Full tank, or fill by value?
"₹500 of petrol" or "fill it up"? Drivers argue about this at every pump. The honest answer: both are fine — they just measure different things. Here's how mileage actually gets calculated, why partial fills blur it, and the rhythm that keeps every number you track honest.
How tank-to-tank mileage works
No app can see inside your tank. Mileage is arithmetic on two full fills: drive until near empty, fill the tank completely, note the odometer. Later, fill full again. The distance between the two fills, divided by the litres it took to top up the second time, is your real-world mileage for that stretch.
This is why the method is called full-to-full (or tank-to-tank). The full fill is the calibration point — it's the only moment the tank's actual level is known.
What happens when you fill a fixed amount
Filling by value — "$40, please" — is genuinely good for some things:
- Budget discipline. A fixed weekly fuel amount is a spending line you can actually stick to.
- Riding fuel prices. Smaller, more frequent buys let you lean toward cheaper days and stations.
- Less cash parked in the tank. Fuel in the tank is money sitting still (and slowly aging, if the car sits for weeks).
The catch: a partial fill leaves the tank level unknown. If you filled $40, drove 300 km, and filled $40 again, you can't know the mileage of that stretch — maybe the second fill topped you higher than the first. Your cost tracking stays perfectly accurate (money out, litres in, odometer — all recorded), but per-fill mileage has to wait. The next full fill reconciles everything: the distance across all the partial fills gets divided by all the fuel since the last full fill.
The rhythm that gets both
You don't have to pick a side:
- Log every fill, whatever kind. Full or partial, amount-based or brimful — the record is what matters. A tracker that only handles "perfect" fills isn't tracking your real life.
- Fill full every third or fourth visit. That's often enough to keep mileage current, and the stretches between still reconcile correctly.
- One caveat on tiny fills: pump meters have a small tolerance, and it looms larger on a $8 fill than a $45 one. If you fill very small amounts, expect a little more noise.
And the weight thing?
Yes, a full tank is heavier, and heavier cars burn more. But a few dozen litres adds tens of kilos — well under a percent of consumption, smaller than the effect of slightly soft tyres or one week of aggressive driving. Fill full for the maths, not for the physics.
Common questions
Which gives more accurate mileage — full tank or fixed amount?
Full tank, by construction: full-to-full is the only method where the tank level is known at both ends. Fixed amounts are fine for cost tracking and reconcile at the next full fill.
Should I run the tank low before refilling?
No need. The old "pump cooling" advice mattered for some older cars; modern ones are fine. What actually matters is not gambling on the fuel gauge on a highway — and logging every fill you make.
Does a full tank waste fuel because of the extra weight?
Technically yes, practically negligible — well under one percent. Tyre pressure and driving style matter far more than tank weight.
FuelCap logs full and partial fills, marks which is which, computes mileage whenever the maths allows, and stays honest when it can't. Free, forever — see the app.