In a word, yes. 225/40-18 tyres will fit on a Primera P11/Infiniti G20 but with some provisos.
You need to have the correct offset and you need to accept some alterations to the fenders depending on the tire you are using.
I am using Kumho V70A 225/40-18. These tires have the text book definition of a “square shoulder” ie the corner on the edge of the tyre is square. 225 cross section includes all of the tread. The tread is just as wide as the rest of the tyre, the sidewalls do not bulge out. At all.
These tyres are probably the worst case scenario and they fit a Primera P11/Infiniti G20. Just.
18 inch wheel offset.
I am running 40mm offset and while this works, a 37mm offset would be better. The problem is at the front. The tire will slightly rub the third link on full lock. I think a 37mm offset would solve this so I will be fitting some spacers in the future.
I am sure this is going to cause some rubbing on the front fender but nothing the phone book method will not be available to solve (I hope)
The plastics in the front fender well also rub on full lock.
At the rear, the 40mm offset is about perfect but there is rubbing. You will need to roll the fender not just at the top but all of the way round, 90 degrees on each side. And I still have a slight problem with the tyres rubbing the rear bumper brackets. These will have to be bent upwards and the bumper attached with cable ties.
I should also mention my Primera P11/Infiniti G20 is lowered by around 40mm front and rear on coilover suspension.
Driving the Primera P11/Infiniti G20 on 18s
You notice the weight of the wheels. You have to push the brake pedal harder and you have to put in more initial effort to turn the wheel. This generally gives the car a heavier feel.
I am using an SR20VE motor. While the acceleration is not as sharp as before, the engine has no problem getting on top of the gears. The car is a more relaxed cruiser now and the increased diameter has give the car a bigger range in each gear. Which is nice. Especially when you are driving at 80mph. Less than 4000rpm now.
Driving in town is even worse now because of the longer first gear.
Before the car could hit the rev limiter in 5th gear, basically Nissan absolutely nailed the gearing on the car from the factory, now it wont. A plus because you save on fuel when driving on the freeway. Top speed seems to be about the same. Maybe it has lost 3 or 4mph top speed.
All in all, if you are looking to increase the acceleration of your Primera P11/Infiniti G20, 225/40-18 tires are not the way to go. If you want to give the car better range in 3rd and 4th, increasing the rolling diameter is the way to go. Again, I stress the car uses an SR20VE, I am not sure if the DE has the power to work with a 225/40-18 tire. Maybe it would effect the acceleration too much.
Now the ride is appalling. Again I use coilover suspension, but the effect of the tires has been massive. The car is rock hard now. And again, I am using Kumho V70A tyres, they have an ultra stiff sidewall.
This is the biggest downside to fitting 225/40-18 tyres over the 195/60-15. Road based 225/40-18 tires will probably be better.
Stick likes glue. I have been driving the car for a few weeks now and I still do not know where the limits are. My bravery is the limiting factor not the car. One thing I have noticed is the car is more sensitive to changes in surface now. It can go into snap oversteer over surface changes, nothing drastic but it gets your attention.
If you want to give your Primera a more big car feel with more reach in the gears and as long as your Primera P11/Infiniti G20 is making 180bhp plus it is hard to argue with the bigger rolling diameter.
Downsides are the ride quality and the ultimate acceleration. Also the fender modifications.
If you want to keep your car’s bodywork standard, you need to stick with the original rolling diameter because with a 225/40-18 tyre on a lowered car, you need to modify the fenders.
If you want more grip, In a perfect world I think you would go with a 15 inch track day tire, 225/50-15 and paint the wheels black. That way you get the best of all worlds, except the extra range in the gears.