Gear

[srosario]

How can u tell wat gear do I have, the previous owner said he put 4.88, and a friend said if I had 4.88 on an auto, my r vs should be high at highway speed, but I do 70 mph at 2500 rpm's. And my speedo says 60 mph but I'm really going 64.2, I wanna get the trail dash but I would need to know what gear I'm running.

 

[MarkW13]

How can u tell wat gear do I have, the previous owner said he put 4.88, and a friend said if I had 4.88 on an auto, my r vs should be high at highway speed, but I do 70 mph at 2500 rpm's. And my speedo says 60 mph but I'm really going 64.2, I wanna get the trail dash but I would need to know what gear I'm running.

The total gear ratio as it translates to rpm in a certain gear is dependent upon several inputs. Transmission gear ratio. 1 transfer case ratio. In high it is 1 to 1. Differential gear ratio. Tire size. There are several gear ratio calculators online. Simply input your specs and it will give you a number. Don't forget that tire size is actual rolling diameter, not advertised size on the sidewall.

An easy way to check your gear ratio is to mark the tire, jack up the rear end, then spin the tires. The drive shaft should spin just less than 5 times for 4.88 gears for every single revolution of the tire.

Here is a link to a gear ratio calculator site. It didn't have my 2013 jkur manual, but you can input your own numbers with a little research.


http://www.grimmjeeper.com/gears.html

Some has done all the work if you want to look at a chart.

http://www.wanderingtrail.com/Mechanical/gearratio.html

 

[aermotor]

What year is your JK and what size are your tires? The following will vary slightly depending on your engine.

Use a free speedometer app for your phone or a GPS and make sure you are going exactly 70mph, note your RPM, then compare to the appropriate chart below, 3.6 Pentastar or older 3.8 engine.

 

[srosario]

Thanks, we'll do, I have a 2wd 2010 auto, 35" advertised 33" true