The A4069 runs for about 20 miles between Llangadog and the valleys around Gwaun-Cae-Gurwen, and the 12-mile mountain section between Llangadog and Upper Brynamman is the part people come for. It climbs over the western edge of Bannau Brycheiniog to a summit of about 493 metres, with open moorland on both sides and a famous hairpin, Tro Gwcw, the "cuckoo's bend", on the northern approach.
Why the economy figure drops
Compared with a gently rolling road, the Black Mountain Pass asks a lot of an engine for its length. The climb is steep in places, the hairpin forces a slow first- or second-gear turn, and there's little chance to settle into a steady speed. The page's estimate of up to a quarter lost to the climb and bends is about right for a car driven briskly. Smooth drivers who carry momentum into the climb and avoid repeated hard acceleration lose much less.
A worked cost
Picture a hatchback that normally manages 45 mpg, crossing the whole 20-mile road with petrol at an illustrative 140p a litre. On the flat that would cost about £2.80. With a 25% economy penalty it's nearer £3.80. Even at the worst, crossing the pass is cheap; the planning issue isn't the cost, it's making sure you aren't running low when you start the climb, because there's no fuel on the mountain.
Weather and livestock
The summit is exposed. Cloud can sit on the top while the valleys either side are clear, and visibility can drop very quickly. Sheep and ponies graze the open land and often stand on or beside the road. Drive at a speed that lets you stop for an animal around the next bend, especially in fog or after dark.
Fuel and charging around the pass
Fill up in Llandeilo or Llangadog to the north, or around Brynamman, Gwaun-Cae-Gurwen and Ammanford to the south. Electric car drivers should treat the pass the same way: start with a comfortable charge, since the climb uses more energy and the nearest chargers are in the towns. In Welsh towns and villages, remember that the default urban speed limit is 20 mph, which changes journey times more than fuel use.
Seasons on the mountain
In summer, the pass is busy with drivers, motorcyclists and cyclists tackling the climb, and the laybys near the summit fill on sunny weekends. In winter, snow and ice can make the road dangerous, and it isn't always gritted as quickly as the main roads in the valleys. Check the weather forecast for the high ground, not just for the towns below.
Cycling and sharing the road
The climb is popular with cyclists, and on the steeper sections they'll be travelling slowly. Overtake only where you can see well ahead and can give at least 1.5 metres of space, as the Highway Code advises. On the descent, cyclists can reach high speeds, so be prepared for them to need the full width of the lane on the bends.
Exploring beyond the pass
The pass sits on the edge of Bannau Brycheiniog National Park, and short detours lead to Carreg Cennen Castle near Llandeilo and the reservoirs and waterfalls further east. Combining the pass with a circular route through the surrounding valleys makes a full day of driving.
A second example: a scenic circuit
A circular drive from Llandeilo over the pass to Brynamman, returning through the Amman and Tywi valleys, covers roughly 45 miles. In a car averaging 45 mpg at an illustrative 140p a litre, that's about £6.40 of fuel, or around £7.50 allowing for the climb.