My car is a G37 sport sedan with the stock wheel and tire sizes (Fronts 7.5" + 225/50/18 and Rears 8.5" + 245/45/18).

I've tried searching for answers but I can't find a specific answer. I'm in the market for new tires kinda soon and I'll be keeping the stock rims, but I don't like the sidewall size very much. I've played with tire size calculators to compare different sizes but it's hard for me to wrap my head around how it will actually affect the looks/performance of the G.

My goal by changing the tire size is to reduce the sidewall size a little bit without changing the dynamics or balance of the car too much. Now I'm thinking of two different options.

Option 1: Change front tire size to 235/45/18 and keep the rear the same at 245/45/18.
This option will lower the front tire height by 0.6". In stock form, the front usually sits HIGHER than the rear by 0.2", so if I changed the front tires then the front would sit LOWER than the rear by 0.4". This also gives the front a 0.4" wider contact patch and it reduces the sidewall size by 0.2". So if I did this, the front sidewall size would be 4.2" and the rear would be 4.3" so they would look pretty much the same.

Option 2: Change front tire size to 235/45/18 and also change the rear to 255/40/18.
This option will lower the front tire height by 0.6" and the rears by 0.7". This also gives the front a 0.4" and the rear a 0.4" wider contact patch and reduces the sidewall sizes by 0.2" and 0.3" respectively. So if I did this, the front sidewall size would be 4.2" and the rear would be 4".

Any opinions would be greatly appreciated! I'm fine with the speedo being off a bit btw, if necessary. And would doing this make my wheel gap bigger or stay basically the same?

 

The ECU checks the front/rear rotation rates. If there is more than a 3% difference than the OE value, the dashboard will light up.

You first option gives a 2% difference from stock in the front.

You second option gives a 1% difference between front and rear but the rear is a hair over 3% different from stock and may trigger the ECU.

 

While it's more costly, the simpler solution is to acquire a set of coupe 19s. Then you can run 245/275 staggered and enjoy both more tread on the ground and lower sidewalls.

 

Isn't there a way you can reset or change that sensor?

 

You have to change the value stored in the ECU. None of the tuners care about that aspect so they don't expose it.

 

As far as the two options themselves go, what do you think about either Choice as far as how it will look or affect the performance?

 

I calculated about 2% difference from stock for the Fronts and 2.5% on the Rears

 

My spreadsheet sez 3.08 percent on the rear. Your first option is "safer". Be aware that lower profile tires tend to accentuate off-road look.

 

To the OP, if you really want to get technical about it, you shouldn't be making comparison based on tire size, but rather revs/mile. You can find this info on tire rack and most mfg spec sites. You know the stock size rotates X times per mile, so you don't want to deviate significantly.

Also, the deviation is relative between the wheels, not absolute. For example, hypothetically install 16" wheels with tiny tires, say 20% small diameter than stock. Such wheel/tire combo is installed at each corner. The car has no idea what tires/wheel sizes are installed. All it knows is revolution per unit time of each wheel based on the speed sensor at each wheel. The computer is happy so long as each wheel is spinning at a rate within an acceptable tolerance of the other 3 wheels (3%?).

Further, going wider/stickier will result in poor gas mileage. This may or may not be a concern. Not to mention, shorter sidewalls will affect ride quality and be more susceptible to damage. When I got the PSS's for my sedan, I kept stock sizes. I figured for the type of driving I'll be doing, it makes no sense to change. The car still rails quite well in the corners while keeping it decent in around town driving.

 

The G apparently monitors wheel speed for more than just differential (antilock). That's where the ±3% hard limit comes from. I'll dive into my FSM to see if it mentions anything about this.

 

Good point. If the computer is smart enough to know that it takes between x-y revolutions (tolerance) of the wheels per z speed then there may be other issues such as traction control. If so, then one must truly adhere to near stock diameters. I realized that might be the case after posting last night.