The Truth about Gears and Tires

[Timmy]

With 4.88s and 37s, I struggled to maintain 75mph.

Hey Ross, you've mentioned this a few times now and I was curious... Why do you feel it struggled to maintain 75mph with 4.88's? I'm guessing at 75mph you were around 2900 RPM in sixth gear? (based on that tool I published and punching in your measured tire size.) I'm very surprised that at 2900 RPM it was struggling at 75 Mph? I actually would have thought the exact opposite would happen with the 4.56 gears? 4.56 Gears appears to be hitting 75 Mph at 2700 RPM so I would have thought it would be more of a struggle say if you hit an uphill section? I mean, technically, isn't this like saying you couldn't hit 75Mph by downshifting to 5th because the RPM's went to 3200?

Not doubting your personal experience as I fully trust you when you say 4.88's were not as good as 4.56's on the highway, but I'm wondering if you could elaborate more on why you feel the 4.88's struggled so much?

[JeepLab]

Hey Ross, you've mentioned this a few times now and I was curious... Why do you feel it struggled to maintain 75mph with 4.88's? I'm guessing at 75mph you were around 2900 RPM in sixth gear? (based on that tool I published and punching in your measured tire size.) I'm very surprised that at 2900 RPM it was struggling at 75 Mph? I actually would have thought the exact opposite would happen with the 4.56 gears? 4.56 Gears appears to be hitting 75 Mph at 2700 RPM so I would have thought it would be more of a struggle say if you hit an uphill section? I mean, technically, isn't this like saying you couldn't hit 75Mph by downshifting to 5th because the RPM's went to 3200?

Not doubting your personal experience as I fully trust you when you say 4.88's were not as good as 4.56's on the highway, but I'm wondering if you could elaborate more on why you feel the 4.88's struggled so much?

When i say "struggled" thats probably not the best word. The Jeep was fine getting to those speeds, but the pressure i had to hold on the pedal to get there was uncomfortable, keeping the truck at 2600 rpm is not her sweet spot for cruising. So if i wanted to do 75 for a long distance with 4.88s, I had to really hold the pedal down farther than normal. She didnt comfortably cruise at 75. Or 70 for that matter.

2900 rpm to do 75mph is a LOT of rpm to me. I dont think the truck wants to spin at that level for an extended trip either. Or maybe i dont like to hear the engine working harder than it should in a cruising situation.

With 4.10, I would cruise at 75 at about 2k rpm. She was a road trip killer. Now she isnt quite the same cruiser she was, but the overall acceleration is better.

Now I cruise at 70 at around 2200 rpm. Or somewhere around there. I thought 4.88 was unlivable. Could i get to high speed? yes. Did I have to flog BB to get there? also yes. In NJ people do 80 mph on every highway. If your not up there, you are a slug.

[dalearyous]

but the pressure i had to hold on the pedal to get there was uncomfortable, keeping the truck at 2600 rpm is not her sweet spot for cruising. So if i wanted to do 75 for a long distance with 4.88s, I had to really hold the pedal down farther than normal. She didnt comfortably cruise at 75. Or 70 for that matter.

2900 rpm to do 75mph is a LOT of rpm to me. I dont think the truck wants to spin at that level for an extended trip either. Or maybe i dont like to hear the engine working harder than it should in a cruising situation.

With 4.10, I would cruise at 75 at about 2k rpm. She was a road trip killer. Now she isnt quite the same cruiser she was, but the overall acceleration is better.

also yes. In NJ people do 80 mph on every highway. If your not up there, you are a slug.

couple things here. holding the pedal down would imply you require more throttle and maybe the jeep wasn't in its optimal power band (HP)? glancing at the pentastar chart this would appear to be true and you were asking a lot from it at that RPM. i dunno, spit balling here.

not wanting to hear the engine working harder than it should? isn't that what a supercharger basically does? i also don't see how you could cruise at 75 mph at 2k rpms. my jeep stock had trouble maintaining 75 because in 6th gear because the RPMs were so low.

now yes, in NJ, you better be driving fast to survive on the highways and i bet it sucks.

anyway a lot of this is circumstantial and i have to agree with snarf, drive what feels best. clearly with your setup 4.56 drives best. it would be very hard to duplicate that as i don't know anyone in person that has a turbo or super charger. and maybe you can get away with 4.56 on 37s BECAUSE of the super charger ... or if you have a free RIPP supercharger i would be happy to be a guinea pig

[JeepLab]

anyway a lot of this is circumstantial and i have to agree with snarf, drive what feels best. clearly with your setup 4.56 drives best. it would be very hard to duplicate that as i don't know anyone in person that has a turbo or super charger. and maybe you can get away with 4.56 on 37s BECAUSE of the super charger ... or if you have a free RIPP supercharger i would be happy to be a guinea pig

"Drive what feels best" is no answer at all. Anyone can agree with that. The driver's goals will dictate what feels best, and with gears you rarely know how its going to feel until you've spent the money and ripped your diffs apart.

