I noticed recently that the tires fitted to the front of my Coupe are the wrong size: Should be 225/40 R18 but they are 255/35 R18 (same width and aspect ratio as the rears). There is still plenty of tread left on the tires so I don't want to replace them unnecessarily and, due to the smaller diameter, I could not use them on the rear. My question is this: What should I set the tire pressures to? The book/door sticker says 32psi on the front, but that is for the recommended tire size. The ride is very firm and steering slightly vague (when compared to my FWD Renault Megane!) Any comments/suggestions would be welcome. I love the car by the way!

 

The tire pressure on the door is not anything other than a recommendation. It is a good place to start with a new set of tires, but the brand, type of tire, style of driving and the weather conditions will determine the correct pressure for the situation. With a smaller sidewall one would think that you could use less pressure compared to a similar tire because the stiffness of the sidewall should increase. I have a set of recently purchased Falken tires on the rear of my SRT6. They are very squirrely at 32 psi and must be run at 40 psi to keep the rear stable for spirited driving.
If your steering feels "vague" then you may need to change the steering stabilizer. Easy job most of us do ourselves and not expensive.

 

Try tiresize.com

 

I've got a set of OEM-sized tires on my coupe. And I run 35-36 psi in both fronts & rears.

 

Many thanks Zip. When I bought the car a few months back and drove it home (approx 140km) I did not notice any vagueness in the steering. A few days later when I checked the tire pressures, they were up around 45psi. I figured that the previous owner had simply let the garage attendant check the tire pressures (very common here in SA), hence the over-pressure. I deflated them to the recommended pressure which resulted in a smoother ride but some vagueness in the steering. That's when I also spotted the incorrect tire size on the front. Perhaps the previous owner had deliberately over-inflated them to give more precise steering?

 

Many thanks 180. Will do.

 

Many thanks dedwards0323. I plan to experiment with tire pressures over the next few weeks. Other members have referred to the Steering Stabilizer to cure the vagueness. I will investigate that too.

 

Tyresize.com suggests 30 psi (vice 32 psi) for the larger tires that are currently fitted to the front of my Coupe. I will give that a try and report back.

 

Some feedback: With the lower pressures (30psi) on the front (due to the incorrect tire size fitted by a previous owner) the car feels much more pleasant to drive - smoother and tracks better. So I will stick to those pressures until I replace the tires with the correct size.