How do I determine my axle gear ratio?

New Jeep owner with a question about my 2002 TJ Sahara. My speedo reads 9 over. So when it shows 60 on the guage, GPS shows 51. I'm running (supposed to be 31") tires (worn and more likely closer to 30"). I pullled the speedo gear and it is a 33 tooth gear. I had ordered a 33 tooth off the chart before pulling what was in there (my bad). By calculations, I think I'll need a 40tooth gear, which is on order....but what the heck would my gear ratio be for this??? 4.10? Sorry for the long post. Just would like to know what's in there. Thanks so much!

will
 
New Jeep owner with a question about my 2002 TJ Sahara. My speedo reads 9 over. So when it shows 60 on the guage, GPS shows 51. I'm running (supposed to be 31") tires (worn and more likely closer to 30"). I pullled the speedo gear and it is a 33 tooth gear. I had ordered a 33 tooth off the chart before pulling what was in there (my bad). By calculations, I think I'll need a 40tooth gear, which is on order....but what the heck would my gear ratio be for this??? 4.10? Sorry for the long post. Just would like to know what's in there. Thanks so much!

will
 
New Jeep owner with a question about my 2002 TJ Sahara. My speedo reads 9 over. So when it shows 60 on the guage, GPS shows 51. I'm running (supposed to be 31") tires (worn and more likely closer to 30"). I pullled the speedo gear and it is a 33 tooth gear. I had ordered a 33 tooth off the chart before pulling what was in there (my bad). By calculations, I think I'll need a 40tooth gear, which is on order....but what the heck would my gear ratio be for this??? 4.10? Sorry for the long post. Just would like to know what's in there. Thanks so much!

will
51 vs 60 is a 15% difference. If you have a 33 tooth, that is 85% of 38.8 teeth.

You likely have 4.10 and need a 38 or 39 tooth for 31/4.10. 38 tooth has always been accurate for me on 31's and 4.10. Keep in mind the tire being worn has little to do with it. Most tires are considerably smaller than their advertised measurement. My 31's are worn and naturally small, they measure about 29" when you measure to the center of the hub x2. That lines up with my rpm calculators so I say I have about 29" tires (31x10.50 nominal).

38/4.10 was always spot on.

Also, the percent error is exactly that, a percent. If you are off by 51 at 60, then at 100 mph on the gauge, you would be off by 15, doing a real world 85 mph on the GPS. It is not off by a set mph (in your case 9 mph) unless you have a bad speedo gauge that isn't responding appropriately to the speed input being reported to it.

For a more scientific test, go 60 on your GPS and report back what RPM your engine is spinning and what gear you are in. Be as accurate as you can possibly be when reporting the rpm. Or, go to a certain rpm, like 2500 or 3000, and report what speed the GPS shows.
 
Will do! Thanks for the reply! And a correction to my previous post, the gear I pulled out when the speeds were taken, was a 34 tooth gear. Not a 33. Fumble fingers. Thank you again!
 
Sorry, my initial math was a little off. 51 x 1.17 equals 60. So 33 x 1.17 would have been 38.6. just a small difference but I wanted to be consistent.

Now that you have updated the gear tooth count to 34, the math is coming to 39.78, or 40 teeth.

On 31's though, this isn't adding up. 31's and 4.10 needs 38 teeth. Your tires would have to measure REALLY small to need a 40 tooth gear on 4.10. 31's of the same diameter that needed 38 teeth for 4.10, would need 42 teeth with 4.56 ratio.

There is no gear ratio between 4.10 and 4.56 except for 4.27 but you can't get that for the front axle. So something must be wrong. I'll await your rpm reporting to further investigate.

Does it feel pretty zippy around town and slightly revvy on the highway?? Are you sure the tires are 31's? 31x10.5R15, or a metric equivalent?

Just some items to investigate to get you fixed up.
 
Sorry, my initial math was a little off. 51 x 1.17 equals 60. So 33 x 1.17 would have been 38.6. just a small difference but I wanted to be consistent.

Now that you have updated the gear tooth count to 34, the math is coming to 39.78, or 40 teeth.

On 31's though, this isn't adding up. 31's and 4.10 needs 38 teeth. Your tires would have to measure REALLY small to need a 40 tooth gear on 4.10. 31's of the same diameter that needed 38 teeth for 4.10, would need 42 teeth with 4.56 ratio.

There is no gear ratio between 4.10 and 4.56 except for 4.27 but you can't get that for the front axle. So something must be wrong. I'll await your rpm reporting to further investigate.

Does it feel pretty zippy around town and slightly revvy on the highway?? Are you sure the tires are 31's? 31x10.5R15, or a metric equivalent?

Just some items to investigate to get you fixed up.
The tires are 31x10.5 R15....just like you said. It's great aound town, but seems like I need to upshift early. Well, maybe that's just me trying to save gas LOL. I'd be tickled if it is a 4.10. It just doesn't make sense to me. The build sheet shows it shipped with a 3.73. And obviously, it's not lifted, so it wasn't to run bigger tires. A mystery I guess. I'll get back when I can get the new measurements. Your help is appreciated.
 
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Yeah, could be a few things...I'm sure they are not measuring that small, You'd have 31's in the 27" range.....surely they can't be that worn or manufactured that small.....You very well may have 4.10, just that it still doesn't add up. Maybe the speed comparison was off by a mph or two. I really don't know, but once you look into it more we'll figure it out.

If you're bored, you might just pop off the diff cover and visually verify what gears you have, that way there's no question there. Then the speedo can be evaluated and fixed accordingly.
 
(y)
Yeah, could be a few things...I'm sure they are not measuring that small, You'd have 31's in the 27" range.....surely they can't be that worn or manufactured that small.....You very well may have 4.10, just that it still doesn't add up. Maybe the speed comparison was off by a mph or two. I really don't know, but once you look into it more we'll figure it out.

If you're bored, you might just pop off the diff cover and visually verify what gears you have, that way there's no question there. Then the speedo can be evaluated and fixed accordingly.
 
34 is the stock gear for a 3.73 ratio which you probably have. You can check just by rotating the driveshaft, you don't have to pull the diff cover. Your speedometer is off because the cluster is probably off, the needle spring is old and weak. The new gear will cause your odometer to read incorrectly. If you get a cheap ELM327 OBD scanner you can check the speed the cluster thinks it is supposed to show or simply do a cluster test by holding down the trip meter and turning on the Jeep. There are details around here about doing that test. You are probably stuck having one or the other read wrong.