You can either - bore her with details about it (like she does about her shopping, home design, kid stuuf) or just regrow the cajones and get them. Tell her what happens in the garage stays in the garage
And if she gets out of line, remind her it is cheaper than a divorce
The other approach would be to get her out on the track so she can learn how much fun (and educational) it is, so when you say you "need" something for the car, she agrees, or even better, tells you whata you need (like a trailer, motorhome, cup car etc)