Im not against 5.13, please dont take it that way, Tonka, our JK on 40s has 5.13 and no SC and she feels fast. there are different opinions with regard to this related to driving style and location. In NJ you want to be able to drive fast as well as wheel. I also have a 4 to 1 transfer case that gives a little more freedom to run a little less gear.

the best information is for those of us who have actually done it to share with others what our trucks do mathematically now. Then others can get in their trucks and run their RPM up to where we say ours go to maintain highway speed, and see if thats livable for them.

[Timmy]

When i say "struggled" thats probably not the best word. The Jeep was fine getting to those speeds, but the pressure i had to hold on the pedal to get there was uncomfortable

That makes more sense, thanks for elaborating on that. And I do totally understand what you are talking about. I was driving down the road this morning at 40mph, with my 3.73 gears, and I had shifted into 5th gear and I was at 1400rpm just to save some gas. The 3.6L has enough power that it could not only maintain 40mph, but I could actually accelerate slightly if I wanted to (and I do mean slightly.) If I needed a hair more power, I downshifted to 4th which brought me to 1600-1700rpm and I could accelerate without much issue. Maybe that's why I'm getting 16.5mpg in my city driving on my 37" tires (and yes, my speedo is calibrated) versus a lot of other guys only getting 13mpg.

As I was driving, I was thinking about our gearing conversations and how if I was at 75mph on the highway in 6th gear, I would be pissed off that I was at 2700 RPM when clearly the 3.6L can cruise at a much lower RPM. I remember when I had my 35" tires on and I would drop into 6th gear all the time on flat surfaces and cruise at 1700-1800 RPM (55mph-60mph) very happily. 6th gear is suppose to be an "overdrive" gear, not a passing gear, right? I think that's why I see so much conflicting information on the web and when I talk with people. It's like people want 6th gear to be able to accelerate up a descent hill on the highway. Maybe in a little 4 door sedan that has a low drag coefficient you could do that, but in a mobile brick with 37" tires that just doesn't seem a good use of the power band.

I've gone back and forth on the 4.56 and 4.88 decision. One day I think I want to try the 4.88's, then, like today, I'm thinking I'd be dumb to do that when like you and I want to see my RPM's spinning lower as it is quieter, smoother and more gas friendly. The reality is, on 35" tires, my 3.73 gears were *almost* fine for me. I was mostly happy, wishing I had just a hair more low-end. I probably would have been happiest at 4.10 gearing with those tires. When I calculate out what 4.56 gearing looks like with my 37" tires, it is actually just a hair more aggressive than what 4.10 gearing would have been with my 35" tires. The one thing I'm worried about with the 4.88 gearing is first and second gear. I'm not worried that they would have enough power, but actually that they have too much power and you have to shift out of them too quickly. I really don't want to pull a left hand turn from a stop light having to shift not only through 2nd gear, but possibly all the way to 3rd gear at the exit of the turn. I watched a video of a guy on-line that showed the difference between his 3.73 gears on 35" tires versus 4.88 gears on 35" tires. It was his first drive after getting 4.88 gearing. It was interesting to hear his surprise at how quickly he had jumped up to 4th gear. It was one of those "oh crap, I didn't expect that, hope I didn't make a mistake" type of reactions.

I think for me, what I'm looking for is that gearing that almost let's me start from a dead stop in 2nd gear. I say almost because I don't want it to be a guarantee in all circumstances, just your usual flat grade, 1/2 second stop with a slow pull out. To me, that would be just about perfect. What I'm basing that on is driving my other vehicles. I've noticed that my other vehicles (all auto's) are calibrated not only in their gearing but also in their programming to actually have you launch in 2nd gear from a stop if the conditions are right. 1st gear is used if it is a full-on stop where you actually sit there for more than 1 second and your previous driving style coming up to the stop sign was more aggressive (because remember, gear programming these days is now adaptive and it knows when you were racing around versus driving like a grandpa.)

i also don't see how you could cruise at 75 mph at 2k rpms. my jeep stock had trouble maintaining 75 because in 6th gear because the RPMs were so low.

I'm not sure about your experience, but as I said above, I was maintaining 40mph at 1400rpm. I used to cruise at 70mph at 2000rpm all the time and it maintained it just fine. The 3.6L has a lot more torque than people give it credit for. I get tired of reading people say all the time that the engine is useless below 1800-1900rpm. That is just crazy! I can accelerate at high speed from 1500rpm!