My manual 2011 v6 speedometer and mileage is completely out of whack. It shows and registers between 15 and 25% more than what I'm actually doing. What is the fastest fix to get it back in line? As far as I know gear ratio is stock. Would a tune fix it?
Sounds like the previous owner installed 3.73s (vs. 3.15:1 stock). This means that your speedo and odo would be off by about 18%. But it is easy to check the rear gears.
* Lift the rear and put it on jackstands.
* Put a piece of masking tape on the driveshaft and diff housing and a piece of masking tape on the inside side of the tire at 6 o'clock.
* Put the car in neutral and get under the under the car.
* Rotate the tire one revolution and count the revolutions of the driveshaft.
* Repeat to confirm the result.
If the car has a tune installed then it could just be a 'bad number' in the tune (that is, somebody entered 3.73 by mistake or revs/mile were not entered correctly). I can't imagine that revs/mile could be faulty just because 15 to 20% is a big change in tire height (revs/mile). Do you have a tuner (SCT, HPT, etc.)? If not, go to a reputable tuner and order a tuning device and tell your tuner you need an option to adjust rear gears and revs/mile.