TDY Fly or Drive Decision When Both Are Allowed

The TDY fly or drive decision has gotten complicated with all the conflicting advice flying around — pun intended. I learned everything there is to know about this the hard way, on a trip to Fort Knox where finance handed me back a voucher short by $200. Today, I will share it all with you.

I drove to Fort Knox because I figured I’d pocket the mileage money. Straightforward enough plan. What I didn’t do was run the actual numbers first. The government had already built a cost comparison before I ever backed out of my driveway, and I got reimbursed at the lower of the two figures. That stung. This article walks through the exact math finance uses — so you can run it yourself before your orders are even finalized.

How the Government Decides Which Mode Is Cheaper

But what is Constructed Cost of Travel? In essence, it’s a side-by-side comparison of what it would cost you to fly versus what it costs to drive your POV. But it’s much more than that — it’s also the ceiling on what you get reimbursed. Whichever number is lower, that’s your cap. Full stop.

The flying side of the ledger pulls the GSA city-pair airfare for your specific route, adds ground transportation to and from the airport — think Uber, shuttle, whatever reasonable estimate the system uses — and tacks on any mandatory baggage fees. The driving side multiplies the current POV mileage rate by round-trip distance. As of 2024, that rate sits at $0.67 per mile when no government vehicle is available. That number shifts periodically, so confirm the current rate at GSA.gov before you run your estimate.

For the airfare side, pull your number from GSA’s City Pair Program lookup tool. These are contract fares negotiated between specific airport pairs. Almost always lower than what shows up on Google Flights. That matters because finance isn’t comparing against whatever Southwest is selling leisure travelers that week — they’re using the city-pair rate, and so should you.

When Driving Actually Wins on Reimbursement

Burned by the Fort Knox situation, I started doing the math before every trip. Here’s a concrete example of a drive that genuinely wins.

Say your TDY is 400 miles from your duty station. Round trip is 800 miles. At $0.67 per mile, your mileage reimbursement comes to $536. Now build the flying cost: city-pair fare on that route is $380 — plenty of domestic routes hit this, especially smaller regional airports — ground transport runs $50 round trip, and you’re packing light enough to skip checked bags. Constructed flying cost: $430. Your mileage reimbursement at $536 is higher, but since flying costs more, DTS pays you the full mileage rate uncapped. You keep every dollar of that $536.

Shorter distances under 300 miles are where driving almost always wins outright. At 250 miles round trip, your mileage reimbursement is $167.50. Many city-pair fares — once you layer in Uber and a checked bag — clear $200 easily. Drive it, document it, keep the difference. It’s essentially a small bonus for your time behind the wheel.

That’s what makes the short-to-mid-range drive endearing to us road-trip-willing service members. The math actually works in your favor, and you don’t have to sprint through an airport at 0500.

When Flying Wins and Driving Costs You Money

Probably should have opened with this section, honestly — because this is the scenario that quietly wrecks people’s vouchers.

Long-distance TDY might be the worst option for driving, as the mileage reimbursement requires a favorable city-pair comparison to pay off. That is because once the constructed airfare drops well below your mileage figure, DTS caps you at the airfare number no matter how many miles you actually drove. At 1,200 miles one way — 2,400 miles round trip — your mileage reimbursement looks like $1,608. Sounds solid. But if the city-pair fare is $310, Uber is $60, and one checked bag each way runs $35 per leg, the constructed flying cost lands at $440. That’s your ceiling. You drove the equivalent of roughly 48 hours of road time and you’re getting reimbursed for a plane ticket.

Short TDYs under two days create a separate problem. Drive instead of fly on a 1.5-day TDY, and you’ve potentially added two full travel days — more time driving than actually working the mission. Per diem for those extra days helps a little. Not enough. Flying wins on both time and money in that situation.

The most common finance kickback I’ve seen — and I’ve seen it plenty — is a service member who selects POV in DTS, adds zero justification, and watches the system flag it automatically because the mileage reimbursement exceeds the constructed airfare. DTS caps it with no warning. You just see a smaller number on your voucher when it comes back. Don’t make my mistake.

Extra Days of Per Diem When You Drive Instead of Fly

Here’s a mechanism most travelers never use — and honestly, it can flip the entire math on a borderline trip.

Orders can authorize additional travel days when you’re using your POV. General rule of thumb: one extra travel day per 400 miles, each direction. Those extra days aren’t dead weight on the calendar. They generate per diem and lodging reimbursement at the full TDY rate.

Run it back through the numbers. If your per diem location pays $157 per day in meals and incidentals and you’re authorized one extra travel day each way, that’s an additional $314 sitting on the table. A drive that looked like a wash financially suddenly becomes a meaningful gain. I’m apparently someone who actually reads the JTR footnotes, and that section works for me while skipping it never does.

To request this, add a clear note in your DTS authorization remarks — something like: “Traveler requests POV authorization for 800-mile round trip. Requesting one additional travel day each direction IAW JTR 020204. Estimated POV cost does not exceed constructed airfare cost.” Your approving official has discretion here. This is not automatic. Some commands are stingy about it. Get verbal buy-in from your supervisor before you ever build the authorization — a surprise request rarely goes smoothly.

How to Document Your Choice So Finance Does Not Push Back

Frustrated by a kicked-back voucher years ago, I built a quick pre-departure checklist using nothing more than a Google Maps screenshot and three sentences in a remarks field. Every voucher I’ve submitted since has come back clean.

• Screenshot the city-pair fare for your route at the moment you’re building your authorization. Save it as a PDF. Attach it directly to your DTS authorization before you submit.
• Document your mileage using Google Maps or MapQuest — grab a screenshot showing the route and the total mileage. Finance needs to see the exact number you used, not a rough estimate.
• Write two to three sentences in the DTS remarks field stating explicitly that you compared POV cost to the constructed airfare and chose to drive because it was equal to or less than flying — or because mission reasons required it.
• If your mileage reimbursement exceeds the constructed airfare, say so outright and note that you understand you’ll be capped at the lower amount. This prevents the confusion behind most resubmissions.
• Confirm your approving official actually sees and approves the authorization before you leave. A verbal “yeah that’s fine” does not protect you if they later adjust what they approved in the system.

Selecting POV in DTS with no explanation is the single most common reason finance returns a travel voucher. Three minutes of typing saves days of back-and-forth after you get home. So, without further ado — run the numbers, attach the screenshots, write the remarks.

Bottom line: drive when the city-pair fare plus airport costs exceed your mileage reimbursement, the distance is under 500 miles one way, or you can get extra travel days authorized. Fly when the distance is long, the TDY is short, or the city-pair fare is under $300 on a major corridor. The math isn’t complicated — it just has to actually be done before you leave the driveway.