Speedometer is 10 mph too slow
I bought a used Mustang. Driving home my
spouse was following. He wanted to know if I
afraid of the car because I drive so slow. We used
Waze and when it reads 80 mph, car shows 70
mph. Would this cause the mileage to be
olincorrect as well? Fyi, checked speedometer
against other vehicles using Waze and it was spot
tires and rims not original ? a little bigger maybe ?
Yes, not only will the actual mileage and speed be incorrect but your fuel mileage results will br incorrect also. I have tried al different ways to check actual speed and the best way is to find a speed check point on the freeway and set the speed at exactly 60 mph for 5 miles and time it, you should do 5 miles in 5 minutes, if it is off then the speed sensor will need to be set up or down depending on your results, if too slow it needs to be increased, if too fast it need to be decreased. You may also want to check the tire place card inside the vehicle, some are on the drivers door pillar, some on the passenger and even some on the inside glove box or door, make sure you have the specified tires on the vehicle, if not that maybe your whole problem. HTH
Maybe a previous owner swapped out rear end to one with a different gearing. Excessively oversized or undersized tires/rims can affect speedo reading. Ford does make a speedo recalibration tool if the rear end has in fact been swapped out.
Talked with my brother and he thinks someone must have changed gears in rear end. Going to take to our trusty mechanic to diagnose/fixed. This should correct problem as well as whine coming from back.
If you have a gear whine in the rear end then it wasn't setup properly and would not last very long anyway, so it is best you have it checked out.
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