If you take the initiative to plan a surprise trip for your partner's birthday, the responsibility for the logistics—including the vehicle—should fall on you. When someone tells me they’ve 'planned a surprise,' I expect to be treated as the guest, not the provider of the primary transportation. Using my own car means I am the one absorbing the depreciation, the mileage, and the potential wear and tear on a trip I didn't even choose. It takes away from the feeling of being pampered and celebrated when I have to worry about my car's maintenance schedule or cleaning it out for a long drive. If your own vehicle isn't suitable for the trip you planned, you should look into a rental or adjust the plan rather than just assuming your partner will offer up their car. It is about the principle of the gesture; a gift should not come with a hidden cost or a logistical burden for the recipient.
While the sentiment of a surprise trip is a gift, the logistics should always be based on what makes the most sense for the couple as a unit. If one partner has a gas-guzzling truck and the other has a fuel-efficient sedan, it is objectively better to take the car for a long journey. Arguing over whose car it is feels petty when the goal is to enjoy a weekend away together. Planning a trip involves a massive amount of effort—researching locations, booking stays, and managing a schedule. Expecting the planner to also provide the perfect vehicle, even if theirs is less practical or more expensive for the journey, is an unnecessary burden. We should be looking at this as a team. If my car is the better tool for the job, I am happy to contribute it to ensure our trip is safer, cheaper, and more comfortable. The 'gift' is the experience and the effort put into the planning, not just the mechanical transport.