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Message: <blockquote data-quote="lutter94" data-source="post: 1454332" data-attributes="member: 10528">Of course it will be inaccurate based on a percentage not a certain mph.  It's not weird, its how ratios work.  If you go from 2.73 to a 3.27 you will gain 20% in engine RPM's for the same given speed.  Or loose 16.5% of your speed (reading 90 mph would truly be 75 mph).  Yes I know 20 doesn't equal 16.5.  2.73/3.27 = 83.5%.   3.73/2.73 = 1.20%Here's some math for ya:With your 285/35R18 tires, so 25.9" tall.  And a T5 trans, your final drive ratio is 0.68 IIRC.If you have 2.73 rear end gears, a true 70MPH will be right at 1690 RPM.  See below for all of them.  Not saying your tach is 100% perfect, but you should be able to tell between the 200-300 RPM steps.2.73 @ 70 MPH = 1690 RPM3.27 @ 70 MPH = 2025 RPM3.55 @ 70 MPH = 2200 RPM3.73 @ 70 MPH = 2310 RPM4.10 @ 70 MPH = 2540 RPMSounds like you are currently setup for 2.73 gears, but actually have 3.27.  If you're doing 90 reading 75.....math is working out perfectly at the moment.What drive gear do you have?  You had it all apart correct?  Did you count teeth?</blockquote>