Differences in Speedometer Gear Calibration Recommendations

[CharlesHS]

After replacing the tires on my brothers Jeep; I wanted to calibrate the speedometer to the new tires.
The tires installed are 31" (manufacturer states 31.1") and the differential gears are 4.10/4.11.
Online websites have found several recommendations for the speedometer gears; some sellers recommend 35 tooth, some sellers recommend 34 tooth, another recommends 36 tooth and Quadratec recommends 37 tooth.
I have several speedometer gears laying around from my '98 XJ when I ran different tires and gear ratios combinations.
Testing the different gears; I found the 34 tooth speedometer 5 mph faster, the 35 tooth is 3-4 mph faster, the 36 tooth is 2-3 mph faster and the 37 tooth is about 1 mph faster.
The only conclusion I have come to is with the 31" on the charts; there is a range of 31.1"-31.9" which can make a difference in speedometer calibration.

 

[freedom_in_4low]

After replacing the tires on my brothers Jeep; I wanted to calibrate the speedometer to the new tires.
The tires installed are 31" (manufacturer states 31.1") and the differential gears are 4.10/4.11.
Online websites have found several recommendations for the speedometer gears; some sellers recommend 35 tooth, some sellers recommend 34 tooth, another recommends 36 tooth and Quadratec recommends 37 tooth.
I have several speedometer gears laying around from my '98 XJ when I ran different tires and gear ratios combinations.
Testing the different gears; I found the 34 tooth speedometer 5 mph faster, the 35 tooth is 3-4 mph faster, the 36 tooth is 2-3 mph faster and the 37 tooth is about 1 mph faster.
The only conclusion I have come to is with the 31" on the charts; there is a range of 31.1"-31.9" which can make a difference in speedometer calibration.

part of the issue may be that charts are probably designed using "nominal" size (the number on the sidewall), instead of actual size. A typical 31 is more like 30.5, and has an actual rolling diameter of 29.6" (nobody publishes this, but some publish revolutions per mile and you can back into it from that). If your tire manufacturer says the actual diameter is 31.1 you might need to be looking at the column for 32s.

 

[CharlesHS]

part of the issue may be that charts are probably designed using "nominal" size (the number on the sidewall), instead of actual size. A typical 31 is more like 30.5, and has an actual rolling diameter of 29.6" (nobody publishes this, but some publish revolutions per mile and you can back into it from that). If your tire manufacturer says the actual diameter is 31.1 you might need to be looking at the column for 32s.

The chart states with 4.10 and a 31" tire; the recommended gear is 35 and 32" tires the gear is 34.
You don't see the 37 tooth recommended until the 29" tires; so your idea of actual rolling diameter has merit.

 

[machoheadgames]

I ran a 38 tooth on Duratrac 31x10.5 and 4.11 gears and it was perfect, confirmed with GPS. I use that as my baseline for calculating all other proportions, since I know it was right. That tire was 681 rev/mile, so 29.6" actual.

That same tire I used a 32 tooth gear on 3.55 gears based on a chart, and it was NOT perfect, it read fast by about 3-4 mph at 70. I was a noob at the time and so I didn't really know any better about how to figure out what to use, so I just trusted the chart. Pretty sure I used the Quadratec chart with the links to the gear to purchase, but that was 11 years ago so I don't really remember.

I am now running 30x9.5 on 4.56 with a 43 tooth gear and all seems good from the limited bit of driving I have done.

 

[Rickyd]

After replacing the tires on my brothers Jeep; I wanted to calibrate the speedometer to the new tires.
The tires installed are 31" (manufacturer states 31.1") and the differential gears are 4.10/4.11.
Online websites have found several recommendations for the speedometer gears; some sellers recommend 35 tooth, some sellers recommend 34 tooth, another recommends 36 tooth and Quadratec recommends 37 tooth.
I have several speedometer gears laying around from my '98 XJ when I ran different tires and gear ratios combinations.
Testing the different gears; I found the 34 tooth speedometer 5 mph faster, the 35 tooth is 3-4 mph faster, the 36 tooth is 2-3 mph faster and the 37 tooth is about 1 mph faster.
The only conclusion I have come to is with the 31" on the charts; there is a range of 31.1"-31.9" which can make a difference in speedometer calibration.

