OK...here is the deal: I have been planning to go with aftermarket wheels and tires. I plan to go a fat as possible, but want to know the implications. First, I figure a 245 series up front should help a bit with the understeer issues. Then in the rear 295's are the widest I can fit without rolling the fenders (may have to just a bit depending on my offsets anyway) I'd even consider running 305's, but no one makes a tire that size that would work

Anyway....here is what has me worried: when I talk to aftermarket distributors...they all tell me this and that and can all get me a great deal and mention nothing about any problems with what I want to do. However, I just spoke with a guy from TireRack who said they don't have any fitments recommended for my car as aggressive as I want to go....and that I need to be careful about all the "important electronics" on this car that rely on rolling diameters etc. WHAT the hell is he talking about? Isn't the only thing that may be off is my odometer and speedo if I have slightly incorrect tire diameters? If I went with 295/35R18's, wouldn't my speedo say 66.5 mph when I'm actually going 65mph? Is that so bad? Or if I went with 295/40R18's, wouldn't it say I'm going 63.6mph at 65? Can either really mess up the VDC or EBD? Do I have my math wrong? Would this alteration feed back any incorrect information to the ECU making for a reduction in performance?

I just want to be informed before I make a dramatic modification which is not only expensive, but potentially damaging to my performance.

Thanks

 

it will also through off your odometer.. when you go to get rid of your car may show more / less miles than it really has on it
and i dont know if we had any verdict on this forum as to whether or not it will interfere with VDC. i think vdc reacts to what is happening at the wheel not what is "supposed" to happen at the wheels. so you might be alright. However stoptech released a good whitepaper on why bigger brakes can screw up abs. so i guess anything is possible

 

Well, just to be safe (and this is presuming I don't change my mind again) I am likely going to go with 245's up front, and 285/40/18's in back. This takes me up only .3" from stock, and don't feel it will mess with things very much. The downside to 295's is that a 40 series will put the diameter .6" over (too much in my mind) and teh 35 series would be .6" under. Since the weight is going to be more...I'd opt for the 35 series of the two, but then I'd also be getting a greater span between the top of the tread, and the fender. It would almost look like I raised the suspension! NOT the direction I want to go. 285/40/18's will keep me within .3 of stock diameter, and slightly decrease the wheel to fender distance. I can accomplish the "fatter" look with my offsets, and the 10mm won't be something I can't live without. Now if someone successfully runs 315's....please get ahold of me and give me the details! The 20mm might change my logic!

 

http://www.miata.net/garage/tirecalc.html

you should stay within 3% of your stock rolling diameter to avoid any issues with speedo calibrations, ABS, VDC and all that other stuff.

So:

Stock rear: 245/40-18
Sidewall: 3.9"
radius: 12.9"
diameter: 25.7"
circumf: 80.8
Revs/mile: 784.0

A good upgrade is 275/35-18
(aspect ratio (35 in this case) should decrease as tread width (275 in this case) increases.)
Sidewall: 3.8"
radius: 12.8"
diameter: 25.6"
circum: 80.4
Revsmile: 788.0

Speedow readin will be 0.5% too fast. So, when you're speedo reads 60, you will be traveling 59.7.

The 294/40-18 will be off by 6.1%. Way too much. Look for a 295/30-18 and you will be right on the money, only 2.9% off.

Hope that helps.

PeteH

 

your math is a bit off on yoru stock wheel your radius x2 doesnt equal your diameter.. fwiw

 

I didn't go through Audito350Z's math...but he used the wrong stock tire size which may explain my confusion as I read his post. Our car's (with 18" wheels) come with 245/45/18's not 245/40/18's on the rear with a diameter of 26.7". If I wanted to go with 275's, then actually a 275/40/18 would be dead nuts at the same 26.7" diameter, but then it would be thinner than I like. I think I'm pretty settled on the 285/40/18's...but even the 295/40/18's would only be a smidgin (sp?) over 2% difference (2.1978 if my math is correct).

 

no check your math again too.. theres no way that 245 x .40 = 275 x .40

remember the sidewall measurement is a relationship to the width of the tire

edit: another way to find what size Sidewall you need :
2(.40x245)+18=2(S x 275) + 18
.40x245=Sx275
98=275S
.35=S
you want a 35 series sidewall

 

Reread my post...it may be somewhat confusing, but I am saying that a 245/45/18 is EXACTLY the same as a 275/40/18. If you tell me this is wrong, I'm going to have to shoot the people that provide all those tire calculators on various sites! Additionally, I am saying I don't want to go that thin, and although I'd like to run 295x18's, the closest combination is either a 35 or 40 which are both .6" off (one under and one over). THUS...I'll probably go with a 285/40/18 which is only .3" over.

