How we check the math
Every number is checked before it ships.
A fuel tax tool is only worth using if its numbers match what the state expects. So the engine has to agree with real returns, worked examples, the state manuals and a second calculator, every time we change anything.
What we test against
- Tennessee sample returnPass
A completed return with 33 states, credits, debits and two surcharges. Every state's gallons and tax are compared line by line.
Tennessee Department of Revenue33 states - Penske four-state examplePass
100,000 miles across Maryland, Pennsylvania, Tennessee and Virginia, including the Virginia surcharge. $473.00 owed.
- Argo filing guide examplePass
120,000 miles and 20,000 gallons across three jurisdictions. $116.65 owed.
Argo Software IFTA guide3 states - 2026 rates with the Kentucky surchargePass
Illinois, Indiana and Kentucky on real 3rd quarter 2026 rates. $139.72 owed, $50.40 of it surcharge.
Transportial IFTA calculator example3 states - Rounding rules from state manualsPass
Every rounding example the instructions print: MPG to two decimals, whole miles and gallons, money to the cent.
Wyoming CC-104, West Virginia IFTA-13, Tennessee IFT 506 and Pennsylvania IFTA-101 instructions9 examples - The rate tablePass
Every diesel rate and both surcharges, typed in a second time from the official matrix and compared.
- 10,000 random returns, solved twicePass
Random trips through 1 to 12 states, checked against a second calculator written with whole-number math, so a floating point slip can't hide. Each one also has to follow the rules below.
Built in house10,000 returns
Rules every return must follow
Checked on all 10,000 random returns.
- The total is exactly the sum of the states.
- Surcharge is charged on gallons burned and never turns into a credit.
- A credit is never bigger than the tax you paid at the pump.
- A state you never drove in or bought fuel in owes nothing.
Why solve it twice?
Computers store money as binary fractions, and 5.765 can quietly become 5.76499999. To make sure that never changes an answer, we wrote a second calculator that only uses whole numbers: miles, gallons, hundredths of an MPG and tenths of a cent. Both have to land on the same cent for every return.
We also break the engine on purpose to make sure the tests notice. Change one rounding rule and the checks fail straight away.
The one spot we flag
Tennessee's instructions show MPG 5.127 becoming 5.12, which drops the last digit. Every other manual we checked, and Tennessee's own sample return, rounds instead, so we round. If your base state is Tennessee and your MPG lands on that edge, check it by hand.
Rates come from IFTA's 4th quarter 2026 matrix, which IFTA calls final on December 4, 2026. We check the table again after that date and every quarter after.