Rubicon Speedo and RPM

Mike_H

Mike_H

autos are better - WRWD508
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Feb 28, 2017
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Grand Rapids, MI, United States
My Rubicon has quite a bit of speedometer error, like it reads 3-5 mph fast at highway speed (meaning I'm going 60 when it read 65). I also have stock size tires (245/75R16). It seems odd to me that the speedo would come that mis-calibrated from the Factory. I did some math:

Speedo error = 1.085 (65/60 = 1.085)

Tire Size (current) = 30.5" nominal
Tire size to make Speedo correct 30.5 x 1.085 = 33.09"

Its weird, cuz I didn't think this jeep had ever been lifted, but maybe it has...and then the lift was taken off at some point...Its definitely not lifted now. 33s won't fit on a stock height jeep so the error doesn't really make sense.

Also, I'm turning about 2500-2600 RPM at 70'ish. Is that about right for the Rear End? I haven't done the math yet...thought I'd ask anyone else with a stock Rubicon what their RPM's are.

Anyone have access to carfax and want to pull a report for me?
 
I bet the speedo gear was changed out by a PO and when went back to stock tires it now needs to be changed back.
 
I run about 2500 rpm at 70 in my Rubicon with 265/75R16's (31.7") and stock 4.10 gears
 
Last edited:
Ranger_b0b said:
My Rubicon has quite a bit of speedometer error, like it reads 3-5 mph fast at highway speed (meaning I'm going 60 when it read 65). I also have stock size tires (245/75R16). It seems odd to me that the speedo would come that mis-calibrated from the Factory. I did some math:

Speedo error = 1.085 (65/60 = 1.085)

Tire Size (current) = 30.5" nominal
Tire size to make Speedo correct 30.5 x 1.085 = 33.09"

Its weird, cuz I didn't think this jeep had ever been lifted, but maybe it has...and then the lift was taken off at some point...Its definitely not lifted now. 33s won't fit on a stock height jeep so the error doesn't really make sense.

Also, I'm turning about 2500-2600 RPM at 70'ish. Is that about right for the Rear End? I haven't done the math yet...thought I'd ask anyone else with a stock Rubicon what their RPM's are.

Anyone have access to carfax and want to pull a report for me?
Click to expand...

Do you have a 5-speed? If so, with stock gears and stock roughly 31" tires, 70 mph would be about 2650 rpm. I don't know why your speedo is off by so much.
 
Yup
Brantley said:
Do you have a 5-speed? If so, with stock gears and stock roughly 31" tires, 70 mph would be about 2650 rpm. I don't know why your speedo is off by so much.
Click to expand...

Yup, 5 speed. That makes sense. I feel like I should confirm the gears by counting, but for indicated 70 mph is in that range.
 
Ranger_b0b said:
Yup


Yup, 5 speed. That makes sense. I feel like I should confirm the gears by counting, but for indicated 70 mph is in that range.
Click to expand...

Why would you need to count your gears? By 5-speed, I meant the manual transmission in which there would be nothing to count. If you have the automatic (which is a 4-speed), then your rpm would be lower than I said unless somebody regeared your Jeep and didn't calibrate the speedo. That would cause the speedo to spin faster than you're actually traveling, which is the problem you have.
 
Brantley said:
Why would you need to count your gears? By 5-speed, I meant the manual transmission in which there would be nothing to count. If you have the automatic (which is a 4-speed), then your rpm would be lower than I said unless somebody regeared your Jeep and didn't calibrate the speedo. That would cause the speedo to spin faster than you're actually traveling, which is the problem you have.
Click to expand...
By count the gears, I mean count revolutions, to verify my gear ratio in the axle. I was pretty out of it this morning when I woke up! :)
 
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