Get odometry from wheels encoders

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Hi all, I have a problem to compute odometry from wheel encoders, I mean, i don't have a real problem, I just don't understand a step.
I get the ticks from the encoders (they are about 2626 ticks for a full wheel rotation) and I perform the odometry calculation in the following way(of course this is in a loop...i paste just the important lines about the odometry calculation)

current_time = ros::Time::now();
double DistancePerCount = (3.14159265 * 0.13) / 2626;
double lengthBetweenTwoWheels = 0.25;

// extract the wheel velocities from the tick signals count
deltaLeft = tick_x - _PreviousLeftEncoderCounts;
deltaRight = tick_y - _PreviousRightEncoderCounts;

omega_left = (deltaLeft * DistancePerCount) / (current_time - last_time).toSec();
omega_right = (deltaRight * DistancePerCount) / (current_time - last_time).toSec();

v_left = omega_left * 0.065; //radius
v_right = omega_right * 0.065;

vx = ((v_right + v_left) / 2)*10;
vy = 0;
vth = ((v_right - v_left)/lengthBetweenTwoWheels)*10;

double dt = (current_time - last_time).toSec();
double delta_x = (vx * cos(th)) * dt;
double delta_y = (vx * sin(th)) * dt;
double delta_th = vth * dt;

x += delta_x;
y += delta_y;
th += delta_th;

geometry_msgs::Quaternion odom_quat = tf::createQuaternionMsgFromYaw(th);

geometry_msgs::TransformStamped odom_trans;
odom_trans.header.stamp = current_time;
odom_trans.header.frame_id = "odom";
odom_trans.child_frame_id = "base_link";

odom_trans.transform.translation.x = x;
odom_trans.transform.translation.y = y;
odom_trans.transform.translation.z = 0.0;
odom_trans.transform.rotation = odom_quat;

// send the transform
odom_broadcaster.sendTransform(odom_trans);

// Odometry message
nav_msgs::Odometry odom;
odom.header.stamp = current_time;
odom.header.frame_id = "odom";

// set the position
odom.pose.pose.position.x = x;
odom.pose.pose.position.y = y;
odom.pose.pose.position.z = 0.0;
odom.pose.pose.orientation = odom_quat;

// set the velocity
odom.child_frame_id = "base_link";
odom.twist.twist.linear.x = vx;
odom.twist.twist.linear.y = vy;
odom.twist.twist.angular.z = vth;

// publish the message
odom_pub.publish(odom);
_PreviousLeftEncoderCounts = tick_x;
_PreviousRightEncoderCounts = tick_y;

last_time = current_time;

The thing that I don't understand is why i have to multiply vx and vth for 10 (it is a value that i set randomly and seems to work) to make the odometry consistent with the movements of the robot (if i dont multply, the odometry will change very slowly compared to what the robot does in terms of meters done).
Maybe I'm doing some errors calculating the velocities.
An help would be very very appreciated, thank you.