How smooth is scored

The Smoothness Composite Index

Several times a second, while the car is moving, Gas Math checks three things. Each reading either passes all three (smooth!) or it doesn’t (not smooth!), and the Index is the percentage of time in motion that passed.

Stopped time is excluded because sometimes there's a red light or stop sign.

The three checks

1. 1

   Gentle on the throttle?

   Acceleration under 0.17 g up to 30 km/h, tapering to 0.06 g from 60 km/h.

   A brisk pull-away is normal, probably unavoidable, and it lets the transmission reach a high gear sooner. The same push at highway speed needs way more power, and thus fuel.

2. 2

   Engine unbothered?

   While accelerating, fewer than 2,300 rpm at speeds up to 30 km/h, tapering to 1,800 rpm from 60 km/h.

   Above that, the transmission kicks down, which costs ~3–5× the fuel per km/h gained. (Phone-only drives can't measure this as there's no tachometer data.)

3. 3

   Braking with foresight?

   Deceleration under 0.10 g.

   Hard braking converts fuel you've already paid for into heat. Gas Math records this as “Lost to braking”.

Allowances for ordinary driving

• Hills. The phone's barometer measures the gradient. Speed gained going downhill isn't counted as your push, and speed lost going uphill isn't counted as braking. A kickdown on a climb still counts.
• Pull-aways. i.e., starting from a stop or low speed. Up to 3 seconds over the line below 60 km/h still counts against the score, but your streak survives.
• Stops. Idling and red lights are excluded from the score. The drive report does, however, show what idling cost.

What you see

The Pedal view draws the same rules as two lines: Ease off above, Brake below. Stay between them and the score (and your streak) climbs.

Depending on settings, crossing a line may trigger a discreet audio cue.

The limits were derived from tests on The Knucklemobile, a 2016 Dodge Grand Caravan in Ontario w/ 285k+ km. Obviously highly representative of all vehicles everywhere. I'm still working on my calibrations. Feedback is very welcome: support@gasmath.ing.

How fuel use is calculated

1. Every second: an estimate

   Through the OBD adapter's data, Gas Math calculates the air the engine draws in (from engine speed, intake pressure, and temperature) and calculates fuel consumption from that. With the phone alone, it uses physics formulas and averages: speed, push, and drag.

2. Every fill-up: a correction

   Fill the tank to full and log it. The litres pumped are what the car burned since the previous fill-up. Gas Math compares that with its own estimate for the drives in between, and scales every fuel, cost, and CO₂ figure accordingly.

   Correction = litres pumpedlitres estimated

   Include odometer readings and it will also detect drives you forgot to record, and report your true L/100 km.

3. Every drive: a comparison

   Each drive gets its own L/100 km and cost, measured against your usual consumption at the same speeds, so a slow city crawl isn't judged against an open highway. Repeat a trip and you'll see this run beside your typical one.

Accuracy improves with every logged fill-up. Until you've logged two, please treat fuel figures as educated guesses.