VEX Odometry Not Correctly Calculating Distance

[TreeFrog]

I am working on odometry for my robot with PROS. My code is currently as follows:

void position_track() {
	auto plus = (right_mtrs.getPosition() + left_mtrs.getPosition()) / 2 * M_PI * wheelDiam;
	distanceMoved += plus;
	right_mtrs.tarePosition();
	left_mtrs.tarePosition();
}

void moveDistance(double i) {
			while(distanceMoved < i) {
				left_mtrs.moveVoltage(2400);
				right_mtrs.moveVoltage(2400);
				
				position_track();
				pros::delay(20);
			}

			left_mtrs.moveVoltage(0);
			right_mtrs.moveVoltage(0);
			while(true) { position_track(); pros::delay(20); }
		}

(Note that distanceMoved and wheelDiam are class variables.)

However, the odometry is not accurate. When I command it to move 12 in. (wheel diameter 3.25), it actually moves over twice that distance. However, this is rarely consistent — sometimes it will move as much as three times that distance. I have also tried to use libraries like Okapi, which all face the same issue.

The robot itself uses 6 motors (three on each side) and gears — 8 wheels touch the ground.

Please advise if there are any errors that I should fix.

[computingpotatochips]

Are your motor groups set to report their position in terms of rotations and not any other units using right_mtrs.setEncoderUnits(AbstractMotor::encoderUnits::rotations); left_mtrs.setEncoderUnits(AbstractMotor::encoderUnits::rotations);

[TreeFrog]

They are indeed set as that. I also verified that both encoders are counting similarly (i.e. one side isn’t broken).