Do I need to re-gear?

[Ed Dell]

OK, so I have a 3.73 Axle ratio on my Jeep beach Gladiator which is based on the Willys and I only have put 285/70/17 bfg ko3 tires and a 1.5" Terraflex front stabilizing lift. Nothing in the back, no overlanding gear or nothing extra weighing the truck down.

Now My question is like at times, the truck seems to struggle to maintain speed, not so much on regular driving, but really noticeable with the Cruise Control on (like when it needs to catch back to the set speed it would go as low as 3rd or 4th gear like going really hard rpms) and at times it feels kinda sluggish at regular driving.

Now, I don't know if it is a normal thing with these trucks and engines, or if indeed is due to the tires, or maybe its the gear ratio?? Is anyone having a similar issue with the cruise control like I'm having or think its due to the Gear Ratio on my truck?

Any ideas or recommendations?

EDIT** SInce I forgot to mention it originally , I did change the tire size with the Jscan in the truck.

Also, I might be thinking of going into 35's next tire change (not soon tho) so that might also influence my decision into re-gearing down the road if it indeed affects performance.

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[Badunit]

Answers you will receive: Yes you definitely need to, no you definitely do not need to, and all answers between.

I regeared my Rubicon from 4.10 to 4.56. Either of those would be better than 3.73 in my opinion. 4.56 limits top cruising speed (before the cams go into high lift mode) to around 85 mph or, with a margin of error, more like 82mph. That, to me is the only drawback. It might have cost me 1 mpg also (when driving at 80mph) but I wasn't keeping that good of records. Otherwise I prefer it to 4.10 because it minimizes the problems you mentioned.

Edit:
I have the same size tires as you. Auto transmission. Manual transmission high gear is different and might affect what differential gearing someone prefers. Often it is not mentioned.

 

[Reddout99]

I would say no. You're only at a 33" tire there would be ne real need to regear. You will find people consider regearing when they get up in tire size, 35" & 37" are pretty typical tire size upgrades. I'm still running my stock gear set with 37" tires. Mine, like yours, is a daily driver and I have zero issues. Even took a long roadtrip from Nebraska to California and back using cruise control almost the whole time.

 

[ShrimpHappens]

I've been running 285/70R17 on my 3.73 gears for two or three years now, and I have no complaints. It holds 75mph on the interstate just fine. Did you use a programmer (JScan, Tazer, whatever) to tell your computer about the larger tire size?

 

[Hootbro]

Have had two JT's with 3.73 gears and also putting on 33's, kind of understand the issue. If you have not already reprogrammed for the increased tire size, you are leaving a lot not being utilized by the PCM programming to account for it. Even if you do have it programmed correctly, you can get by with decent performance but there is going to be more hunting and pecking between the gears.

I eventually found a set of 4.10 geared wide axles and swapped to them and feel it is better suited to 33's.

So to answer the question, you do not need to but you will probably eventually want to swap gears if you cannot find the happy medium of your current setup.

 

[Stan H]

I've been running 285/70R17 on my 3.73 gears for two or three years now, and I have no complaints. It holds 75mph on the interstate just fine. Did you use a programmer (JScan, Tazer, whatever) to tell your computer about the larger tire size?

This
Additionally you could go to 4.10 they are adequate on hardtop.

 

[Thunderspud]

If you have not already reprogrammed for the increased tire size, you are leaving a lot not being utilized by the PCM programming to account for it.

Not to derail the thread......but is this actually true? As in, does the tire size that's set in the PCM affect things OTHER than the MPG calculations? And if so, what are they? Logically, it seems like it would, but I am not sure that isn't just wishful thinking.

Now that I think about it...I seem to recall a thread about just this thing over on the Wrangler forums.....off to find that now. Carry on.

 

[Hootbro]

Not to derail the thread......but is this actually true? As in, does the tire size that's set in the PCM affect things OTHER than the MPG calculations? And if so, what are they? Logically, it seems like it would, but I am not sure that's just wishful thinking.

Now that I think about it...I seem to recall a thread about just thing thing over on the Wrangler forums.....off to find that now. Carry on.

Vehicle speed accuracy is required for the transmission shift points. It does make a difference. There is a measurement of the wheel speed sensor and the transmission Output Speed Sensor Signal. Accurate measurement of tire size is needed to control shift points.

One correction, tire circumference is actually stored in the BCM.

 

[Reddout99]

Not to derail the thread......but is this actually true? As in, does the tire size that's set in the PCM affect things OTHER than the MPG calculations? And if so, what are they? Logically, it seems like it would, but I am not sure that isn't just wishful thinking.

Now that I think about it...I seem to recall a thread about just this thing over on the Wrangler forums.....off to find that now. Carry on.

I believe by adjusting for actual tire size (I used Jscan) like I did when I went from 35s to 37s I feel the PCM readjusted the tranny shift points to compensate for the larger size. Runs and shifts fine on mine.

 

[Stan H]

Not to derail the thread......but is this actually true? As in, does the tire size that's set in the PCM affect things OTHER than the MPG calculations? And if so, what are they? Logically, it seems like it would, but I am not sure that isn't just wishful thinking.

Now that I think about it...I seem to recall a thread about just this thing over on the Wrangler forums.....off to find that now. Carry on.

It % does dude it affects shift points in the transmission it affects SPEDOMETER ALSO.
No offense where you been.

 

[Thunderspud]

Good to know. The current truck is of the manual variety, so those shift points are purely up to my discretion, lol. Probably why I never noticed it. I feel like in the previous two auto trucks, I never noticed much of a difference either way. I was definitely guilty of never adjusting for tire size changes.

FWIW, I still haven't found the forum post I initially referenced. I recall there being a full blown discussion of this on there though. I was thinking that maybe they had discussed whether things other than shift points were being affected is what I am saying.

 

[MPMB]

Since 33s are standard on Rubi and Mojave, with 4.10s, there's likely little difference with 33s and 3.73 gears.

Sidetrack: I wouldn't doubt the proper calibration of tire size would/could have an effect. Before I changed my tire size, I downloaded a GPS-speedometer to compare. I also used Waze to compare speeds between the three sources. The truck was off, the other two matched. Changed the tire size until speed matched. 36.5" or something like that. Comparing to the radar trailers, my speeds match those.

 

[Thunderspud]

Sidetrack: I wouldn't doubt the proper calibration of tire size would/could have an effect. Before I changed my tire size, I downloaded a GPS-speedometer to compare. I also used Waze to compare speeds between the three sources. The truck was off, the other two matched. Changed the tire size until speed matched. 36.5" or something like that. Comparing to the radar trailers, my speeds match those.

No doubt. Adjustments for speedo, MPG, and now shift points (as pointed out above) make total sense. I was just curious if someone had evidence that that change also affected more performance-oriented things that would affect how the truck feels to drive.

 

[Stan H]

No doubt. Adjustments for speedo, MPG, and now shift points (as pointed out above) make total sense. I was just curious if someone had evidence that that change also affected more performance-oriented things that would affect how the truck feels to drive.

It must also effect ABS ,ESC and traction control to some degree and I would reckon power band curve in the computer as well due to making it lug and crap like that.

 

[Panthers65]

on 285's I probably wouldn't, unless you are routinely pulling a heavy trailer.

Fine a set of Rubicon axles on the cheap, id snatch them up, but 3.73 to 4.10's is such a small jump you probably won't feel much

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