Agreen
NAXJA Forum User
- Location
- Charleston
So my 1998 (4.0, 2wd, Aw4) did not have an axle tag, so I've never really known the rear ratio. I converted it to 4WD in 2017 and instead of swapping the rear end I just counted the number of input vs output turns. I got about 3-3/4 turns on both axle yokes when the tires made a full revolution. Yes, I've had the rear differential cover off several times, but I've never once considered counting pinion/ring teeth
At 60 MPH the tach has always read 2000 rpm. I also verified this with GPS and the OBD scanner. Without any question, it's 2000 rpm and 60 MPH.
Using an RPM/Ratio/speed calculator I get 60 MPH when I input the following data:
3.73 axle ratio
28.3 inches tire height (225/75r15)
0.75 transmission ratio
2000 RPM
Fast forward to today. I put a 3" lift, 31" tires, and want to correct the speedo. Easy enough, the ratio is still fairly forgiving for 31" tires, so I'll just pop a 33 tooth speedo gear in, right?
So I pull the old gear out and it's a 34. Weird. I expected a 36. Determined to know the answer, I prepare to pop the front diff cover and count teeth. Then I noticed the tag (which I somehow missed before). Ok, so I have been a little lazy and only topped the front diff off when I did the swap. Anyway, there it is. It says 3.55 on it. Makes sense. A 34t speedometer is accurate for 3.55 ratio gears.
So what's the deal? If it's 3.55 I should be reading 63 mph at 2000 rpm, or 1900 rpm at 60. So now I have to order a different gear, and I'm still confused as to why everything else points to having 3.73 ratio when I most likely have a 3.55.
I'm really only asking because I don't want to order the wrong gear and still have the speedo off still.
At 60 MPH the tach has always read 2000 rpm. I also verified this with GPS and the OBD scanner. Without any question, it's 2000 rpm and 60 MPH.
Using an RPM/Ratio/speed calculator I get 60 MPH when I input the following data:
3.73 axle ratio
28.3 inches tire height (225/75r15)
0.75 transmission ratio
2000 RPM
Fast forward to today. I put a 3" lift, 31" tires, and want to correct the speedo. Easy enough, the ratio is still fairly forgiving for 31" tires, so I'll just pop a 33 tooth speedo gear in, right?
So I pull the old gear out and it's a 34. Weird. I expected a 36. Determined to know the answer, I prepare to pop the front diff cover and count teeth. Then I noticed the tag (which I somehow missed before). Ok, so I have been a little lazy and only topped the front diff off when I did the swap. Anyway, there it is. It says 3.55 on it. Makes sense. A 34t speedometer is accurate for 3.55 ratio gears.
So what's the deal? If it's 3.55 I should be reading 63 mph at 2000 rpm, or 1900 rpm at 60. So now I have to order a different gear, and I'm still confused as to why everything else points to having 3.73 ratio when I most likely have a 3.55.
I'm really only asking because I don't want to order the wrong gear and still have the speedo off still.