I had a similar situation. My wife lives in another part of the state, and I commute to be with her when I can. This reason makes mpg very important.
She is having a hard time financially, and we mostly try to stay out of each others finances.
I told her, I will mod her car. No expense flr labor, or parts. Anything above EPA, she pays me. The 3 dollar role of duct tape I bought has since paid foritself. When she wants somethin in a store, I bring it up. A new car? Better internet? Cable? A kitty cat?
Less fuel used, means more money. Your experience will depend on your wife, and her goals, and your situations. Surely you are happy. If you had problems, you most likely wouldn't be on here commenting on this lol. But if she doesn't care about money, fuel, dinosaurs, so on, then it won't work. I taught my wife not to race to red lights, as step one.
People fly past as I coast to a red light. Get there, it is turned green, second gear, and gone. Less fuel, less brake, and past them. When she realized momentum is better than being the first to a red light, the seed was planted amd she started growing.
I hope the circumstances work out to your liking
Post Script: what if you challenged her in that you matched her fuel savings on here? For a trip, or whatever. If she sewed, a new machine, so on. After the first gift giving holiday you put it towards, tell her she can keep saving money by just driving that way. College fund for your young lady? Fees for sports? Either way, I hope it works out. I know the anguish when someone puts on air con when , in my personL opinion, God has blessed us with free air con outside.