Fix #6920: Make 9.8m/s^2 a common constant for TE-calculation

bin/ai/regression/tst_regression/result.txt

@@ -1292,7 +1292,7 @@ ERROR: IsEnd() is invalid as Begin() is never called
     GetRunningCost():   820
     GetPower():         300
     GetWeight():        47
-    GetMaxTractiveEffort(): 139
+    GetMaxTractiveEffort(): 136
     GetVehicleType():   0
     GetRailType():      0
     GetRoadType():      -1
@@ -1436,7 +1436,7 @@ ERROR: IsEnd() is invalid as Begin() is never called
     GetRunningCost():   1968
     GetPower():         1000
     GetWeight():        131
-    GetMaxTractiveEffort(): 388
+    GetMaxTractiveEffort(): 381
     GetVehicleType():   0
     GetRailType():      0
     GetRoadType():      -1
@@ -1454,7 +1454,7 @@ ERROR: IsEnd() is invalid as Begin() is never called
     GetRunningCost():   2296
     GetPower():         1200
     GetWeight():        162
-    GetMaxTractiveEffort(): 480
+    GetMaxTractiveEffort(): 471
     GetVehicleType():   0
     GetRailType():      0
     GetRoadType():      -1

src/engine.cpp

@@ -431,9 +431,9 @@ uint Engine::GetDisplayMaxTractiveEffort() const
 	/* Only trains and road vehicles have 'tractive effort'. */
 	switch (this->type) {
 		case VEH_TRAIN:
-			return (10 * this->GetDisplayWeight() * GetEngineProperty(this->index, PROP_TRAIN_TRACTIVE_EFFORT, this->u.rail.tractive_effort)) / 256;
+			return (GROUND_ACCELERATION * this->GetDisplayWeight() * GetEngineProperty(this->index, PROP_TRAIN_TRACTIVE_EFFORT, this->u.rail.tractive_effort)) / 256 / 1000;
 		case VEH_ROAD:
-			return (10 * this->GetDisplayWeight() * GetEngineProperty(this->index, PROP_ROADVEH_TRACTIVE_EFFORT, this->u.road.tractive_effort)) / 256;
+			return (GROUND_ACCELERATION * this->GetDisplayWeight() * GetEngineProperty(this->index, PROP_ROADVEH_TRACTIVE_EFFORT, this->u.road.tractive_effort)) / 256 / 1000;
 
 		default: NOT_REACHED();
 	}

src/ground_vehicle.cpp

@@ -58,7 +58,7 @@ void GroundVehicle<T, Type>::PowerChanged()
 
 	this->gcache.cached_air_drag = air_drag + 3 * air_drag * number_of_parts / 20;
 
-	max_te *= 9800; // Tractive effort in (tonnes * 1000 * 9.8 =) N.
+	max_te *= GROUND_ACCELERATION; // Tractive effort in (tonnes * 1000 * 9.8 =) N.
 	max_te /= 256;  // Tractive effort is a [0-255] coefficient.
 	if (this->gcache.cached_power != total_power || this->gcache.cached_max_te != max_te) {
 		/* Stop the vehicle if it has no power. */

src/vehicle_type.h

@@ -17,6 +17,8 @@
 /** The type all our vehicle IDs have. */
 typedef uint32 VehicleID;
 
+static const int GROUND_ACCELERATION = 9800; ///< Acceleration due to gravity, 9.8 m/s^2
+
 /** Available vehicle types. */
 enum VehicleType {
 	VEH_BEGIN,