Ok so when I bought my Jeep, driving home it was reading about 52-53. Wife was following in her Durango and said I was doing about 65. Her GPS Confirmed 65-66. MY Rpms were just a hair below 2000.
So I knew I had to replace my Speedo Gear. Last couple days the speedo will not read at all when starting out, then sometimes read 80 continuously. Other times it will max. Then after about 5 minutes it will start reading "correctly" (Meaning it goes up and down with regard to road speed). After VSS replacement it still wont work initially. I am going to test the wires AND read a couple other tests I can do on the Speedo itself and will get to ya'll on that.
SO I replaced my VSS sensor today and noticed I have a 40 tooth Speedometer gear (Orange with a big fat 40 on it). The "housing" is correctly set with the 39-45 mark facing down.
The Tag on my Diff says 3.55, I have 31" tires. I have ran a calculator that says at 65 I should be about 1956 rpms with that setup which IS VERY close to what I was at when the wife said I was doing 65.
According to the Gear Selector Chart, I need a 32 tooth Gear and that 93 electronically driven speedos were a short shaft. But I have pulled the gear out and it's most definitely a Long shaft.
Would a 40 tooth Gear on 31" and 3.55 read 12-14 mph slower than actual speed or is something seriously wrong lol.
For the record I ordered a 32 tooth long shaft gear tonight.
After researching a ton of info, confirming charts and calculators. I'm going on the ASSUMPTION that the PO must have put in a used transfer case from another vehicle and didnt bother changing the Gear.
To recap, my Speedo wouldn't work for the first 5ish minutes after starting in the AM. Then read 80 mph and actually go up to MAX when I'd stop. Then back to 80 as I rolled faster. After about 10ish minutes it would start working as it should but about 12-14 mph off.
I have 31" tires, 3.55 gears and I found I had a 40 tooth speedo gear which I know was wrong. I KNEW it would account for a speed discrepancy but couldn't figure out why the speedo would half ass work.
I replaced the VSS sensor. Still had problems. I troubleshooted the 3 wires. All read according to specs. I pulled out the VSS and Gear and "drilled" it and the speedo responded instantly. Checked the facing (39-45) and that was correct.
Anyways, my new 32 tooth speedo came in......and everything works PERFECTLY. Starts working immediately when I move and is accurate according the my wife's GPS.
So putting the proper tooth gear not only fixes a calibration issue (which I knew) but can also fix a speedo half-working problem(which I didn't know).
Take it for a test drive after you finish up and see how the speedo behaves. You can bring a GPS with you on your test run to get a more accurate speed just in case the gear is in fact off. Here's a quick chart I found on quadratec when I needed the speedo pinion. Definitely helped out a lot.
"Some days you eat the bear, and some days the bear eats you"
My Build: '92 YJ 4.0, AX15 with 4" RC lift, SYE, Tom Woods drive shaft, 33x12.50 Hankook Dynapro MT, 4.10's