i recently got the coupe 19s and i know the ideal tires size would be 225/40f or 235/40f and 255/35r.... but i cant seem to find this size for the achilles atr sports, does anyone know if its fine to go 245/35 all around?? Thank you for your time in advance

 

yea thats fine, you will still be in spec. im running 35 series on my oem 19s. 40s would be a much nicer ride though.. just so u take that into account. i dont complain, the ride is fine for me.

 

Yeah you can run that size no problem. Hopefully you are lowered a bit because that tire size is a bit smaller than OE tire diamters.

 

im gonna be lowered on tanabes df210s. Is it true that if the tire sizes arent correct, you will start to have gear problems?

thanks again for all the fast replies.

 

If the rolling diameters of the tires are more than 3% your VDC will freak out.

 

Which Tanabe df210's? coupe, 350z or sedan? People run different springs because they offer different drops. and no you will not have gear problems because of tire diameter.

 

exactly, your slip will start going off at random times... will get super retarded.

 

im running the df210s from the coupe, is this fine? just got the fenders rolled and i was told it lowers the sedan about 1.5

awesome, so when you say " in spec" that means under 3%?

 

^thats right, u will be in spec.

 

So just for clarification, when you say within 3% are you refering to 3% overall diameter difference front to rear.

 

No. 3% from factory settings

 

^^I think 4D is correct. From what I've read factory diameters aren't the guide, it's the difference between the rolling diameter of the front and rear tires.

This link has a great calculator where you plug in the different tire sizes and it will spit out the specs for comparison and actually also create an image comparing the 2 sizes. http://www.miata.net/garage/tirecalc.html

 

Can you elaborate on that? I dont understand how that would affect VDC. The car has no way of knowing if you are running 26.3" 24.0 or 27.8" tires. Obviously the diameter change affects the effective gear ratio, but I dont understand how it can affect VDC with all things similar. Meaning the same overall diameter front to rear (if the car came that way) or the same overall diameter variance front to rear (IE 225/40f and 245/40/r just larger or smaller in overall diameter).

I have heard others say this and never understood. If you are running larger than OE tires then the car will just think its going slower than it actually is and the opposite for smaller tire diameters, which is true and makes sense.

I guess I can relate this to doing a pumpkin swap. I went from a 3.3 to a 3.5 which is about 6% difference, or the same affect as going with a tire that is 6% smaller than OE. Others have gone with 3.7, on up (15+% change) with no VDC issues.
I have played around with tire sizes (same front to rear, rear larger than front etc) and I have noticed that the tires will slip more before VDC interferes with a larger rear tire than the front. My understanding is that the car compares the front/rear tire speeds for VDC (when it sees the rear spinning x% more than the front VDC takes over).

Thats all I know. You are not the first person to state it this way so I hope to learn something. Thats all I am trying to do

Thanks.

 

I'll do the best I can to explain what I know, but hopefully someone like DaveB that has more technical knowledge might be able to explain it better.

The VDC is programmed to work when the tires are w/in 3% of each other in diameter. For example, the stock tires on an 06 coupe are 225/40/19F with a diameter of 26.1". The rear tires are 245/40/19, diameter 26.7". The difference is .6" or a 2.23% difference in size. If you were to swap out say the rear to a 275/40/19 you increase the diameter of that tire to 27.7" which at a 1.6" difference would give you 5.78% difference in size and your dash would light up and most probably slip into limp mode.

However, you could go down in size to a 225/35/40-245/35/40 set up which would set your diameters to 25.2" and 25.8" which is a 2.33% difference, but would give you tires that were around an inch shorter but still play nice with the VDC. And you're right, when you start playing with sizes your speedo will be off a little, but your car will drive just fine.

Hope this makes sense and helps you understand.

 

Cool, I'm on the same page.
So if the rear tire is drastically larger than the front it will effect VDC as well? Using your example of 5.78%.
I know having a smaller rear tire (aesthetics aside) is not good because it tricks the computer into thinking that the rear tire is slipping due to it spinning faster than the front. I have never ventured that far (rear more than 3% larger).
And yes anyone with experience please chime in, this is a good learning experience for all.