Tire size question
Burning Brakes
Joined: Jun 2008
Posts: 1,214
Likes: 3
From: CA
I had 255/35/18 fronts & 305/30/19 rears before i did my ZO6 conversion. I don't see why not. Might want to stay close to the diameter of the stock tires. Otherwise you might have issues w/ your AH & ABS
Race Director
Joined: Mar 2005
Posts: 10,399
Likes: 26
From: Austin Texas
You can do it, but why would you want to? 20mm wider tire on stock width wheels can be done but handling will suffer.
Just like you could put a size 9 foot in a size 11 shoe, it can be done.
The size stagger F to R would be wrong also, up 20mm with the same aspect ratio as stock in the front and up 20mm with a lower aspect ratio in the rear will probably throw off your Active Handling.
Compare stock diameters to your diameters on;
http://www.miata.net/garage/tirecalc.html or
http://www.1010tires.com/TiresizeCalculator.asp
Just like you could put a size 9 foot in a size 11 shoe, it can be done.
The size stagger F to R would be wrong also, up 20mm with the same aspect ratio as stock in the front and up 20mm with a lower aspect ratio in the rear will probably throw off your Active Handling.
Compare stock diameters to your diameters on;
http://www.miata.net/garage/tirecalc.html or
http://www.1010tires.com/TiresizeCalculator.asp
Thread Starter
Instructor
Joined: Aug 2008
Posts: 184
Likes: 0
From: PITTSBURGH PA
You can do it, but why would you want to? 20mm wider tire on stock width wheels can be done but handling will suffer.
Just like you could put a size 9 foot in a size 11 shoe, it can be done.
The size stagger F to R would be wrong also, up 20mm with the same aspect ratio as stock in the front and up 20mm with a lower aspect ratio in the rear will probably throw off your Active Handling.
Compare stock diameters to your diameters on;
http://www.miata.net/garage/tirecalc.html or
http://www.1010tires.com/TiresizeCalculator.asp
Just like you could put a size 9 foot in a size 11 shoe, it can be done.
The size stagger F to R would be wrong also, up 20mm with the same aspect ratio as stock in the front and up 20mm with a lower aspect ratio in the rear will probably throw off your Active Handling.
Compare stock diameters to your diameters on;
http://www.miata.net/garage/tirecalc.html or
http://www.1010tires.com/TiresizeCalculator.asp
Race Director
Joined: Mar 2005
Posts: 10,399
Likes: 26
From: Austin Texas
Speedo reading is from a reluctor on the ring gear in the differential, the only way it can be changed is by a computer tune.
Previous posts have said that if you keep the same diameter difference between front and rear that the A/H won't interfere. IE; If you change the R. diameter by 0.50 inch then change the F. by the same amount.
I don't understand your question about calculating all 4.
Previous posts have said that if you keep the same diameter difference between front and rear that the A/H won't interfere. IE; If you change the R. diameter by 0.50 inch then change the F. by the same amount.
I don't understand your question about calculating all 4.
Thread Starter
Instructor
Joined: Aug 2008
Posts: 184
Likes: 0
From: PITTSBURGH PA
Speedo reading is from a reluctor on the ring gear in the differential, the only way it can be changed is by a computer tune.
Previous posts have said that if you keep the same diameter difference between front and rear that the A/H won't interfere. IE; If you change the R. diameter by 0.50 inch then change the F. by the same amount.
I don't understand your question about calculating all 4.
Previous posts have said that if you keep the same diameter difference between front and rear that the A/H won't interfere. IE; If you change the R. diameter by 0.50 inch then change the F. by the same amount.
I don't understand your question about calculating all 4.
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Safety Car
Joined: Sep 2007
Posts: 4,359
Likes: 6
From: Los Alamitos California
St. Jude Donor '09
Burning Brakes
Joined: Nov 2008
Posts: 862
Likes: 0
From: Gaithersburg MD
How is the speed computed?
