Ferry or drive on? Do the maths

1. THE CORE COMPARISON. Do not set 'ferry' against 'driving' as separate line items, but calculate two COMPLETE door-to-door route scenarios: (A) entirely by road, (B) using the ferry. Both scenarios get the same five cost items: fuel, tolls/vignettes, overnight stays en route, the ferry/tunnel ticket, and a value placed on time. Without items 3 and 5 the ferry systematically comes out too expensive — the most common mistake in existing calculation tools. 2. FUEL PER COUNTRY SEGMENT, not with an average. The price difference within Europe is extreme in August 2026: €1.73/l in Estonia and Iceland against €2.46/l in Denmark, a factor of 1.4. Over 2,000 km with a motorhome doing 12 l/100 km that is a €175 difference. Split the route into country segments (km × l/100 km / 100 × the country price) and add a refuelling-tip module: 'fill up in Luxembourg (€1.90) before you drive into Germany (€2.25)' saves ±€32 on 90 litres. For NO, SE, CH, CZ, PL, HU, RO and GB you have to work with the daily exchange rate, not a fixed factor. 3. MAKE THE 3.5 t THRESHOLD AN EXPLICIT SLIDER, not a footnote — this is the biggest cost for motorhome travellers and the tool's distinguishing feature. Above 3,500 kg something drastic happens in five countries: France jumps from toll class 2 to 3 (+±60%), Slovenia replaces the €32 weekly vignette with a DarsGo unit and distance charging, Austria replaces the €12.80 vignette with a compulsory GO-Box, Switzerland replaces the CHF 40 vignette with the PSVA, and Norway automatically puts you in the expensive class 2 without an AutoPASS tag. Italy, Spain, Croatia, Portugal and Greece by contrast do NOT look at weight but at height or axles — nothing changes there. Show those five warnings only if the user is above 3.5 t AND the route touches that country. 4. TOLLS AS A HYBRID MODEL. Vignette countries (AT, CH, CZ, SI, HU, BG, RO, SK) you calculate as fixed amounts per transit, with a choice of vignette duration. Toll countries (FR, IT, ES, PT, GR, HR, PL) you calculate as km × the class rate. Base rates: FR class 2 ±€0.135/km, IT class B ±€0.120, PT C2 ±€0.130, GR category 3 ±€0.130, HR class II ±€0.120, ES ligeros ±€0.115 (but with a low tolled-road fraction). Do NOT count the full route length as toll road: estimate per corridor what proportion really is toll road (classic north-south corridors through France ±80–85%, Spain more like 15–25%). 5. OVERNIGHT STAYS TIP THE RESULT. An overnight ferry replaces 1 to 2 days of driving AND 1 to 2 overnight stays. Work with the regional mid-range benchmark (western Europe €20, southern Europe €27, Scandinavia €35, eastern Europe €18) and let the user slide between budget, mid-range and comfort. Show it explicitly: 'the ferry saves you 2 overnight stays at €20 = €40'. 6. TIME AS A SEPARATE, VISIBLE RESULT — not inside the euros. Alongside the costs, always show: total journey time, number of days of driving and net driving hours. Use 82 km/h on motorway and 58 km/h on secondary roads, plus 30 minutes' break per 2.5 hours and 45–60 minutes a day of overhead. Let the user choose between 1 and 2 drivers (450 versus 700 km/day comfortably). Optionally offer a value for time (€0–25 per hour saved) but set it to zero by default, so that the result stays verifiable. 7. FERRY PRICES: RANGES AND A SEASONAL FACTOR, not a point price. The project data show that the seasonal effect is 35–40% (the Finnlines motorhome package €711 in season I against €968 in season III) and that a motorhome on the Channel costs 1.3× the car fare on a quiet Thursday and 1.9× on a peak weekend. So build: base car fare × motorhome multiplier (1.3–1.9) × seasonal multiplier (1.0–1.4) × day type (midweek/weekend). Always show the next early-booking window as well: Grimaldi closes at the end of April for the summer, Finnlines Hej gives up to 55% for booking in the winter, Moby/Grimaldi 30–40% for booking in January–March, TT-Line waives the length surcharge above 6 m entirely for bookings made before 31-08-2026. 8. PRESENT UNCERTAINTY HONESTLY. Put a confidence indicator against every figure. The diesel prices and vignette rates are solid; the derived toll class rates, the tunnel prices and above all the ferry CO2 are not. A calculator that shows its own margins earns more trust than one that implies three decimal places. 9. CACHING STRATEGY. Refresh diesel prices weekly (the EU Weekly Oil Bulletin appears every Thursday and stood at 13 August 2026), vignette rates annually in December/January, ferry fares live per booking or otherwise quarterly. Put a visible reference date on every block.

Aerodynamic drag grows with the square of speed and, on a motorhome (frontal area 4.5–6.5 m², cd ±0.45–0.60), is by far the biggest consumer above 70 km/h. Practical rule: 80 km/h is the most economical cruising speed, 100 km/h costs ±15–20% more and 120 km/h ±40–50% more than 80 km/h. In concrete terms for a low-profile: ±10 l/100 km at 80, ±11.5–12 l at 100 and ±14–15 l at 120. Because average journey speed including stops barely rises above 100 km/h, driving faster on a long motorhome run buys you a lot of extra fuel use and very little time: 100 instead of 120 km/h costs roughly 45 extra minutes over 1,000 km but saves 20–25 litres of diesel (±€45–55 at August 2026 prices). Accessories: a bike rack with 2–4 bikes costs ±0.3–0.8 l/100 km (+3–7%), a roof box ±0.5–1.0 l/100 km (+5–8%), and the combination runs up to 1.0–1.5 l/100 km. Bikes on the back sit in the slipstream and cost less than the same bikes on the roof. Note: with many ferry operators, and at the Storebælt/Øresund, a bike rack and roof box count towards the measured length and height — so they cost you money twice.