Yes, tell her

You take Sarah's hand and look into her eyes very seriously. "This is going to sound weird, but it's completely true. I didn't actually buy the car."

Sarah's eyes go wild. "You stole it!?"

"No. I used my magic powers to create it."

Sarah looks shocked. She squeezes your hand tightly. "Um... okay."

"No, really. I had this crazy dream last night and now whatever I want to happen, happens."

She tilts her head and smiles at you consolingly, as if you're a small child having trouble with single digit addition. "Honey, I love you. But we both know that if you could make anything happen, my breasts would be twice as big."

You smile and make the wish. Sarah looks down at her chest as it starts expanding. In a few seconds it goes from B-cup to D-cup, stretching out her t-shirt.

Sarah drops your hand. "Oh my god! How...? They don't even feel weird!"

"That's because I also resized your bra and strengthened your back muscles a little." You lean back to get a broader view or your work. "So, what do you think?"

Sarah runs her hands over her new bust. She feels every curve through her tight shirt. Her eyes close and her mouth parts slightly as she explores her new chest. "I can't believe this is real! You really can change the world! And of course, you have to demonstrate it on my chest!"

"Actually, you're the one who brought up your breasts. You are always making jokes about how much better I'd like you if you had bigger breasts."

Your girlfriend hugs you, pressing her new rack hard against your chest. "That's just because all guys like the same things. Everyone knows what makes girls popular."

"Well, do you like them?" you ask.

"Absolutely! Except... my clothes don't really show them off. Maybe you could give me something with a little more cleavage? I've never been able to show much cleavage before."

You step back away from her and examine her t-shirt. You change it into a lot cut tank-top. It isn't quite slutty, but it shows a nice splash of cleavage.

"Perfect!" says Sarah. "Thank you so much! Now let's go for that spin!"

The two of you hop into her new car. The tires squeal as it pulls out of the parking lot, making Sarah squeal with them. She stomps down on the accelerator and takes off.

The speed is incredible, though the car can only go so fast in the tight city streets. Sarah blazes through a stop sign without slowing down, enjoying her new toy immensely.

Then a siren blares behind you.

"Oh no!" says Sarah as she pulls over. "This is so bad. Wait, you can fix this!"

You smile, thinking about where you want to teleport the cop to.

Sarah bats her eye lashes. "I mean, I can fix it, now that you've given me some help up top. But I might need a little something more to flirt my way out of this. There's another thing I'd like you to wish for me, please."