I just installed 2 new rear tires on my S2000. Nankangs NS-25, 245/40/17. The car drives perfect in city driving (under 50mph), but on the highway (around 70mph+) I noticed the car started swerving left and right. (Drives unstable at high speeds.) Steering wheel shakes slightly. Pressure is 40psi on both rears. I've never had this problem before, just now after I installed the tires. Car has 30k miles. MY03. I was told that the shop most likely didn't balance the wheels correctly, and that I might need an alignment (I actually need an alignment.) Do these suggestions sound about right? Thank you. :-)

 

Tyres should be balanced perfectly and the suspension alignment should be spot on if you drive higher speeds. 40 PSI is 2,75 Bar, that seems a little bit to much for me. Try 32 PSi / 2,2 bar.
Wheels are bolted on correctly with the proper torque setting?

 

I got my tires balanced at some horrible shop, they gave me alot of problems and I feel like they might have misbalanced my tires. I honestly wouldn't be surprised if they incorrectly balanced them. Im going to my local Honda dealer for an alignment tomorrow. And also no, I need to torque them correctly, it's 80 lb•ft correct?

 

80 is correct. Definitely have them re-balanced. Tires are no joke when it comes to safety.

 

Do you have the same tires on the front? At one point I ended up using using two different types of Bridgestones on my car. I think there were Re71s on the front and So2s on the rear. The car drove the shits. They were ok on perfectly smooth pavement, but were terrible if there were imperfections. There are very few ideal roads around here, so driving the car at speed was no fun. Mounted four Firestone 500s this spring and the car drives great again. These cars are very sensitive to tire pressure also. Forty psi is too much.

 

Tires could be out of balance, but that's not going to cause the dangerous left/right swaying at freeway speeds, rather just some vibrations at a particular speed. Its likely the tire choice and or combination of poor alignment acerbating this handling problem. This car needs very stiff 2 ply sidewall tires to handle correctly, not cheep flimsy ass all seasons.

 

I wrote my first posting here under the impression that the tyres are appropriate for the car.

 

The tread on those tires probably moves around a lot. And, as you said, you need an alignment.

Any time you go from worn tires to new ones, you'll you'ly notice *some* amount of tread squirm.

They're just UHP all season tires. Do you drive in snow?

Make sure they put the correct wheels on the correct positions. If your rear wheels were carelessly put on the front, they won't fully bolt up.

Then try an alignment.

 

The vibration is most likely associated with poor balancing. Nankang is not the most quality tire out there and with cheaper tires like the Nankang, balancing is often a trait the average consumer doesn't notice or care about since they expect the installer to fix any balance issues. So your shop may have done a perfectly fine job or as best they could in trying to balance them.

The swerving is something different altogether. As others have mentioned, make sure the right wheels are on the right end of the car. The shop may have confused the two since you went to a square setup. If nothing weird pops out, try tire pressure first, then look at an alignment.

 

Seems like you've got a lot of good suggestions here already - here are my $0.02. Please don't take any offence to this - I'm just being frank.

Going cheap on tires I think it one of the biggest mistakes ppl make. On a car like the S2000 especially this is a mistake as it is very sensitive to proper tires, alignment and tire pressure.
Mix-matching tires on a car like this is just a bad idea plain and simple. You didn't mention if the front tires are the same brand and model tire but I will assume they are not. Just that alone will give you undesired handling.
Tire pressure - as many mentioned - you have them too high. 32psi is what I'd recommend.
New tires will have initial break in - they will feel squirmy at first.
Those tires look like they are directional - with an outside tirewall and inside tirewall. Are they installed on the correct side of the car? Are they BOTH correct? Or do you have them on the wrong sides, or two right tires, or two left tires? If you look at the tires both should say "outside" on the outter wall.
I would not recommend all season tires on the S2000. It should have proper summer tires and if you plan on having to drive it in the winter (I'd hope not), I'd have dedicated snow tires... when I bought mine there were cheap all season tires on it and the handling was terrible.