Okay, another regear question. Hear me out!

[Len]

Okay so, I re-geared my JTR to 5.13’s and I have 37” tires. It runs great but, I’ve noticed that the shifting points seem different, not in a bad way, just different. Don’t know how to really explain it. It almost feels like it holds the gear like a half second too long.
So other day I wanted to see what would happen if I changed the gear ratio in the computer to 5.38’s. Not sure what compelled me to do this but I did. I drove it for about 10 miles with a mix of highway and city streets. I was completely floored, it felt smoother and just a bit punchier.
After I drove the 10 miles I reset the computer again to 5.13 ratio. Once I switched it just seemed to not shift as smooth and it felt like I lost a little bit off the line and in the passing gear.
So, my question is, would I cause any damage running. 5.13’s with the ratio set to 5.38’s? Or is there something else going on that I should be concerned with?

Sponsored

 

[Escape.idiocracy]

Interesting…..

And then play with the tire size to calibrate the speedo to correct the speed? ? or would that wash everything?

 

[Len]

Interesting…..

And then play with the tire size to calibrate the speedo to correct the speed? ? or would that wash everything?

The speedo still seems to be accurate with it set to 36.74. As that is what my 37” tires measure. I really want to leave it set for the 5.38 gear ratio but I don’t want to damage anything by doing that. But yeah, interesting right!

 

[Stan H]

What would it do or have you tried setting tire size to 37 and then gear ratio to 5.13 ?

 

[AmishMike]

If it shifts better, then go with it.
Interesting fact about A/T's, a solid shift is better than a smooth shift for the transmission. The smooth shift is typically a result of two transmission bands (gears) overlapping between shifts, a solid shift signifies that one gear released and the next engaged. The smooth shifting creates extra wear over time and definitely causes heat.
We used to call them stage I, II, III; stage I was stiffer shifts, II was stiffer shifts + manual shifting, III was manual shift only. Stage I was a great towing upgrade as slippage between gears under load is bad.

 

[toddarama]

I re-geared my JTR to 5.13’s

Sorry - not trying to be a smart-a$$ here, but did you do the regear yourself? and/or did you verify that the gear set was 5.13 and NOT 5.38? I am guessing that you've already ruled out any possibility of confusion over the actual gearset ... but given your explanation of the symptoms you are experience in I just have to ask.

 

[KevinC]

I'm curious what 37" tire you are running that nets you 36.74" on your truck under load?

 

[Len]

Sorry - not trying to be a smart-a$$ here, but did you do the regear yourself? and/or did you verify that the gear set was 5.13 and NOT 5.38? I am guessing that you've already ruled out any possibility of confusion over the actual gearset ... but given your explanation of the symptoms you are experience in I just have to ask.

No , your good ! I went to a Jeep mechanic that came highly recommend. I called him and asked if he did install 5.13 vice 5.38. He was very adamant about installing 5.13’s.
im going to reset my computer to 5.13 just to be safe. The difference really isn’t much if I’m being honest, it just feels a little better.
I grew up working on cars with my dad but I knew doing gears is out of my wheel house and wanted to be sure it was done correctly.

 

[Len]

I'm curious what 37" tire you are running that nets you 36.74" on your truck under load?

I’m running MT Baja Boss AT. If you go to there website it gives you the actual tire size vice the printed 37. Actual size is 36.7. But the tazer only lets me set it to 36.74.

 

[Len]

Did you measure the tire while sitting on the ground under load? Measure from the ground to center of axle then x2. The rolling diameter is a lot different than what the manufacturer states.

I will. Check that out as compared to what Mickey Thompson website says actual tire size is. I went with what they determine “actual tires size”. Which is 36.7.

 

[bleda2002]

Did you measure the tire while sitting on the ground under load? Measure from the ground to center of axle then x2. The rolling diameter is a lot different than what the manufacturer states.

Height from the ground has no real bearing on rolling diameter other than the loose connection of it being semi-corcular. If you air down to 20 or air up to 40 the rolling diameter stays the same as the perimeter isn't changing just how oval the tire is due to pressure. Best bet is to measure the actual circumference of the tire and divide by pie to get the "height" to enter.

 

[bleda2002]

I disagree

That's fine, you can, but mathematics disagrees with you. The air pressure will stretch the tire a bit more as it increases but squish is a product of weight and softness of the compound, the outside of the tire still has to make a full revolution every time it goes around (unless severely deflated) so it will still cover basically the same distance.

Basically a tire that is 36.7 inches unladen has about a 116 inch circumference, mounted on the truck and squished it's still 116 inch circumference if not a little more, not a little less.

 

[Lunentucker]

I did mine ground to center x2 and my speedometer matches Waze and a GPS app on my phone perfectly. Pie at Christmas was good though!

 

[bleda2002]

I did mine ground to center x2 and my speedometer matches Waze and a GPS app on my phone perfectly. Pie at Christmas was good though!

How far off is ground to center vs the manufacturers actual circumference? Ground to center should get you close enough for government work. Unless you are off by an inch it should be good enough but that doesn't mean it's actually dead on. Drop your pressure to 15 so that it's an inch or 2 lower, the speedo will still be right. This also becomes less and less pronounced as the tire gets bigger since half an inch makes less overall difference.

On my 39 km3s (actually circumference about 38.5, ground to height upfront is 37.6 at 28 psi) if I use the ground to height instead of the actual circumference I end up off by 1 mph at 80. Again not a huge deal but the whole idea that the ground to height is more accurate than measuring the actual rolling circumference of a tire is crazy.

 

[bleda2002]

That doesn’t make any sense. The closer the axle is to the ground, the faster the axle will spin.
It’s mathmagical

Huh? The axle being close or far from the ground makes no difference to how fast it spins. The outer circumference of the object is how much distance it covers in 1 revolution and that determines how fast it spins for a given speed. A big oval and a little oval can both spin at 1000 rpms, but the big oval will cover more ground than the little oval over those 1000 rpms and that's all mph is. How much ground is covered by the oval.

Your assumption is the oval's circumference shrinks due to the squish, in reality it stays the same it just becomes more oblong.

Edit: relatively the same I should say, as in with in a few 32nds of an inch or less.

Let's put it another way, I can't air down a 39 to be the same height as a 37 and expect it to spin at the same rpm for a given speed, the 39's oval circumference is just bigger.

Sponsored