A qualified medical expense for which you can reimburse yourself with a distribution from your HSA is any medical expense, incurred after the establishment of your HSA, by you, your spouse, or your dependents that would otherwise be includible on line 1 of Schedule A (even if you end up using the standard deduction). IRS Pub 502 describes deductible medical transportation expenses:
https://www.irs.gov/publications/p502/ar02.html#en_US_2016_publink1000179021
You would use the actual expenses or the standard rate for the year in which the transportation occurred. The standard mileage rate for medical purposes in 2017 was 17 cents per mile. For 2016 the rate was 19 cents per mile.