Calculator 06 / Trip Planner
Trip Planner
Arrival time, tyre pressure and stopping distance all shift with conditions — temperature moves pressure, and road surface moves the distance you need. Drive time and arrival, tire pressure change with temperature, and stopping distance by road condition.
How this calculator works
Units below default automatically — Metric for non-English pages (or if your device's language is set to one we support), US customary otherwise — and convert automatically if you switch the toggle above the form. Changing language resets to that language's default unless you toggle again.
Tire pressure vs. temperature uses real gas-law physics (constant volume), not a flat rule of thumb — converting to absolute pressure (PSI + atmospheric) and absolute temperature (Rankine) before scaling gives an accurate result across any temperature swing, not just the commonly quoted "~1 PSI per 10°F" approximation. Stopping distance uses standard physics with published approximate friction coefficients (μ) for different surfaces — actual values vary with tire tread depth, vehicle weight, and brake condition. A loaded tractor-trailer cannot use all of the road surface's grip: its brakes are sized to a legal maximum stopping distance rather than to the tires, and air brakes lag before they apply. Both figures here are set to reproduce the FMCSA CDL manual's published stopping distances, and the vehicle selector is what switches between them.
Worked examples: one from each tab
ETA. 380 miles at an average 58 mph with 45 minutes of stops, leaving at 07:00:
- Drive time = 380 ÷ 58 = 6.55 hr
- Plus 0.75 hr of stops = 7.30 hr total
- Arrival = 07:00 + 7:18 = 2:18 PM
Tire pressure. 80 PSI set at 70 °F, checked at 30 °F. Working in absolute pressure and absolute temperature: (80 + 14.7) × (489.67 ÷ 529.67) − 14.7 = 72.8 PSI, a drop of 7.2.
Stopping distance. A loaded tractor-trailer at 65 mph on dry asphalt, 1.5 s reaction: 143 ft reacting, 38 ft of air brake lag, 238 ft braking — 419 ft in total, about a quarter of a mile.
Stopping distance by speed and vehicle
Both columns are this calculator's own output on dry asphalt with a 1.5 second reaction time. The truck figures include air brake lag and the reduced braking a loaded combination actually achieves; the car figures do not, because a car has neither.
| Speed | Car / light truck | Loaded tractor-trailer | Difference |
|---|---|---|---|
| 35 mph | 132 ft | 167 ft | +35 ft |
| 45 mph | 189 ft | 240 ft | +50 ft |
| 55 mph | 256 ft | 324 ft | +68 ft |
| 65 mph | 331 ft | 419 ft | +88 ft |
| 75 mph | 416 ft | 526 ft | +111 ft |
Braking distance rises with the square of speed while reaction distance rises in a straight line, so the gap widens the faster you go. At 75 mph a loaded truck needs 526 ft — well over a tenth of a mile, and 111 ft more than the car beside it. These are dry-road figures with brakes in good adjustment; wet, worn or poorly adjusted brakes are all worse, and none of it accounts for a downgrade.
Road surface and the friction coefficient
| Surface | μ used here | Truck at 65 mph | vs dry |
|---|---|---|---|
| Dry asphalt | 0.75 | 419 ft | — |
| Wet asphalt | 0.40 | 628 ft | 1.5× |
| Packed snow | 0.20 | 1,074 ft | 2.6× |
| Ice | 0.10 | 1,968 ft | 4.7× |
On ice at highway speed a loaded truck needs more than a third of a mile. The reaction and lag distances do not change with the surface — only the braking distance does — which is why the multiplier is not simply the ratio of the friction coefficients. These coefficients are representative values for planning, not measurements of any particular road.
Tire pressure against temperature
Gas in a fixed volume changes pressure with absolute temperature, which is why tires read low on a cold morning without having lost any air. Starting from 80 PSI set at 70 °F:
| Ambient | Gauge reads | Change |
|---|---|---|
| 70 °F | 80.0 PSI | — |
| 50 °F | 76.4 PSI | −3.6 |
| 30 °F | 72.8 PSI | −7.2 |
| 10 °F | 69.3 PSI | −10.7 |
| 0 °F | 67.5 PSI | −12.5 |
The familiar "about 1 PSI per 10 °F" holds for car pressures around 30–35 PSI. At the 80–110 PSI a commercial tire runs, the same physics gives closer to 1.8 PSI per 10 °F, because the change is proportional to the absolute pressure you started from. That is the trap in the rule of thumb, and the reason this page does the arithmetic properly. Always set pressure cold — a tire warmed by an hour's running reads high, and bleeding it down to the placard figure leaves it badly under-inflated once it cools.
Where this goes wrong
- Using stopping distance to choose a following distance. It does not include the vehicle ahead also braking, a downgrade, or anything about the road you are actually on. Follow the interval your training and the conditions call for, with margin.
- Picking the car setting on a truck. The vehicle selector changes the answer by 20% or more, and the default is the loaded tractor-trailer for a reason.
- Using an optimistic reaction time. 1.5 seconds is an alert driver. Fatigue, distraction and darkness all push it higher — try 2.5 s to see what that costs.
- Setting tire pressure hot. Set it cold, or use this page to work out what the cold figure should read at the temperature you are measuring in.
- Averaging optimistically on the ETA tab. Average speed over a whole leg is well below the speed limit once fuel, scales, traffic and terrain are in it. It is also not a schedule you may legally drive — that is what the hours-of-service page is for.
- Forgetting the ETA has no hours-of-service in it. A 12-hour drive time is arithmetic, not a legal plan.
Where these numbers come from
Drive time is arithmetic; stopping distance and tire pressure are physics with a legal floor underneath them. The rules below set that floor.
- 49 CFR Part 393 — Parts and Accessories — Brake system requirements, the source of the stopping-distance performance a truck must meet.
- U.S. Tire Manufacturers Association — Why cold inflation pressure is the one that is specified, and how ambient temperature moves it.
- 49 CFR Part 392 — Driving of Commercial Motor Vehicles — Includes the requirement to reduce speed for hazardous conditions, which no planned average accounts for.
Why does tire pressure drop so much in cold weather?
Air contracts as it cools — a 40°F temperature drop can easily cost 3–5 PSI, which is why tire pressure warning lights often trigger with the first cold snap of the season even with no leak.
Is 1.5 seconds a realistic reaction time?
1.5 seconds is a commonly used average for an alert driver; fatigue, distraction, or older age can push real reaction time meaningfully higher — try the calculator with 2.5s to see the difference.
Stopping-distance guide
For why the stopping-distance tab defaults to a loaded tractor-trailer instead of a car, and how the numbers check against the FMCSA CDL manual, see how to read a loaded-truck stopping-distance chart.
Estimates only. Stopping distance and tire pressure figures vary by vehicle weight, tire condition, and load — treat these as planning references, not guarantees.