max for car is $18k for 100% business use

You sure? I read that on Tesla forum that it is possible. Article 179 aka Hummer tax loophole basically says you can deduct 100% of the cost of your Model X since it is more than 6k lbs. 

For new and pre-owned vehicles put into use in 2018 (assuming the vehicle was used 100% for business):

• The maximum first-year depreciation write-off is $10,000, plus up to an additional $8,000 in bonus depreciation.
• For SUVs with loaded vehicle weights over 6,000 pounds, but no more than 14,000 pounds, 100% of the cost can be expensed using bonus depreciation.

Sec 179 

IRS TCJA