Fuel Log & Efficiency Analyser

What this tool does

CM8-101 is a working fuel log for a small fleet — vans, trucks, pool cars, electric vehicles, or a mixture. You record one row per fill: the vehicle, the distance covered since the last fill, the quantity and cost of fuel, and whether the tank or battery was filled to full. From that the tool works out cost per unit of fuel, consumption, and cost per unit of distance for every fill; totals and averages per vehicle; the fleet's overall cost per distance; and it flags the fills whose consumption strays outside a tolerance you set — the early sign of a dragging brake, soft tyres, a fuel leak, or a fuel card being used somewhere it shouldn't be.

Everything runs inside this single file. There is no account, no upload and no network request of any kind, so registrations, drivers' names and card spending stay on the computer you are using.

Recording a fill

One row is one visit to the pump or the charger. Record it while the receipt is in your hand:

• Date and vehicle — the vehicle name must be spelled the same way every time, because the per-vehicle summary groups on the exact text. "Van 12" and "van 12" are two vehicles to a computer.
• Odometer reading — optional, for your records and for spotting typos in the distance.
• Distance since last fill — entered by you, in the distance unit chosen in Settings.
• Quantity and unit — what went in, in litres, UK or US gallons, or kilowatt-hours.
• Full fill — tick it only if the tank or battery was genuinely filled to full.
• Cost — the total paid for this fill, in your currency. The tool derives the price per unit from it, which is often the first place an odd receipt shows up.

Use the notes field for anything that will explain the numbers later: towing, a loaded roof rack, a cold week, long idling, a dearer motorway station. A flagged fill with a note against it is a question answered; a flagged fill without one is an hour of arguing three months later.

Why you enter the distance yourself

It is tempting to have the tool subtract each fill's odometer reading from the previous one. This tool deliberately does not, and asks you for the distance directly, because derivation across rows fails quietly in real fleets: a missed or unlogged fill silently doubles the next interval; rows entered out of order, or deleted, or filtered out, shift every figure after them; a mistyped odometer poisons two intervals, not one; and a vehicle that changes hands mid-month has gaps no subtraction can see. A wrong distance produced by arithmetic looks exactly as confident as a right one.

Entered directly, the distance is one honest fact per row, each row stands on its own, and a typo damages only itself. Most drivers read it straight off the trip meter — reset it at each fill and the figure to type is on the dash. The odometer field stays available as a cross-check: if the distance you typed is nowhere near the change in odometer, one of them is wrong.

Full fills, and why they matter

Consumption can only be measured between two full fills. Filling to full fixes the fuel level at a known point; the quantity that goes in next time is then exactly what was used over the distance between. Put in a partial "splash and dash" and nobody knows how much was in the tank, so this tool shows "—" for consumption on any fill not marked full, and on any fill without a distance or a quantity — a dash is more useful than a confidently wrong number.

Partial fills still count fully in every cost figure, and note one subtlety: the first full fill after a partial one produces a consumption figure that is only approximate, because the interval started at an unknown level. The sample data includes exactly this case.

Litres, gallons and kilowatt-hours

Each fill carries its own fuel unit, and an electric charge in kilowatt-hours is treated exactly like any other unit — same fields, same formulas. What the tool will never do is add quantities in different units together: 40 litres plus 30 kWh is not 70 of anything. The per-vehicle summary therefore totals fuel separately per unit ("212.0 L + 96.5 kWh"), and every combined figure across the fleet — the monthly chart, the fuel-type donut, the fleet average — is stated in cost terms only, because money is the one column that genuinely adds across diesel, petrol and electricity.

Keep one unit per vehicle. UK and US gallons differ by about twenty per cent, so mixing them within one vehicle's history makes its consumption trend meaningless. The tool also expects a charge marked as electric to be in kWh, and will say so if it is not.

