My Firebird needs new shoes. The tires are way past their good till date, and tread is coming off of them. I looked at the current tires; they are P235/60 R15. America’s Tires has one stock size for them: P225/70 R15. I’m not a tire guy; but are my current tires lower profile than stock? Would that be enough to make the speedometer read about 5% slow?