2006 Pontiac Vibe Tire Cupping

Hi, I took my car in for it's routine Oil change and mentioned to the service writer the steering wheel was vibrating at 75mph. He said I needed a tire rotation and balancing. He then informed me 3 of my tires were cupping. The car is 2wd and has 37,000 miles on it. I bought it with 13,286 miles on it. It was a program car. If I don't replace all tires right away, will the cupping affect the gas mileage or performance of the car? I was told that many of the GM tires have been cupping after 24,000 miles. Any information you can give me will help. I plan on replacing all 4 tires with Kuhmo's as I had those on my Mustang and I think they are much better than Goodyear's.

Thanks! AmazonRia