I am not sure but I think the speed as displayed is computed by the front wheels using the wheel speed sensors of the traction control system. If it were not then when you are spinning tires in first gear the transmission controller would shift you directly to 2nd since clearly your wheel speed is far above the 1->2 shift points.
I think you can see this by looking at the speed display when doing a burnout.
I think you can see this by looking at the speed display when doing a burnout.
Former Vendor
Joined: Aug 2005
Posts: 16,897
Likes: 412
From: Lewisville TX
St. Jude Donor '03-'04-'05-'06-'07-'08-'09-'10-'11-'12-'13
The car has problems when the tire's rolling diameter is changed. This is based off of the four wheel speed sensors based inside the wheel bearings.
You have roughly 5% variance to work with here between front and rear as the car does have to account for tire wear. You can go shorter or taller, again slightly but you need to keep the rolling diameter close to the same.
For example I have a 255/35/18 front and 305/30/19 on the rear. While both of these are slightly shorter than stock, they still read correct up to a tested 150 mph.
Now if they are different this is going to grow with speed, so if the car does not see speeds over 125-140 mph with the TC/AH on no worries. If you are going to run the car over that with TC/AH the car doesn't care so it isn't going to effect anything.
You have roughly 5% variance to work with here between front and rear as the car does have to account for tire wear. You can go shorter or taller, again slightly but you need to keep the rolling diameter close to the same.
For example I have a 255/35/18 front and 305/30/19 on the rear. While both of these are slightly shorter than stock, they still read correct up to a tested 150 mph.
Now if they are different this is going to grow with speed, so if the car does not see speeds over 125-140 mph with the TC/AH on no worries. If you are going to run the car over that with TC/AH the car doesn't care so it isn't going to effect anything.
Pro
Joined: Jan 2005
Posts: 615
Likes: 2
From: Mill Creek WA
Anthony,
I'm don't understand your 5% number. It was my understaing that the OEM "rake" is 1.2 inches, where rake is the difference in height of the rear wheel and the front wheels. As long as this rake is maintained the AH/TC doesn't car. Is that correct?
Alternatively, it has been stated on this forum that HP-tuners can be used to allow use of "any" tire combination front-to-rear.
I'm don't understand your 5% number. It was my understaing that the OEM "rake" is 1.2 inches, where rake is the difference in height of the rear wheel and the front wheels. As long as this rake is maintained the AH/TC doesn't car. Is that correct?
Alternatively, it has been stated on this forum that HP-tuners can be used to allow use of "any" tire combination front-to-rear.
Former Vendor
Joined: Aug 2005
Posts: 16,897
Likes: 412
From: Lewisville TX
St. Jude Donor '03-'04-'05-'06-'07-'08-'09-'10-'11-'12-'13
Anthony,
I'm don't understand your 5% number. It was my understaing that the OEM "rake" is 1.2 inches, where rake is the difference in height of the rear wheel and the front wheels. As long as this rake is maintained the AH/TC doesn't car. Is that correct?
Alternatively, it has been stated on this forum that HP-tuners can be used to allow use of "any" tire combination front-to-rear.
I'm don't understand your 5% number. It was my understaing that the OEM "rake" is 1.2 inches, where rake is the difference in height of the rear wheel and the front wheels. As long as this rake is maintained the AH/TC doesn't car. Is that correct?
Alternatively, it has been stated on this forum that HP-tuners can be used to allow use of "any" tire combination front-to-rear.
If you want to get right down to it, the car is going to be looking at the rolling diameter of the tire or rev's per mile. Tire height is a quick and dirty way to do it though.
Racer
Joined: Sep 2007
Posts: 393
Likes: 0
From: North Bend WA
To answer the origional question. Many users on the forum have run 265/35-18 fronts and 305/30-19 rear size tires on the factory wheels and had no problem. Yours may react differently but I was planning on doing the same thing until I got my wheels widened.