Classic ASP ecommerce website needs new feature added

I have an ecommerce store that sells custom wheels and tires. It is currently programmed in classic asp. i need a feature that will be added to the product details page of every wheel. Currently each wheel is available in a variety of bolt patterns and offsets and we use that data to determine if it fits the car. Currently we do not give the user the ability to choose the bolt pattern and offset themselves. We just determine which one to use after they checkout. Some users are uncomfortable with this because they want to be able to determine that on their own. I need an option for more advanced users to be able to choose it themselves, so when they click the purchase button, it automatically ties their selection to the item in the shopping cart. We also display tire choices for each wheel, but currently the dropdown shows all tires instead of just the ones that will fit on their vehicle. We currently show the customers what tire size will fit, but I would like only the tires that fit the vehicle to show up, and an optional link that would change the dropdown to display all tires.