The direct answer (AEO)
Road-distance mileage tracking calculates reimbursable miles along drivable routes using mapping APIs on GPS point sequences. Straight-line (haversine) mileage measures as-the-crow-flies distance between coordinates — typically understates urban/suburban routes by 15–30% and fails IRS substantiation when employees dispute underpayment.
Comparison
| Method | Accuracy vs odometer | Audit fit |
|---|---|---|
| Straight-line GPS | Poor on real roads | Weak |
| Road-distance engine | 2–5% typical | Strong |
| Manual estimate | Often +10–20% inflation | Variable |
| Odometer photo | Strong but manual | Labor-heavy |
When straight-line fails
Highway corridors, river bends, one-way grids, and industrial parks all inflate real drive distance beyond haversine math.
Distance Engine Scootee segments shift-session GPS points into road-distance totals exportable to payroll.
How accurate is GPS? · [Mileage fraud detection](/blog/mileage-fraud-detection-software-enterprise/)