For the last couple of years, I did most of my long drives along NH48, the Delhi-Jaipur Highway, in a Suzuki Swift. Cruise control on, Rafi and Kishore Kumar on the speakers, because that’s the only music that survives four hours of highway without becoming annoying. Over months of nudging the cruise control dial a notch at a time, I kept landing on the same number. Not 70. Not 75. Just 72.

Until today, if you’d asked me why 72, my answer would have been that it feels right. That it sounds cool. Nothing I could defend with a straight face.

So I spent an entire evening doing the math to defend a “vibe”. The question: take the things that actually matter on a typical Indian highway (stopping in time, good fuel economy, safe distance from the vehicle ahead, traffic flow), work out the speed range each one allows, and see where they overlap. I expected a wide, boring band, maybe 60 to 90, with 72 sitting somewhere in the middle. But that isn’t what happened.

72 km/h 20 m/s
exactly twenty metres a second

Stopping distance

Stopping distance is reaction distance (how far you travel while your brain registers “brake now”) plus braking distance:

stopping distance = v·t + v² / (2·g·f)

With t = 2.5 seconds (the standard cautious value from road design), g rounded to 10, and f = 0.40 for a decent NHAI surface:

60 km/h  →  76 m
70 km/h  →  96 m
72 km/h  → 100 m
75 km/h  → 106 m
80 km/h  → 117 m
90 km/h  → 141 m

Look at 72. Reaction: exactly 50 m. Braking: exactly 50 m. Total: exactly 100 m. I spent ten minutes staring at those numbers, feeling like the second coming of Ramanujan, convinced my vibe was actually a latent mathematical genius.

NH48 is not an expressway, or even a proper highway by definition. It has craters bigger than the moon, crests with enough altitude to spill your coffee, curves blind from all angles, and the occasional cow crossing. It’s not a place to be doing 100 km/h. Past roughly 100 metres of required stopping distance, you’re gambling. That knocks out 85 and above. The first boundary is set: 60 to 80.

reaction: 50 m braking: 50 m 100 m
reaction plus braking, flush at the wall

Small engines and gearing

Most writing about fuel-efficient cruising speed is American. They talk about peak efficiency near 55 mph, measured on lazy, turbocharged engines with tall gearing. Indian cars are different. My daily driver is a 1.2L, naturally aspirated Swift. The ARAI-rated fuel economy is 23 kmpl, a number produced in a lab by a machine that had nowhere to be. On actual highways, owners report 19 (pre-E20 era) or 17 (post-E20).

What the Team-BHP ownership threads agree on is that a small NA petrol peaks lower than you’d guess, somewhere between 60 and 80 km/h, and starts sagging well before 100. A 1.2-litre engine holding 100 km/h isn’t cruising. It is me at the end of a 10k run: out of oxygen, gasping, and probably at 80 dB if measured. The second boundary: 60 to 78.

20 15 kmpl 40 60 80 100 km/h barely 1 kmpl steep
the whole 60 to 80 band is one flat kmpl

The closing-speed trap

Previously, my instinct was to cruise well above truck speed so I was never stuck near them. That is backwards. On the busy stretches of NH48, the ones that cause all the braking, trucks settle into 60 to 70. If I’m doing 88 behind a truck doing 65, I’m closing at 23 km/h. The gap vanishes fast and I get a forced decision: brake now or overtake now. That is exactly the ragged driving I am trying to escape.

A small closing speed does the opposite. I gain on the truck slowly, see the decision coming from far back, and half the time the truck’s own gap opens up before I touch the brakes. The target isn’t a safe distance from trucks, it’s a gentle closing speed: 5 to 10 km/h above their band. On empty stretches, trucks do 80 or 90, but overtaking there is easy. The third boundary: 70 to 75.

0 s 6 s 12 s closing 23 km/h brake now closing 7 km/h ease off early
same gap, same clock: at 72 you still have room when 88 has run out

Dividing by 3.6

Inside 70 to 75, here’s what made me sit up. Converting km/h to m/s means dividing by 3.6, so a speed is a whole number of m/s only if it’s a multiple of 3.6. The nearby multiples of 3.6 in our range: 68.4, 72.0, 75.6. Only one sits inside our acceptable range.

72. Exactly 20 m/s. From there every mid-drive calculation comes out whole: reaction distance 50 m, braking 50 m, two-second gap 40 m, three-second gap 60 m, 100 m every five seconds. Nothing left over anywhere.

70 72 20 m/s 75 1 second reaction, 2.5 s two-second gap five seconds 19.4 48.6 38.9 97.2 20 50 40 100 20.8 52.1 41.7 104.2 all whole, in metres
the neighbours you could have dialled instead, and their leftovers

The compromise

I noticed, partway through, that I took a lot of liberties with the numbers. They are realistic, but I selected them because they were clean and suited my vibe, not because I ran a braking test on NH48.

The three boundaries are mine too. Nudge each by a few km/h and you get a slightly different window. But 72 remains.

I still set the cruise control to 72. Not because of the math, but because at exactly 20 metres a second, the highway stops feeling like a combat zone. The overtaking decisions make themselves, the car doesn’t complain, and I can listen to Kishore Kumar in peace. The math is just the excuse I write down so I don’t have to admit I just want the quiet. But at exactly 20 metres a second, the road stops being a threat. It is just space moving past at a speed I can keep up with.