The Truth about Gears and Tires

[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!