therefore need to be reset as well?

 

Ash: great post, I wasn't aware that we could do this. (or would even be willing to)
I now have 19x10 wheels on the rear but I'm wondering exactly how much Mazda would charge for this little update.

No way to update this with an AccessPORT or ProTuner?

 

+1 great write up! wasnt trying to knock it just had a question

 

Sorry, can't read your mind mate of 2 years ago

In your case I don't think the half inch wider tires would make that much if any to the Speedo reading...it is the Diameter of the Wheel.

I just came across this in my S2 Factory Workshop Manual, and recall others asking about Speedo errors is wheel rims that are larger (or smaller), and thought little of it until I just read this.

The information in Black, is directly from the WS manual and what a Dealer would Read...if you want to call as ask it is Page..

Page 01-40-26 Caution
Page 01-40-26-1 Procedure
In 2009 Mazda RX-8 Workshop Manual (1927-1U-08C)

 

Ahh is this only for S2 RX-8's or would this work for all series 8's?

 

Yes, if you use the standard 18" rim and used a Higher Profile Tire which would then make the Overall Diameter larger..then yes it would also have a slight change.

PLEASE NOTE GUYS......

It may be (and I think it will be the case) that you will ONLY be able to Reset the Tire Settings to those which Mazda use or those that I have shown above which is in the WS manual..

So Alfy, if you have 19" rears but are 10" wide, I would still change it to the
225/40R19" Setting..because at the moment you would be on the 18" one.

I repeat that is only if Series 1 owners can do so with their PCM.

Sorry, I don't have a Series 1 WS Manual to look, does anyone else??..

It is listed in S2 in "Control Systems"

 

I just checked the Factory Parts Manual and they show just two rim sizes for SERIES 1 RX-8's...would you believe a 16"x7.5JJ Rim and the standard 18", but NO Larger 19"!!.

So sorry, guys unless someone has the S1 Factory WS Manual they can look up, it may not be possible...you could go to 16"...possibly, but what for??

Looks like it may be only for Series 2 RX-8 Vehicles.

 

Ash, I am pretty sure I have seen a parameter while working on ProTuner that I thought was this exact setting. I lost all my emails a few months back and my discussion with Hymee about has gone. Will go and have another look and get back to you.

 

Cool..Thanks Andrew, It would be good to know this can be done on Series 1 RX-8's, for 90% of members..

Owners of either Series 1 and 2 who have purchased the 19 inch rims from the 09 R3 with 225/40 R19 89W Bridgestone RE050A Potenza's.
Obviously a different tyre than the 225/45 R18 91W on the 18 inch rims.

 

Or, you could just use the proper aspect ratio tire and not worry about it.

 

Sorry, cant find it

 

Has anyone tried to confirm if only the stock wheel/tire combinations can be selected?