What your records have to contain to survive a check by Revenue Department (กรมสรรพากร), and the rate DriveHub applies.
฿4.00 per km
In force 2007-08-03 onwards.
Revenue Code s.42(1) and s.42(2). (1) exempts transport expenses spent "honestly, necessarily, exclusively and wholly in carrying out his duties"; (2) exempts transport expenses and travelling per diem at the rates prescribed by the Government for its own officials. The 4,00 THB/km car figure reaches the private sector through that ceiling — Ministry of Finance letter กค 0409.6/ว 42 (26 Jul 2007) under the Royal Decree on official travel expenses B.E. 2526 s.25, read across by Revenue Department ruling กค 0702/1831.
No logbook and, notably, no receipts — but a written travel authorisation stating the work to be done and the period is the condition. The evidential burden is that the travel was genuinely and wholly for work, which is what s.42(1)'s wording is doing.
The Revenue Code obliges an individual to make a daily record and to produce documents on a summons, never to keep them for a stated time. The effective horizon is the summons limit — Revenue Code s. 19, "ภายในเวลาสองปี นับแต่วันที่ได้ ยื่นรายการ", two years from filing, which the Director-General may extend "แต่ต้องไม่เกินห้าปี" on suspicion of evasion, and which s. 23 makes unlimited for anyone who never filed. The RD's own FAQ tells individuals to keep documents five years and cites no provision, because there is none. A company keeps its books five years from the closing date of the accounts (Accounting Act B.E. 2543 s. 14, extendable to seven) and its VAT records five years from filing (s. 87/3) — and s. 8 of that Act lists who is bound and contains no natural person, so a Thai sole trader is outside it. The sanction is the harshest here and it is not money: under s. 21, failing to comply with a s. 19 summons lets the officer assess "ตามที่รู้เห็นว่าถูกต้อง" — as they see fit — and "ห้ามมิให้อุทธรณ์การประเมิน", the assessment may not be appealed.
The allowance becomes assessable income.
The requirements above are quoted from Thailand's own tax authority. DriveHub keeps them, and the rate, under version control and re-checks them against the authority's pages — this page is generated from that record, not written separately from it.
DriveHub records this automatically. It tracks each drive in the background, keeps the start and end, the distance and the route, and exports a report in the shape a tax office expects.