I was looking at the gear ratio forum to find out what gearing I should have in my 88 YJ. According to the chart there, I can figure out the gear ration by tire size and RPM at 65 MPH. I tested that out this evening. I am turning 33" tires at 4000 RPM to get 65MPH. That places me off the chart?! Am I not doing something right? Does someone have the calculation that I can figure out myself?
Gear Ratios
- Thread starter pdparker
- Start date
PhattyX
New member
I'm going to take a guess that it has to do with what transmission gear you're in. To the best of my knowledge, the chart assumes a 1:1 transmission ratio. That's 4th gear on both my 4sp & 5sp manuals, but I'm not positive in the YJ.
Are you in 3rd? Junkpile posted a link to a great site with transmission specs. Let me see if I can find it.
{edit} http://www.jeeptech.com/wiki/index.php/Transmission
I do know the 4cyl YJ's have 4.10's from the factory.
-X
Are you in 3rd? Junkpile posted a link to a great site with transmission specs. Let me see if I can find it.
{edit} http://www.jeeptech.com/wiki/index.php/Transmission
I do know the 4cyl YJ's have 4.10's from the factory.
-X
Last edited:
Bounty__Hunter
Super Moderator
Have you changed your speedometer gear to correct the speedo for the larger tires? If not, that formula is useless.
What engine and transmission does your jeep have? For the '88, the 4.2L 5spd most often had 3.07 gears. The 2.5L had 4.10 gears with the 5spd and 3.73 gears with the auto. A 4.2L auto could be anything between 3.07 and 3.73.
You can look at the tag bolted to either diff cover, first three digits will be your gear ratio. Or pull the diff cover (good time to change the gear oil) and look for the pair of numbers stamped on the edge of the ring gear. Divide the larger number by the smaller and you'll have your gear ratio.
What engine and transmission does your jeep have? For the '88, the 4.2L 5spd most often had 3.07 gears. The 2.5L had 4.10 gears with the 5spd and 3.73 gears with the auto. A 4.2L auto could be anything between 3.07 and 3.73.
You can look at the tag bolted to either diff cover, first three digits will be your gear ratio. Or pull the diff cover (good time to change the gear oil) and look for the pair of numbers stamped on the edge of the ring gear. Divide the larger number by the smaller and you'll have your gear ratio.
I am running a 4.2L w/ 5spd trans. I have not adjusted the speedometer. I was in 4th gear when I took the tach reading. Even if the speedometer is off slightly, I was nearing 4000RPM while going 60. That still does not sound right. I think I will take the advise of Bouty Hunter and pull the diff cover. Thanks.
Bounty__Hunter
Super Moderator
You were likely doing about 75mph without the speedo corrected.
Chances are you have 3.07 gears. I too had an '88 4.2L 5spd YJ, now have an '88 MJ and a '95 YJ. Swapped the '88 4.2L for a 350 back in the day.
Chances are you have 3.07 gears. I too had an '88 4.2L 5spd YJ, now have an '88 MJ and a '95 YJ. Swapped the '88 4.2L for a 350 back in the day.
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