What if.... I don't?

[MPMB]

Hey Team,
Time to swap tires. 2021, Sport S, Tow Package, 4.10 gearing, 2.5" RE lift. Have a friend that is swapping his 37's for 40's. 37's (KO2's) are still fresh and shiny. For the sake of argument, If I were to strap the hand me down 37's on mine and NOT regear right now, what are the ramifications/consequences in your mind?
This is a money now money later argument.
Thoughts? and Many Thanks,
JP

I'm on 37s w/ 4.10s and mpgs take a small hit. I can't say definitively how many because there were only about a week between putting on the lift and then the 37s, and I didn't drive it around much with it looking goofy - 33s with a ~3" lift.

I towed a small trailer (<2000lbs) from Indiana to Utah. Anything over 67-ish mph ended up tanking mpgs down to the 9-11mpg range. I tried to stay in the 12-15mpg range for most of the trip.

A recent trip to Glacier NP (about 800mi) and back I averaged about 15mpg with the trailer.

Jeeps aren't built for speed, so driving pavement I don't notice much, if any real difference. Maybe more shifting since it doesn't get into 7th and 8th as easily.

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[Reddout99]

Do you know: Does the tire size calibration keep the same max speed?

The calibration should adjust the speedometer based on the new tire size if that's what you are asking. When comparing my speedometer reading with a GPS app, I am within 2 mph at highway speeds, right on at lower in-town speeds

 

[froril]

I live at 6000 feet elevation had 37s and wheel sometimes up to 13500ish feet in elevation and it definitely needed more gear while wheeling than the stock “max-tow” 4.10 gears and the sport transfer case. The tires were rubbing the fenders at full flex pretty bad with a 3.5 in lift. With a Rubicon, I think I would’ve had enough gear in 4low. Also, I would have had enough Fender clearance. Driving i70 in Colorado during the summer heat the oil temps would get over 250 sometimes. Gas mileage and driving around town wasn't bad though.

I dropped back down to 35s. If I lived somewhere else or just mall-crawled, I would’ve just stuck with the 37s.

 

[Jteakus]

Get the 37's on a deal. Run them awhile and you will decide whether you need a regear OR 37's. My guess is you will love the tires and gather parts for the regear a little along then make the jump in 6 months or so.

 

[Zachanadandy]

Get the 37's on a deal. Run them awhile and you will decide whether you need a regear OR 37's. My guess is you will love the tires and gather parts for the regear a little along then make the jump in 6 months or so.

Unless you're installing the gears yourself, I wouldn't suggest gathering parts. Most shops won't warranty their gear install if you supply the parts for obvious reasons. Almost never will you save any money buying the gears and then finding the shop to do the work.

 

[Jteakus]

Good point.
Doing mine myself so I picked up gears, axles, install kits, diff covers and trusses a little along to spread out the expense. Just waiting for time to get it on the lift.

 

[Chasm]

The calibration should adjust the speedometer based on the new tire size if that's what you are asking. When comparing my speedometer reading with a GPS app, I am within 2 mph at highway speeds, right on at lower in-town speeds

Not exactly what I was asking, that part I knew. The part I'm wondering about is max speed. My current max speed is 105. If I go up to 35's, at the same wheel RPM, I'd be going ~115.
When I say it like that, it seems like the real question is: Is the governor based on speed or RPM?

Yes, I know, Jeeps aren't supposed to be fast. Consider it an academic question.
But yes, I've had mine over 100.

 

[Thunderspud]

I hit 90 on the interstate today with these 37" Firestone MT2's, and you probably couldn't pay me to do it again. All four corners screaming like a full-throttle 747 during takeoff.

Think my old-man ass will stick to rowing gears at twisty mountain road speeds from now on, thank you very much.

 

[Reddout99]

Can't help you there, bud. Never had mine near 100 or any where needing to hit the governor. Not sure what the top speed is on these.

 

[Zachanadandy]

Not exactly what I was asking, that part I knew. The part I'm wondering about is max speed. My current max speed is 105. If I go up to 35's, at the same wheel RPM, I'd be going ~115.
When I say it like that, it seems like the real question is: Is the governor based on speed or RPM?

Yes, I know, Jeeps aren't supposed to be fast. Consider it an academic question.
But yes, I've had mine over 100.

It's speed based, so if you didn't recalibrate you could actually attain a higher top speed. Not sure it would be worth the drawbacks of the shifting being off? Ours are both governed at 100mph and the mojave sees that regularly in the dirt.

 

[Chasm]

I hit 90 on the interstate today with these 37" Firestone MT2's, and you probably couldn't pay me to do it again. All four corners screaming like a full-throttle 747 during takeoff.

Think my old-man ass will stick to rowing gears at twisty mountain road speeds from now on, thank you very much.

Can't help you there, bud. Never had mine near 100 or any where needing to hit the governor. Not sure what the top speed is on these.

Most of the 2025's are limited to 95 (or is it 97?) but the Nighthawks get 105 because the larger rims mean less sidewall, so more rigid, so higher speed rating. If you tried to go 100 on 17" rims and 35" tires you'd probably be doing 115 because of how much the tires would expand, and you'd feel like you were floating!

 

[Chasm]

It's speed based, so if you didn't recalibrate you could actually attain a higher top speed. Not sure it would be worth the drawbacks of the shifting being off? Ours are both governed at 100mph and the mojave sees that regularly in the dirt.

Yeah, I wouldn't really consider going without the recalibration. Just interested in the details!

 

[Stan H]

Can't help you there, bud. Never had mine near 100 or any where needing to hit the governor. Not sure what the top speed is on these.

109 for diesels 110mph for Gas but also varied by year model ranging from 97mph to 110mph on model top speed was 102mph with 2020 and 2024 and 2025 models being the slowest.
https://www.carhp.com/jeep/gladiator/0to60

 

[bleda2002]

I hit 101 on 39 km3s once lol. Was like riding a bucking runaway wagon with how bad km3s are at balancing.

 

[Chasm]

109 for diesels 110mph for Gas but also varied by year model ranging from 97mph to 110mph on model top speed was 102mph with 2020 and 2024 and 2025 models being the slowest.
https://www.carhp.com/jeep/gladiator/0to60

you know what they say: always trust any info you find on the internet! (including my post!)

I was wrong. 2025 Nighthawk gets 110.

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