A bit of Google research suggests that a horse drawing a light buggy could trot for long periods at 10 to 15mph (16 to 24 km/h). The heavier the load, of course, the slower the sustained speed. It should certainly be realistic that a personal buggy would go faster than our company stagecoach. This is not now the case.
There are six horse-drawn "citycars." Some variety might be appropriate, perhaps a range from 14 to 22. We have white and yellow HorseCarriageOpen models, blue and green and cream HorseCarriageClosed (horse-carriage.dat) all currently with speed 12, and a brown and grey HorseCart (horse-cart-citycar.dat). Leaving only the grey horse-cart at 6km/h, I recompiled my local copy with the following changes (diff attached)
diff --git a/citycars/horse-carriage.dat b/citycars/horse-carriage.dat
index fc8a2fb..980bdf0 100644
--- a/citycars/horse-carriage.dat
+++ b/citycars/horse-carriage.dat
@@ -5,7 +5,7 @@ copyright=James
DistributionWeight=3
intro_year=1750
retire_year=1910
-speed=12
+speed=22
length=3
Image[W]=horse-carriage.0.0
@@ -24,7 +24,7 @@ copyright=James
DistributionWeight=3
intro_year=1750
retire_year=1910
-speed=12
+speed=20
length=3
Image[W]=horse-carriage.1.0
@@ -43,7 +43,7 @@ copyright=James
DistributionWeight=3
intro_year=1750
retire_year=1910
-speed=12
+speed=18
length=3
Image[W]=horse-carriage.2.0
@@ -62,7 +62,7 @@ copyright=James
DistributionWeight=3
intro_year=1750
retire_year=1910
-speed=12
+speed=16
length=3
Image[W]=horse-carriage.3.0
@@ -81,7 +81,7 @@ copyright=James
DistributionWeight=3
intro_year=1750
retire_year=1910
-speed=12
+speed=14
length=3
Image[W]=horse-carriage.4.0
diff --git a/citycars/horse-cart-citycar.dat b/citycars/horse-cart-citycar.dat
index 7484cc0..1ce2fc5 100644
--- a/citycars/horse-cart-citycar.dat
+++ b/citycars/horse-cart-citycar.dat
@@ -5,7 +5,7 @@ copyright=James
DistributionWeight=4
intro_year=1750
retire_year=1930
-speed=6
+speed=10
length=3
Image[W]=horse-cart-citycar.0.0