(Current gear/displayed mph)×actual mph= needed gear. Round to closest whole number

 

[mrblaine]

After replacing the tires on my brothers Jeep; I wanted to calibrate the speedometer to the new tires.
The tires installed are 31" (manufacturer states 31.1") and the differential gears are 4.10/4.11.
Online websites have found several recommendations for the speedometer gears; some sellers recommend 35 tooth, some sellers recommend 34 tooth, another recommends 36 tooth and Quadratec recommends 37 tooth.
I have several speedometer gears laying around from my '98 XJ when I ran different tires and gear ratios combinations.
Testing the different gears; I found the 34 tooth speedometer 5 mph faster, the 35 tooth is 3-4 mph faster, the 36 tooth is 2-3 mph faster and the 37 tooth is about 1 mph faster.
The only conclusion I have come to is with the 31" on the charts; there is a range of 31.1"-31.9" which can make a difference in speedometer calibration.

And yet again, the only number that matters is the effective tire diameter. The easiest way to find that is to use the gear calculator and get two numbers for reference. MPH and RPM as it is now then go to the calculator and plug those in. Keep changing the tire diameter until the numbers match what you have on the dash and that is your effective tire diameter.

I just did this very thing yesterday. The 33" tire I'm dealing with has an effective diameter of 31.5".

 

[machoheadgames]

part of the issue may be that charts are probably designed using "nominal" size (the number on the sidewall), instead of actual size. A typical 31 is more like 30.5, and has an actual rolling diameter of 29.6" (nobody publishes this, but some publish revolutions per mile and you can back into it from that). If your tire manufacturer says the actual diameter is 31.1 you might need to be looking at the column for 32s.

The rev/mile has almost always been useful for me, when it's listed of course which it isn't always. Except for Falkens, theirs have been a bit off, but all other tires it has been useful. It has always seemed to me to be a logical number to use, because the distance around the tire doesn't change and that's precisely what that is measuring. The 2x to the hub also works, but I find I'm eyeballing it to get that measurement, so I prefer to just do the math off the rev number.

 

[NashvilleTJ]

The chart states with 4.10 and a 31" tire; the recommended gear is 35 and 32" tires the gear is 34.
You don't see the 37 tooth recommended until the 29" tires; so your idea of actual rolling diameter has merit.

I suggest you forget about the charts. You have real world experience to get you to the closest gear. I’ve accurately changed gears in my rig many, many times - and I’ve never seen a chart.

If you are getting your actual speed from a gps or your phone, compare it to what the speedo is telling you and do the math. For example, if the speedo speed is too fast by 2%, than you need a gear which has 2% more teeth (which slows the rotation of the gear), and vice versa.

But using that approach can only get you to an accuracy of + or - 1 tooth. It may or may not be spot on. If not, something like speedo healer is more accurate, and a bit easier.

 

[freedom_in_4low]

I don't remember what specific tire I based this on but I remember checking a couple manufacturers (of those that actually publish rev/mile) and found it close enough not to worry about brand and model differences.

 

[CharlesHS]

Thank you....
Appreciate all the above responses....

 

[freedom_in_4low]

The rev/mile has almost always been useful for me, when it's listed of course which it isn't always. Except for Falkens, theirs have been a bit off, but all other tires it has been useful. It has always seemed to me to be a logical number to use, because the distance around the tire doesn't change and that's precisely what that is measuring. The 2x to the hub also works, but I find I'm eyeballing it to get that measurement, so I prefer to just do the math off the rev number.

rev/mile is all I use when it comes to gearing and rpm vs speed stuff. In addition to the measurement issue, the hub height is tied to tire pressure, but the rolling diameter isn't, or at least not in a magnitude that matters.

 

[machoheadgames]

rev/mile is all I use when it comes to gearing and rpm vs speed stuff. In addition to the measurement issue, the hub height is tied to tire pressure, but the rolling diameter isn't, or at least not in a magnitude that matters.

Yeah, I use the rev one when it's available and it is pretty close across all brands. The biggest difference I've seen between two brands on one size is Continental 255/75R17 at 647 and Duratracs at 653. Still close enough to be very accurate.

Helps that Grimmjeeper has a spot for that number so you don't even have to do the math for that if you don't want to. I have a similar list of rev numbers for all the common sizes in my phone notes app.