PIC18 Robot – Travelling exact distance straight using two wheel encoders problems

(EDIT: Added TMR0 & TMR1 set-up code)

I have a small robot vehicle controlled by a PIC18f4585. It has two wheels (ball bearing on the front) as well as two rotary encoders. The following function is supposed to travel a specified distance in inches perfectly straight. It travels straight enough, the problem is the distance. the travelled distance has an error of up to 2cm. I was just wondering whether anyone else could see an error I have missed.

Additional info: The PIC is clocked at 40MHz. I am counting the encoder ticks using TMR0 and TMR1 in 8-bit mode.

Any help/suggestions would be greatly appreciated, thanks.

TMR CONFIG:

TRISAbits.TRISA4 = IO_INPUT; 
    OpenTimer0( TIMER_INT_OFF &
            T0_8BIT &
            T0_SOURCE_EXT &
            T0_EDGE_RISE &
            T0_PS_1_1   );

TRISCbits.TRISC0 = IO_INPUT;
OpenTimer1( TIMER_INT_OFF &
        T1_8BIT_RW &
        T1_SOURCE_EXT &
        T1_PS_1_1 & 
        T1_OSC1EN_OFF &
        T1_SYNC_EXT_ON  );

MOVE STRAIGHT FUNCTION:

void moveForwardDistance(double distance, int power)
{
int tickGoal = distance * 5.13348;      //This seems to get ~the correct distance

int masterPower = power;
int slavePower = power;
int error = 0;
int kp = 50;            //error factor
int totalTicks = 0;     //used to keep track of total ticks

resetEncoder();

while(totalTicks < tickGoal)
{
    SetRightMotor(masterPower);
    SetLeftMotor(slavePower);

    error = TMR1L - TMR0L;
    slavePower = power + (error / kp);

    resetEncoder();
    DelayMs(100);
    totalTicks += TMR1L;
}
SetRightMotor(0);
SetLeftMotor(0);

DelayS(1);
}