Read the above carefully, and please correct me if this is wrong....I am basing some decisions on this math

 

looks like the 285/40-18 will work fine...

 

audito hows that work out...???
itd have to be like a 285/33 lol i know 33 isnt real,,, but math wise... run it through my formula

 

It works out like this: a 285/40/18 is diameter of 27" which is only .3" larger than stock and is only slightly over 1% "off". A 285/35/18 which is closest to your suggestion is a diameter of 25.9" and is nearly 3% "off". The 33 you suggest (which doesn't exist) would be even worse with a diameter even smaller yet than the 25.9". Going into 3% and beyond is venturing into uncharted/unrecommended areas.

 

Tyr,

Your formula may or may not be accurate, but the numbers you are plugging into it are messed up! You're plugging in "275" when I want to run a 285 for one. Secondly and more importantly, you keep using "40" as the starting series along with the 245. I WILL REPEAT: STOCK SIZE IS AS FOLLOWS ON A 18" WHEEL: 245/45/18....NOT 245/40/18. And the only variation of this rule is the 17" wheels which use 235/50/17's.

Stop using Audito350Z's incorrect figures from earlier in the post!

Let's all just move on.

 

ya thats my laziness. i didnt bother to make sure those figures were correct,, my formula however is..
just change the numbers

heres the formula again

2(Aspect of tire * Width of tire) + Size of rim = 2(Aspect of tire2 * Width of tire2) + Size of rim2

basically as long as those two equal one another you have the same outter diameter.
you can use the equation to plug in your stock
2(.45 *245) + 18 .... remember the aspect is a percentage
and then say you wanna put a 19 " wheel w/ a 275 on but keep same outter diameter
2(x * 275) + 19

2(.45*245) = 2(x*275) +1
2(110.25)=2(275x)+1
220.5=2(275x)+1
219.5=2(275x)
x= about 39 percent.. so a 40 series sidewall would work

x = what your goal aspect ratio is.
x in this case = 40

 

So for those of us who had our calculator batteries die out 3 posts ago from running all the numbers, if we are keeping the stock 18 inch rims, what would be the best choice for a front and rear size upgrade (given ratio, look, and feel on the road)?

 

i'm most likely to run 245 fronts and 285 rears... rim size and such has not been decided yet. probably going with a 19.. just depends on what suspension stuff comes out soon. as to whether i'll want to lower the car much or not

 

Ok, so my crappy *** REO40's are already bald on my rear after 6k miles. I'm very pissed that Nissan would put such crap on these flagship cars. Anyway, I need new tires asap. From the calculator above, as well as discount tire's recommened sizes, here's what I'm thinking about:

Front
stock: 225/45/18
new: 245/40/18

Rear
stock: 245/45/18
new: 275/40/18

So I'm keeping my stock performance 18" rims. Is there any danger in going to these wider tires? Should I maybe just go wider on the rear and keep the front stock? How will all of this affect handling/steering? Thanks for any input you guys have got!

 

Infinite Z, I agree with you on the crappy tires and also on your size choices.

Although, I asked a Tirerack guy named Clay about the 275/40 in the rear and he could not confirm the fit.

Here is what I asked him along with his response:

My question:
Clay, can I use 275/40/18 in the rear? The stock size
is 245/45/18. (You had recommended 265/40)
According to my calculations, all of the measurements
(radius, sidewall, diameter, circumference and
revolutions) stay the same with a 245/45 and a 275/40.
Am I correct? I like the S-03 Pole Positions and you
carry them in this size.

His Response:
"The 275/40/18 is wider than recommended so I can't guarantee fitment of this size. "


Do you guys agree that 275/40 won't work?

 

Well now I'm scared about the 270s. If I do 255/40 in the rear, I'd feel safer. With a 245/40 on the front, it'd be 0.3" less diameter front, and with 255/40 it's 0.7" less diameter. So overall a 0.4" diameter change, and the rear would be lowered 0.3" over the front. Is this bad, or will I even notice the difference?