The Truth about Gears and Tires

[dalearyous]

should have clarified: ok yeah obviously you can't adjust the tach, i meant the ratio of RPMs to speed. anyway, to clarify the resistance: i was referring to the amount of RPMs need to sustain a speed with the cruise on. your vehicle is obviously going to work harder if you are driving straight into a strong wind and so you get that slight fluctuation (in speed and RPMs). all i was referring to.

i don't like trusting a GPS for my speed. my OBCD dongle has an app that allowed me to basically drive one mile at a constant speed (again as best i could) with a timer and then i did the math. i always found GPS to be 1-2 MPH slower than actual speed.

[Timmy]

should have clarified: ok yeah obviously you can't adjust the tach, i meant the ratio of RPMs to speed. anyway, to clarify the resistance: i was referring to the amount of RPMs need to sustain a speed with the cruise on. your vehicle is obviously going to work harder if you are driving straight into a strong wind and so you get that slight fluctuation (in speed and RPMs). all i was referring to.

i don't like trusting a GPS for my speed. my OBCD dongle has an app that allowed me to basically drive one mile at a constant speed (again as best i could) with a timer and then i did the math. i always found GPS to be 1-2 MPH slower than actual speed.

Ahh, got it.

I agree with you, some GPS can be a little off. Of course, if the GPS is consistent, that's what matters most as that is something you can work with (either consistently above or below.) I have found when I calibrate to GPS and then I cruise past one of those stationary pole mounted radar guns that displays your MPH, that my MPH displayed in the dash matches pretty close to what the radar gun shows on the pole, so I think the GPS is close enough. Also, it's usually good to be a little high on the speedo as it helps save you a ticket. That's why most German cars have a built in 3% margin to the high side ;-)

[KaiserBill]

should have clarified: ok yeah obviously you can't adjust the tach, i meant the ratio of RPMs to speed. anyway, to clarify the resistance: i was referring to the amount of RPMs need to sustain a speed with the cruise on. your vehicle is obviously going to work harder if you are driving straight into a strong wind and so you get that slight fluctuation (in speed and RPMs). all i was referring to.

i don't like trusting a GPS for my speed. my OBCD dongle has an app that allowed me to basically drive one mile at a constant speed (again as best i could) with a timer and then i did the math. i always found GPS to be 1-2 MPH slower than actual speed.

GPS systems have a percentage of error like anything else.

[JeepLab]

i guess i should have said inexperienced instead of ignorant. i only know what i have and i have not driven anything else other than stock rubicons and stock sports.

i have the superchips flashpaq 3872 and have tried to dial in the tach and speedometer as close as possible. i have done the unscientific verification with my wife following me as well as tried to measure distance. basically, at 70 mph im running rite around 2700 rpms. obviously resistance will change that a bit. wind against me its a little higher, wind behind me its a little less. 75mph runs around 2800 ish. 80mph is like right at the 3000 rpm mark. i know the RPMs seem high but the jeep seems to be perfectly fine at those RPMs. like i said, i don't feel like i am taxing it more than lets say 2700 RPMs. i think with 37s it will be perfect which is what i was going for. i am in virginia where the highways are 70 mph speed limit so i go 75 and happy to cruise there.

now if i had your supercharger i would probably be singing a different tune. but most people don't have a super charger so they post about what they have, HOPEFULLY. but yes to your original post, i would completely throw away someone's post about gears when their signature states they are stock or running stock gears on lift and bigger tires. you just don't know until you do it and i think that is the fear and why there are so many threads about it.

btw, snarf and i have had a LOT of conversations on the wrangler forum. seems like a solid guy.

Snarf is more technical than I am. where Im inches, he is millimeters. However, My brain cannot comprehend you having 5.13, AND 35s (which are really less) and still having a comfortable highway cruise. Something is off somewhere. With 4.88s and 37s, I struggled to maintain 75mph.

Thats why I asked if your tach and odometer were calibrated correctly.

I drove for a loooong time with them out of wack and never knew it. Till i tested with a 2 hour drive.

[dalearyous]

snarf helped me with my metalcloak bumpers and suspension when i was making my decision.

i can take a video if you would like. i was thinking about doing one anyway because of all the questions i have gotten with 5.13s.

"comfortable highway cruise" - whats not comfortable to you? we could have a very different frame of reference, what are you comparing it to?

oh also how does it compare to snarf's experience? he went with 4.88s

*edit*

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

the way my local shop explained it to me is that they told me to ignore the garbage on the internet and just go with what it feels like (these guys have been 4x4 racing for 15 years). they said 4.88s with 37s (what i will have eventually) is going to feel like stock rubicon with 4.10s and 31s or whatever is on them. if you want to get back to stock feel with 37s, you go 4.88s. if you want something that feels better, go with 5.13s and 37s. this was tough for me, but he laughed and said let your engineer/comp science brain go for a minute and just trust me. if you want it to feel better than stock on 37s, go 5.13s. so i did on 35s for now.

again, more horsepower changes a lot so i have no idea what a turbo or super charger changes but going with "the feel" feels good right now. i would probably never go 5.13s if i had an LS for example.

[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?

[Snarf77]

My 2 Door with boost will maintain 90 with no problems...not that its a good idea. 70 was a non issue with 37 KM2s and 4.88s. Like Dalearyous said, its probably good advice to go with what feels right. Maybe drive someone else's vehicle with the gearing desired to see if it has the right feel? Might take some time but still, you could potentially avoid a mistake.

[JeepLab]

My 2 Door with boost will maintain 90 with no problems...not that its a good idea. 70 was a non issue with 37 KM2s and 4.88s. Like Dalearyous said, its probably good advice to go with what feels right. Maybe drive someone else's vehicle with the gearing desired to see if it has the right feel? Might take some time but still, you could potentially avoid a mistake.

maintain 90 at what RPM? 3500?

[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