Not petty. You paid good money for a nice car. He has a beater and a company car but wants to put miles on yours to run his kids around....why? Because he doesn't pay the upkeep for your car. He can use one of his TWO cars. Hold your ground against the mooch. ETA plus teens are dirty, esp after sports practice. He doesn't want to mess up his company car. 

Honestly, Rocky, I often wonder how you manage to stay with your SO. No offence meant but he is an unbending bossy-britches who wouldn’t last a week with grannyd. His ex-wife has far too much influence on him and, subsequently, on your relationship.

As is often asked on this site, where do you see yourself in five years; in ten? You’ve already invested time enough and, IMHO, you are flogging a dead horse. 

Of course, you should not be offering your vehicle, that you likely purchased, insure and maintain, in order  to accommodate the demands of your partner's ex-wife! And no doubt, he’ll pout and complain about your refusal. But wait; he already has!

Not that our situation was in any way SKid related. For 6 years while SS was in middle school and HS, I had a company car. I drove that car any time, anywhere I chose to. It was a perq of the role I was in.  Fully funded company car. I schlepped SS and his HS friends around in it countless times.  We took road trip long weekends with that vehicle.

When my boss left and I got a new boss. Suddenly it was a problem, that was not really a problem.  My offer letter included "A fully funded company vehicle will be provided for the duration of the assignment."  I expensed gas, oil changes, repairs, service, etc...   I scanned the offer letter, sent it to my new boss, and never heard another word about it.

I took a transfer and promotion to a home office assignment after 6yrs and surrendere9mos laterd my company vehicle. 

DW could even drive my company car.  Company policy was that per-diem assignments allowed for licensed driver spouses to drive the vehicle.  The last year I was forced to take a relocation. That pissed the client off to no end as relocation costs were highly audited and per-diem roles were covered under discretionary budgets.  So, my bass mandated that I take a relo, that pissed the client off, I transferred about 9mos later.  A year later the company lost the contract.  A contract that I had salvaged and then run successfully for 6yrs.

Anyway, this guy can drive his company car, or his beater to schlep his failed family progeny around.  Why depreciate your personally owned asset  funded with marital money with more miles than neccessary when there are othe options.

On that note, my DW and I have two vehicles. Both the same year.  Hers we bought new off of the dealership show room floor. I bought mine a year later as a low mileage certified program car from the manufacturer. Even if we had bought them at the same time brand new, hers is twice the cost of mine. 6yrs after we bought mine, mine has 50% more miles on it than hers has. Hers has very low mileage. We drive the doors off of mine when we are together. It just makes sense to preserve the value of hers over mine.

DH tried to get me to let SD borrow my old car. I told him, "No, I'm not comfortable with that." The end.

My car is an extension of my purse, my boudoir, my closet. I have "emergency" earrings there, "emergency" mascara, perfume, snacks, a couple of different pairs of shoes and jackets, so no, you don't just get to shove your smelly teenager in there.

if he whines about not using your car remind him "you have two cars you can chose from, mine in not one of those."

Frustrating for sure.  It isn't really even so much about the reason he is doing the driving.. his kid.  This is YOUR car.. YOU pay for. Extra miles decreases the value and increases the frequency of upkeep and repairs (tires, oil changes etc).. it can also impact insurance rates.  It would be one thing if this was a car that was never used and needed occasional use to keep the seals lubricated etc.. (sitting cars can fall apart.. lol).. but it's not.

AND.. the elephant in the room is that you are in a shaky relationship with this guy.. you could be out the door at any time..and that car would have to last you a good long while if you had to live on your own.  So, NOPE.. not racking up miles sir.

Just don't get is. Do you wonder why SS is the way he is. A '''little SO'' .  SS has no respect and SO has no respect.  It's your car. You don't want SS in it,  Nothung wrong with that.  What's wrong is BM controlling your life and time 

Stick to your boundary. To me, it's worth an arguement, if that's the way he wants to go.

It shouldn't be an arguement, though, it is YOUR car. If he were to be in an accident, it would be on you and your insurance, regardless of who was driving. If he were to total it, even with insuranace, it would cost you money out of pocket. If he were in an accident that hurt other people, you and your insurance company would be the ones sued.

My DH and I have been together for almost 29 years. He never drives my car and I only drive his when we are together (I don't like his driving).

Also, if he doesn't like driving the company car at night, what makes driving YOUR car at night okay? Obviously, he's worried about his night driving and getting into an accident with the company car but I guess taking that chance with your car is fine and dandy. Whatever the reason, just, no.

Being in a relationship doesn't mean that everything that's yours, is also his. He needs to respect that, he needs to respect you.

When I was dealing with the Disneyland dad he literally assumed ALL of my possessions could be used by him or his kids if it was in sight and visible to them. 
 

I had to set his a$$ straight like "look here Bob, neither you nor your kids paid for this nor will y'all pay if y'all destroy it so unless I give permission ----> leave MY items alone!!!"

Unless he's paid for your car, maintenance, gas, insurance then he has no business using YOUR car without your consent first
 

Just keep being crystal clear on your boundaries (which unless he's legally deaf / profoundly mentally challenged you shouldn't have to repeat plus he understands very well he's just hoping you will break down and weaken your boundaries for his satisfaction) and if he dare plow past your boundaries then you need to reconsider the relationship 

A man who does not respect your boundaries does not respect YOU ... so why give your prime years and your p00m p00m to someone who doesn't respect you?

did but then one minute after he pulled out he said be ready to get my when I run out of gas(there is plenty to get him where he is going).  He just wanted to make a point to me that I didn't let him use mine

Dude be ready to end up as a 3rd time single dad if you keep pretending like you don't understand basic respect and boundaries 

The A/C has gone out in my car.  It is warm enough that DW told me this AM that I should take her car when I head to the dry cleaners and the grocery store today.

We have two cars. Same year.  For some reason, my AC has given up the ghost. Probably over a month ago but it has been cool and I have not had to use the A/C.

We only take her car when we are going off road or on long road trips.  Mine gets used for any other together driving.  She drives hers to work.

The only time I drive hers by myself is when I am putting gas in it.

This thing with DH and using your car to schlep SS around is odd. Something is definitely off about it.

"Lack of preparation on your part does not constitute an emergency on mine."