Is it permissible to take surplus fuel coupons from the company, beyond what is needed for work, and use them to fill one's personal car?

If the car is owned by the company, then the coupons belong to the company, and it is not permissible for your uncle to give them to you. However, if the car is owned by your uncle, and the coupons are for his fuel, then they are his property, and he has the freedom to dispose of them and give some to you.