Originally Posted by GafferGail
Manual, 6th gear, 35", 3.6. I will give GPS speedo a shot, but if its accurate, this goes against the chart, right?
My suggestion now that I'm not on my phone and I can type a little better is to do an experiment.
Get your GPS and get on the highway and into 6th gear. Pick a speed and get your GPS to register that speed consistently. Read the tach.
Go find out the exact advertised size of the tire you're running. Not just the calculated size of the tire. Go to the manufacturer's website and find the OD (Outside Diameter) measurement they list for that tire.
Go to my gear ratio calculator: Gear Ratio Calculator
Pick the following:
Transmission: NSG370 Close Ratio (2012+ Jeep Wrangler)
Transfer Case: NP241 (NVG241OR if you have a Rubicon)
Axle Gear Ratio: 3.73 to start
Tire Size: Select inch and type in the exact measurement for the tire
Go down to the last set of numbers, the "Road RPMs at given speed in miles per hour" chart. Set the speed in one of the columns to match the speed you were going on your GPS.
Check the RPM in 6th gear. Change the gear ratios between 3.73, 4.10, 4.56, and 4.88 until you find the closest RPM without going over what your tach says. That will be the actual gear ratio you're running.
Keep in mind that the actual tire diameter depends on a lot of little things including air pressure, tread wear, etc. The tire is usually just a little bit smaller than the advertised OD. My 315/75R17 Goodyear Duratracs are advertised at 34.4" OD but I had to enter 33.48" via my programmer to get the speedometer to be accurate.