I would implement both of robtillaart's suggestions. Also, in RPM() don't call Serial.println between clearing frontCount and backCount and assigning lastTime - you want to reset the counts as close to assigning lastTime as possible. You might even need to disable interrupts during these 3 operations. Something like this:
void RPM() {
deltaT = millis() - lastTime;
if (deltaT >=accuracy) {
noInterrupts();
unsigned long currentTime = millis();
unsigned int lastFrontCount = frontCount;
unsigned int lastRearCount = rearCount;
frontCount = 0;
rearCount = 0;
interrupts();
deltaT = currentTime - lastTime;
lastTime = currentTime;
lastRpm_F = rpmFront;
rpmFront = rpmFactor*lastFrontCount/(99*deltaT);
rpmRear = rpmFactor*lastRearCount/(102*deltaT); //3 more ticks on this rotor thus the 102 vs 99
Serial.println(rpmFront);
...