Different Size Tires
 

I got new tires for the rear and I have 18's and I got 245/40/18 and stock is 245/45/18 now when i am doing highway speeds 70+. I feel like the car if flighty in the front when i change lanes abruptly, meaning the car feels loose in the front and swerves side to side. do you think it is because I am running lower tires than spec?

Rear tyre diameter is ~30 mm smaller with your new rear tyres and my guess is the stability control is confused, and thinks the rear wheels are breaking into wheelspin with your steering inputs and it is intervening (maybe applying one brake to avoid a slide.

OEM rear is 245/45R18 and diameter is ~678mm, your new 245/40R18 diameter is ~654mm.

You need to do one of 2 things, and that is fit the proper rear tyre size, OR get smaller front tyres fitted to match the rears (235/40R18 will go close). Be best all round to fit 245/45R18.

The answer to your question is therefore "YES".

are they even the same tires?

^this, different tires, different speed ratings, a lot of factors could be in play. What kind of tires did you put on? speed rating? load rating?

I have 245/40/18 Bridgestone Protenza RE970AS and 225/50/18 Firestone Firehawks on the front.

The place I bought tires said they would switch out my rears to the right ones 245/45/18 for me at no cost. but I have a dilemma my fronts have decent tread on them and I'd hate to have to buy new ones and throw the old ones away.. but I just want the car to handle like it should.

So I am looking for a lil guidance here.

are you saying I should get 245/45/18 for every tire. and not stagger it?

Dude you are driving around with different compounds. Buy 4 tires or deal with it

Zis4me--

I also have the base 18s and suffered from this problem originally..
Not only did the car feel wobbly at higher speeds, but I felt unstable braking at higher speeds as well.

I ended up purchasing new Pirelli P Zeros in the front. Jumped up to a 235/50. Helped fill out the wheel well more. I also added 275/45s in the back. The widest tires possible.

After buying the bigger tires, turning at high speeds is no longer scary, I can no longer spin out the tires in second gear, wobble is gone, and the tires stick to the road.

Thanks I will get 2 new fronts.


you put those sizes on the 18's I might have to do this.. Since I should buy new fronts to match what I have in the back now..

Thanks

I disagree with people saying different compount front and rear affects the handling adversely. In terms of an unsetteled feeling at highway speeds, or with quick left/right manueuvers, it's most likely that the new tire that you purchase has a softer sidewall. All the 370z's, esp. Sport package equipped 370z's, came with sporty, stiff-sidewall tires. Switching to a tire with less agressive characteristics, such as a lower rated performance tire or an all-season tire will affect the stability of the car.

Anecdotally, I personally bought all-seasons for all 4 corners of my car (Bridgestone Potenza RE970AS), and within 30 minutes of driving city and highway, I realized that my car was so unstable compared to the stock RE050A. All I did was change the front tires to Potenza S-04 (extreme summer performance tire), and leave the rears as all seasons, and the car regained it's stability at high speeds and with abrupt maneuvers.

Trust me - it's all about the quaility of the tire, and the sidewall stiffness. Different tire compounds front and rear is not a bad thing. Of course, this only applies to the stability of the car.

I have the RE97AS on my rears now.. and I never have RE050A on my car now (which are super sticky tires.. and have a life span of a butterfly).. I had Firehawks... so Thats all I know.

You didnt like the feel of the RE970AS on all four?

Maybe I'll just get the Potenza RE760 Sport on all four sides.

It honestly wasn't bad, but I knew that the sidewalls were softer, and if you do a quick jerking of the wheel back and forth, you can feel the car unsettle and shimmy. On the stock RE050A, they car just simply darted to the side with no body roll. Grip-wise though, the RE970AS are amazing. Straight line grip is awesome.

I demand a lot from my car handling wise, so I figured I found a perfect balance with stiff, grippy S04 fronts, with the still grippy and long lasting RE970AS in the rear.

Trust me - if you drive in a car with summer performance tires (preferably another Z), it will make sense. Our cars feel like they are on rails with good tires.

One thing that annoyed me about the RE970AS is that I wish they came in 305 wide.

Currently I have re970's in front at a 255/45/18 and Re760's in the rear at 275/40 and I honestly don't feel a difference. At the moment until I upgrade to RE-11's, I'm even auto crossing on the set. People are always amazed that I'm running all seasons on front for autocross but they actually handle pretty damn well. Of course I'm still a beginner so as my skills get better, obviously my opinion of the tires will change. The big difference always tends to be when people mix manufacturers and not just different tread patterns. Each company generally builds all their tires with same speed rating the same so staying same company doesn't make the car feel loose. Running a bridge stone w rated tire with a dunlop w rated tire and sidewalls are constructed differently then the car will probably feel unsteady.

you are running on the stock rims?