// Variables
new EnableSpeedTimer[MAX_PLAYERS];

// Put this in your OnGameModeInit
SetTimer("UpdateSpeed", 1000, 1);

// The function itself, just put it somewhere
public UpdateSpeed(){
	new Float:x,Float:y,Float:z;
	new Float:distance,value,string[256];
	for(new i=0; i<SLOTS; i++){
		if(IsPlayerConnected(i)){
			GetPlayerPos(i, x, y, z);
			if(EnableSpeedTimer[i]){
				distance = floatsqroot(floatpower(floatabs(floatsub(x,SavePlayerPos[i][LastX])),2)+floatpower(floatabs(floatsub(y,SavePlayerPos[i][LastY])),2)+floatpower(floatabs(floatsub(z,SavePlayerPos[i][LastZ])),2));
				// Distance: meters in the last second
				value = floatround(distance * 3600);
				// meters an hour
				format(string,sizeof(string),"~r~~n~~n~~n~~n~~n~~n~~n~~n~~n~                                                  %d MPH",floatround(value/1600));
				GameTextForPlayer(i, string, 1100, 3);
				}
			SavePlayerPos[i][LastX] = x;
			SavePlayerPos[i][LastY] = y;
			SavePlayerPos[i][LastZ] = z;
			}
		}
	}

// Put this in your OnPlayerCommandText

if(strcmp(cmd, "/speedo", true) == 0 || strcmp(cmd, "/speed", true) == 0) {
	EnableSpeedTimer[playerid] = 1;
	SendClientMessage(playerid, COLOR_GREEN, "Speedometer enabled. To disable, use /speedo-off.");
	return 1;
	}
if(strcmp(cmd, "/speedo-off", true) == 0 || strcmp(cmd, "/speed-off", true) == 0) {
	EnableSpeedTimer[playerid] = 0;
	SendClientMessage(playerid, COLOR_GREEN, "Killed the speedo. To enable the speedo again, type /speedo");
	return 1;
	}