Tom is on the money.
I've had a little play...here are the correlations:
Name / Displays As / Filename
CONCRETE CONCRETE taxi_concrete.bmp
GRASS GRASS taxi_grass_su.bmp
WATER WATER ?
UNKNOWN GRASS taxi_grass_su.bmp
ASPHALT ASPHALT taxi_asphalt.bmp
GRASS2 GRASS taxi_grass_su.bmp
GRASS3 GRASS taxi_grass_su.bmp
CLAY GRASS taxi_grass_su.bmp
SNOW SNOW taxi_ice.bmp
ICE SNOW taxi_ice.bmp
UNKNOWN2 CONCRETE taxi_concrete.bmp
UNKNOWN3 GRASS taxi_grass_su.bmp
DIRT DIRT taxi_dirt.bmp
CORAL CORAL taxi_coral.bmp
GRAVEL GRAVEL taxi_gravel.bmp
OIL TREATED OIL TREATED taxi_shale.bmp
STEEL MATS STEEL MATS taxi_steel.bmp
BITUMINOUS BITUMINOUS taxi_bitu.bmp
BRICK BRICK taxi_brick.bmp
MACADAM MACADAM taxi_macadam.bmp
PLANKS PLANKS taxi_planks.bmp
SAND SAND taxi_sand.bmp
SHALE SHALE taxi_shale.bmp
TARMAC TARMAC taxi_tarmac.bmp
UNKNOWNX CONCRETE taxi_concrete.bmp
Sorry my table won't format here.
What I noticed is that some of the texture names, although mapping to the same file, appear to have their own BMP's but don't use them! So there are 25 texture names, but only 17 BMP's used for them.
Examples are Grass, Grass2 and Grass3.
So even if you alter taxi_grass_sp.bmp it has no effect in the sim.
I haven't been able to identify which file maps to "Water"
GM:
be interesting to create a small library of grass with those less-used names.