Speedometer calibration with different tires?

[Ajax MD]

I did a search but didn't find an answer for this particular question:

When the military switched from the old 36" bias tires to 37 inch radials, were the transmissions upgraded with a different speedometer gear to correct the speedometer?
I'm making the swap this weekend and I'm curious as to how far off the speedometer will be. I know I can buy the red/orange speedometer gear from Kascar but I don't even know what gear I have installed now. Maybe I already have that gear?

We're not worried about tearing up the interstate. If anything, we're concerned about accidentally speeding through residential areas and getting nailed. We only have the 3-speed so we tend to stick to secondary roads rather than the interestate.

 

[Chief B]

I know I have 37s on mine and my speedometer is off so I'm thinking they did not change the gears. When my speedometer says 40 I'm actually doing about 47. I actually use a GPS speedometer on my phone and have my phone mounted where I can see it. The location of the actual speedometer is difficult for me to see anyway and since it is off I don't really bother with it. Plus the phone app I use gives me a warning beep once I hit 45 so that I know I have hit my max speed as I don't like to push it much above that.

 

[Ajax MD]

I know I have 37s on mine and my speedometer is off so I'm thinking they did not change the gears. When my speedometer says 40 I'm actually doing about 47. I actually use a GPS speedometer on my phone and have my phone mounted where I can see it. The location of the actual speedometer is difficult for me to see anyway and since it is off I don't really bother with it. Plus the phone app I use gives me a warning beep once I hit 45 so that I know I have hit my max speed as I don't like to push it much above that.

Fudge. I was afraid of that. And 7 mph off at only 47mph is quite a deviation. The question is- is the RED gear that Kascar sells, correct for the 3L80 and meant for the 37 inch tires?

I guess I'll have to call them unless someone here knows offhand.

 

[Mogman]

Dang things aren't very accurate to begin with, especially when you twist them around a couple times..
I am also using a GPS speedometer, it is 3 3/8 just like the original speedo, I put it in a pod (AutoMeter 5204)
on top of the dash, real easy to see and put a mechanical tach where the speedo was. It is Chinese, but so far is working very well and accurate, runs on 9-32V also gives an accurate COG!

 

[Coug]

Speedometer Calibration

Speedometer calibration made easy. Use our speedometer calibration calculator to compare how fast you are traveling and your speedometer reading.

tiresize.com

Input the tire sizes.
Going from a 36x12.5r16.5 to a 37x12.5r16.5
If the speedometer read correctly at 50 mph for the 36" tire, with the 37" tire it will read 51.39mph. At 100 it's 102.78 mph

In other words there is less than a 3% difference in speedometer reading, which is well within the margin of error.


Now, if you are changing other things as well, like the transfer case gearing, then you will probably need to change the speedometer gear, but not if it's JUST the wheels/tires.

 

[Ajax MD]

@Coug ah, ok!

I checked the speedo against a gps app today and it's spot-on. I'm not changing anything but the tires.

 

[Chief B]

@Coug ah, ok!

I checked the speedo against a gps app today and it's spot-on. I'm not changing anything but the tires.

I wouldn't even worry about the speedometer then. With such a minor change you probably will not even notice a difference if yours is correct. You can always change the tires out then compare it again with the GPS to make sure just to make you feel more comfortable.

 

[Retiredwarhorses]

The red gear is for the 242 Tcase

 

[cg25713]

Retiredwarhorses: I have a 242 t-case and a red speedo gear. I lubed my cable and all other parts look good. Speedo is running 5-6 mph fast against gps and road speed signs. Tire diameter is 34.5”. Should I try a 41 tooth gear? I have a M1152A1. Any help would be greatly appreciated. Thank you.

 

[TOBASH]

Why is your tire diameter only 34.5?

that could be your culprit.

 

[86humv]

Orange gear....218 T/C....35 teeth
Red gear..........242 T/C.....36 teeth
And NP2225 T/C has a 41 tooth gear.

 

[cg25713]

Thanks Doc: Good question. I measured it with a tape measure floor to top of tire is 34.5-35. Tires are standard 37x12.50R16.16.5LT Military OZ. Do you see any issues in these pics? How wide should they be? 37"? The truck is an 06' and came with 12 bolt wheels from IP. Now that I'm looking at it - you might be right. Whats Up? Please let me know.

 

[cg25713]

86humv: Thanks and thats what I thought and think what I have. 242 with red gear. Do the wheels and tires look small? Are these what originally came with the truck when new? Any guidance would be appreciated.

 

[cg25713]

URGENT UPDATE: Sorry for three in a row but this is important. Wheels and tires that came with my truck are not the ones pictured in my original IP listing. They were switched at some point and this seems the reason why I've been struggling with the speedo. Here is a link to my original listing https://www.govplanet.com/jsp/s/item/2809536?h=null&do=1 . Can you guys confirm. Thank you.

 

[REF]

with a 242 T case and 3:08 gears on STD tires you need a 41 tooth gear.
larger gear slows the speedometer down. this is the what I used on my M1123 to put the speedometer dead on with the GPS.

 

[Action]

Goodyear MT/R diameter is 36.5"
Goodyear MT diameter is 36.5"
BFG Baja T/A diameter is harder to find, but probably the same.

These are probably unloaded measurements. Mine is nowhere near this tall at 40 psi and about 85-90% tread left.

 

[cg25713]

What about 12 vs. 24 bolt wheels? Is there a preference? Same diameter I assume. My truck had 24 bolt wheels with bfgoodrich tires in the ip listing photos when I bought it but it came with 12 bolt tires and Goodyear tires. Also one of the tires on my truck was totally bald and I had to replace it.

 

[Action]

24 bolts are same size just handle more weight.
It is probably too late for you now. But I would have contacted GP about the tires. What you received is not what you bought. Somebady wanted nicer tires, so they took yours...

 

[TOBASH]

Your pics show standard 37" tires. Check the PSI is at proper level and I would not sweat this too much.

As far as the different tires... That is a violation of their "Iron Clad Policy", and you can threaten them with legal action if they don't make it right. Tires cost about $600 per tire.

 

[cg25713]

Thank you both. I'm very angry about this. U both know how I struggled to change the bad tire and the 24 bolt wheels are to accommodate the extra weight on my truck. Problem is it was picked up by a third party who then worked on it and got it started and roadworthy. I'm not sure who made the switch but I'm investigating. I will create a new thread when I have more information.