WITH THE PROBE AS WITH ANY CAR , START WITH YOUR TIRE SIZE, GOING UP OR DOWN WILL MAKE A DIFFERENCE WITH YOUR R.P.M. ALSO CHANGING WHEEL SIZES WILL CHANGE YOUR RATIO AND YOU'LL NOTICE A DIFFERENCE. IF YOU'RE TURNING YOUR MOTOR TO HARD GO UP ON TIRE OR WHEEL SIZE. SOME PEOPLE CAN FIGURE IT OUT BY KNOWING ALL THE SPECIFICS OF TRANS MISSION RATIOS REAR GEAR RATIOS OR IN YOUR CASE FRONT WHEEL DRIVE ,IF THAT'S WHAT'S IN YOUR CAR. ALSO THEY HAVE TO KNOW THE CIRCUMFERENCE OF THE TIRE . MAY BE YOU CAN FIGURE ALL THAT OUT IF. YOU'RE GOOD IN MATH AND KNOW THE RIGHT EQUATIONS. I HAVE TO DO IT BY TRIAL AND ERROR BUT I'VE HAD A LOT OF SUCCESS AT IT AND ITS EASIER TO CHANGE TIRES OR WHEELS THAN TRANS MISSIONS . I KNOW SOMEONE WITH A FORD 4 CYL. AND HE RUNS 3 TRACKS AND HE'S IN THE WINNER'S CIRCLE ABOUT 90 % OF THE TIME. I HOPE I'VE BEEN OF SOME HELP.