According to the website Heritage Commercials, the Sentinel steam 'bus consumes 50 gallons of water and 110lb of coal for every 10 miles travelled, and can reach speeds of up to 40mph (although can run more sustainably at 25-30mph).
Although the steam 'bus was not a success and only four were ever built, three of which were exported to Czechoslovakia and one of which was used to transport the Sentinel works brass band, the 'bus consisted in effect of a passenger body on a lorry chassis, and the DG4 lorry was one of Sentinel's major types, and a type that we use in the pakset, so these speed and coal consumption figures should be useful for calibrating the steam lorries.
I found Czech page about Sentinels - they were licensed to Škoda, and many were built in Czechoslovakia. http://www.sentinel.cz/
The page is in czech only, but has a lot of photos (incl. blueprint) and the following tech specs:
power: 70 HP at 250 rpm, steam pressure 19 atmospheres
avg. speed: 25 km/h
water tank: 800 l
coal bunker: 300 kg
kerb weight: 7860 kg
cargo capacity: 5000 kg
reach with full tank: 40 km at avg speed 15 km/h (max 25 km/h)
max consumption per 1 km = 30 l of water and 4 kg of coal
prices around 1925: 100 kg of coal = 30 Kč, 1 l of gasoline = 3.40 Kč
running cost was about 1/3 of running cost of comparable gasoline powered lorry
Price of the Škoda-Sentinel lorry was 160000 Kč
Another article about sentinels, has a bit different numbers: http://www.starestroje.cz/historie/parni.vozidla.sentinel.php
It is article from 1923 about the imported sentinels from England (pictures are about rail cars, but text is mostly about lorries)
I admit that the article's language seems to be biased in favor of sentinels, so the comparison with gasoline lorry might not be trustworthy.
75 HP, 19 atm,
max speed 40 km/h
water tank 800 l
test ride: 62.1 km in 3h50min (2h30min without pauses), avg speed 24 km/h, max speed on flat terrain 35 km/h, weight 6t + load 7t + 15 pax, consumption 200 kg of coal, 700 l of water = 3.2 kg/km coal, 11.3 l/km water
revenue calculation and comparison with gasoline lorry:
price for delivering 100 kg of load 0.125 Kc/km
100 kg coal = 22 Kc, 100 kg gasoline = 300 Kc
sentinel load 7 t, 3 hours a day, 30 km/h, 300 working days/year = total income 236000 Kc/year. Running costs (driver + fireman wages, maintenance, oil, fuel, and 10% of purchase cost) are estimated at 98000 Kc/year
gasoline lorry load 4t, 4 hours a day, 20 km/h = estimated income 120000 Kc/year. Running costs (driver wages, maintenance, oil, fuel, 10% purchase) estimated at 83000 Kc/year
The last pat of the article is about steam powered passenger rail car - info taken from factory catalog.
comfortable capacity 60-80 pasengers
coal consumption (at full load) 4 lb/mile, 1.124 kg/km
water consumption (at full load) 2.5 gallons/mile, 5.64 l/km
avg consumption at flat track is 4-5 q (1q=100 lb?) per 120 miles (or 203-254 kg coal per 193 km)
max speed 40 mph, may be geared at 30 mph for extra power (hilly track) - 33 promile at 15 mph
recommended option for czechoslovakia was 17t, 75 HP, avg 40 km/h, 70 pax + 40 pax trailer
top speed on flat track 50 km/h, incline 10 promile 35 km/h, 20 promile at 20 km/h
half-full car (28 pax) - tickets cost 1 pence/mile (1 pence = 0.643 Kc), 150 miles/day, 300 days/year, total income 5250 pounds sterling (809812 Kc)
operating + maintenance costs 778 pounds/year, other costs 1472-2472 pounds/year = net revenue 2000-3000 pounds/year (310000-460000 Kc)
The author was very optimistic about possible imports of these. The reality was that Škoda made one (licenced) piece (class M220.0) that was in use 1925-1948. There were several other similar steam rail cars in use of other designs.