Cab math
Is speeding actually saving you anything?
Punch in your speed, the limit, and your trip — see the minutes you save against the extra diesel you burn. The +20% fuel penalty and the 32 L/100 km default are labelled model assumptions, not measured data.
- Research-based
- Methodology v1.0
- Updated September 2026
Fuel calculator
Set your speed, the limit, and the trip. The math shows minutes saved against extra diesel burned — then you decide if the trade is worth it.
From the CAB MATH episode: 120 in a 100 zone over 100 km saves 10 minutes for about 20% more fuel. Run your own numbers below.
Set your speed, the limit, and the trip. The trade updates live.
You save 10 minutes and burn 6.4 extra litres. That’s the whole trade — your call.
10 min saved
Show your work
Math: 100 km ÷ 100 km/h − 100 km ÷ 120 km/h = 10 min. Fuel: 100 km × 32 L/100 km × 20% = 6.4 L extra.
Estimate only. The fuel penalty is modelled as rising with speed over the limit — real aerodynamics, load, wind, and terrain move it both ways. Research-based; we did not test this hands-on.
The speed trade
The episode said it straight: ten minutes saved for about twenty percent more fuel. Here is where those numbers come from — and where they stop being true.
Time is the easy half. A 100 km trip at 100 km/h takes 60 minutes; at 120 km/h it takes 50. The savings shrink as trips get shorter — at 20 over the limit you gain about 10 seconds per kilometre, no matter how it feels.
Fuel is the hard half. Drag rises with speed, and every truck, load, wind, and grade changes the answer. We model the penalty as rising with speed over the limit — at 120 in a 100 zone that lands around 20%. It is a labelled assumption, not measured data; your real number lives somewhere around it.
The honest footnote: on a 500 km day, that 20 over buys you about 50 minutes. Only you know if those minutes matter more than the diesel. That is what the calculator is for.
Also try
More Cab Math — straight numbers for the driver’s seat.
Research-based. We did not test these hands-on. Scores come from published specs and driver reviews. Every default on this page is a published figure or a labelled assumption — change any input and the math re-runs.