The figures, exactly

Every derived figure is one division, shown here so anyone can check it:

Consumption is expressed as distance units per fuel unit — km per litre, miles per gallon, km per kWh — so a higher number is always better. If you are used to litres per 100 km, divide 100 by the figure shown: 10 km/L is 10 L/100 km, 12.5 km/L is 8 L/100 km.

Outlier flagging

Set an expected consumption and a tolerance on the Settings tab, and every full fill with a distance is compared against it:

Fills whose deviation exceeds the tolerance in either direction are counted in the tile, coloured in the register, and listed in the Outlier fills table with the arithmetic shown. Low outliers suggest a mechanical problem, a driving problem, or fuel not going where the log says it went. High outliers usually mean a data problem — an overstated distance or an understated quantity — and are worth chasing just as hard, because they mean the log is drifting from the truth.

Be honest about the comparison's basis: it is one expected figure for the whole fleet, in distance units per fuel unit. It works cleanly when your vehicles share a fuel unit and a broadly similar duty. A fleet mixing diesel vans with electric cars should set the expectation for the majority unit and read flags on the other vehicles as unavoidable noise — or filter by fuel unit first, which restates the tiles and tables for that unit alone. Set the expected figure to 0 to switch flagging off entirely.

Fleet averages done honestly

The fleet cost per distance is calculated from totals, never by averaging the per-fill figures:

The distinction matters. Averaging each fill's own cost-per-distance would weight a 40 km hop the same as a 900 km run and drift away from what the fleet actually spent per unit of distance travelled. Dividing the totals gives the number that reconciles: multiply it by the distance logged and you get the money spent. The same totals-first rule is applied per vehicle in the summary table, and its footer shows the fleet-level division in full.

Note the basis: the divisor is the distance logged in this register under the current filter. If some journeys were fuelled elsewhere or some fills went unrecorded, the figure describes the log, not the vehicle's whole life.

Reading the charts and tables

Consumption per fill plots every full fill with a distance, in date order, against the expected line. A single bad point is a question; a drift over weeks is a trend — tyres, a service due, winter. Points are only directly comparable where fills share a fuel unit.

Fuel cost by month and by vehicle show where the money goes; the vehicle bars carry each vehicle's own cost per distance so an expensive-but-busy van is not confused with an expensive-and-idle one. Cost by fuel type splits the spend across diesel, petrol, electric and LPG — shares of cost, deliberately not of quantity.

The per-vehicle summary is the management view: fills, distance, fuel totalled per unit, cost, and cost per distance from that vehicle's own totals. The outlier fills table lists every flagged fill with its consumption, the expected figure and the deviation, so the conversation starts from arithmetic rather than impressions.

Printing and sharing

Print Report produces a report from whatever the current filter shows: header, the headline tiles, all four charts, both tables, the full register and your closing notes. Print to PDF to circulate it. The scope line under the title states the filter in force — filter to one vehicle or one fuel type before printing and you have a clean single-vehicle or single-fuel report; clear the filters before issuing anything described as the whole fleet.

Saving your work

Fills, settings and the report header are written to this browser's local storage as you type, and the toolbar shows the time of the last save. That storage belongs to one browser on one computer: another browser, a private window, a second machine or a clean-up tool that clears site data will not have it.

Treat Export .json as the real save — one file containing everything, which Import .json restores anywhere. Export CSV gives you the register for spreadsheet work and includes every filtered record, not only those drawn on screen. Reset asks twice, then erases everything this tool has stored. There is no undo.

Accuracy & disclaimer

This tool calculates only from what you enter. It cannot tell whether a distance was read correctly, whether a fill really was to full, or whether the receipt matches the pump — and every figure it produces depends on those. Consumption figures here are measured over your real duty cycle and will not match a manufacturer's laboratory figure; that is a feature, not an error.

Fuel duty, mileage reimbursement rates, and any tax treatment of vehicle costs differ by country and are entirely outside this tool. It is a record-keeping and calculation aid, not accounting, tax or engineering advice.