i just installed eibach pro kit on my 08 g37s. they feels a little firm compare to stock springs and my interior starting to rattle. so my question is will these tires smooth out my ride quality a little bit??
245/40-19's on the front and 275/35-19's on the back

stock was 225/45-19's in the front and 245/40-19's in the back

 

No. A shorter sidewall rides worse, not better.

 

so what should i use? can't afford coilovers

 

You could do some research to see what other brand of springs are out there. Also users feedback.

 

245/50 in the front 275/45 in the rear IMO.

 

u running on this setup?

 

I am going to respectfully disagree with Black Betty. The second number is the aspect ratio. Normally a lower number means a shorter sidewall, but what you suggested is called plus sizing. Going wider. The sidewalls should be roughly the same. Going with a taller sidewall would be a mistake IMO. I would be concerned that tires that are substantially taller would throw off your speedometer and possibly interfere with your ABS.

Perhaps a tire expert could tell you which brand has a softer sidewall. If you go this way you will be giving up some handeling.

 

You make a valid point. The tires he asked about in plus zero sizing are close to the same circumference and the sidewalls are only .1" shorter both front and rear which isn't a whole lot. But if his primary reason for changing tire size is to get a more comfortable ride it's certainly not going to help achieve that. The larger contact patch from the extra width coupled with the (slightly) shorter sidewall will amplify road imperfections and be less comfortable. I say this because I did exactly that and that's how it felt to me. YMMV.

Changing to a more comfortable spring like Swift will give a more comfortable ride than changing tire size. Getting a tire with softer sidewalls might help a little, but IMO it would just make the handling floatier and not really more comfortable overall.

 

This poor Guy! You guys need to chil!

To the OP, the reason you ride is rough is because you never put aftermarket springs on a old shocks! You have 2008 so that makes your car 6 years old. I would go with new shocks and then pair them with your springs. I bet your car has over 50K miles!

Trust me best thing you can do is get new shocks and your car will feel new again, going with wider tires on your OEM wheels will help with ride and braking! you are going up 20mm in the front and 25mm in the rear.

Also your ride will depend on what kind of tires you are going to put on, going with a softer sidewall will give you a better ride with less steering response.

Hope that helps you!

 

To sum up several posts, there isn't a whole lot that can be done with tire selection to correct the harsh ride. You can change the springs (again) which I would be reluctant to do in your place, you can live with it or try and find smoother roads. As much as I like the lowered look, my G37S is a daily driver. My last car was a 350Z. While the cars are similar in some respects I like the Infiniti better. The 350Z wasn't lowered either, but it was lower than my Infiniti. I had scraped underneath too many times to count. The cat heat shields were all bent up. I also damaged the front bumper (several times). Believe me I was trying to be careful.

It is up to you, if you really like the lowered look you can keep these springs, or replace with Swift springs. Otherwise put the oem springs back in.

 

so Swift springs are softer than eibach?

 

No, not softer. They are actually slightly stiffer (not much). I said they'd ride more comfortably. Those are not the same thing.

With springs, too soft isn't good and neither is too stiff.

 

It's not what you want to hear, but save up for coilovers. Like others mentioned, tire selection will do minimally for ride comfort. You can feel the difference once you have your suspension set up correctly. There are several affordable coilover setups that pop up for sale in the Classifieds section. I am driving on Tanabes and they are extremely comfortable. Much moreso than stock. A used set goes for $600 or so shipped. Swapping springs and getting struts and all that will eventually put you in as much as getting a set of coilovers and doing it one time. It's worth it.

I have Koni/lowering springs on an e34. I didn't do coilovers because I would have had to do extensive modifications to the strut housing. My G37 is much much more compliant and comfortable than the spring/shock setup. Granted they are 2 completely different vehicles, but it generally gives you an idea.

Put up with it until you can save up to get a set of coilovers. Sell your springs, then enjoy your new ride.

BTW - Your car will start rattling regardless of what suspension you have. That's just the nature of things. Over time, tabs and fasteners and what-have-you will eventually start to work loose enough to the point where the piece will vibrate ever-so-slightly. Just enough to start rattling. It's normal.