Jeep Enthusiast Forums banner

RPM's at Speeds on a Rubicon?

4K views 93 replies 15 participants last post by  RDBills 
#1 ·
Could someone who owns ice Rubicon TJ with an automatic that still has stock size tires and stock 4.10 gears please share with me how many RPMs you run at 70 mph or 60 mph or both.

My Rubicon already had larger than stock tires on it when I bought it so I never knew how many RPMs it ran with the stock tires at a certain speed but I need to know that because I'm going with even larger tires than what it had when I bought it and I'm considering a re-gear.

So I'm trying to get an idea of how much gears is needed to correct my RPMs for interstate travel with larger tires.

"CJ" Candice Jolene, "Dashie" 82 CJ-7 [Currently being rebuilt from ground up] and "Rubi-Dash" 04 Rubicon
 
#3 ·
Could someone who owns ice Rubicon TJ with an automatic that still has stock size tires and stock 4.10 gears please share with me how many RPMs you run at 70 mph or 60 mph or both.

My Rubicon already had larger than stock tires on it when I bought it so I never knew how many RPMs it ran with the stock tires at a certain speed but I need to know that because I'm going with even larger tires than what it had when I bought it and I'm considering a re-gear.

So I'm trying to get an idea of how much gears is needed to correct my RPMs for interstate travel with larger tires.
We can tell you what a good ratio would be if you'll tell us what size tires you'll be installing. We can also tell you ahead of time that the axle ratio your Rubicon will require with its automatic will be significantly lower (higher numerically) than you may be expecting. The 4-speed automatic transmission you have has a .69 Overdrive ratio which requires an unusually-low axle ratio to compensate for.
 
#4 ·
biffgnar said:
The grimmjeeper gear calculator will allow you to calculate any combination of RPMs, speed, crawl ratio, etc. Just plug OEM specs in there. With larger tires you will want to run higher RPMs than OEM also.
How does that work?
I know you can put in the tire size in gear ratio on those sites but wouldn't the transmissions final racial also make a difference? Like mine with an automatic four-speed versus someone with a three speed automatic versus someone else's that has a five speed versus someone else's that has a six speed?

"CJ" Candice Jolene, "Dashie" 82 CJ-7 [Currently being rebuilt from ground up] and "Rubi-Dash" 04 Rubicon
 
#6 ·
Jerry I'm running 33 inch tires that I don't plan to go any larger but if I do I will certainly never go large and then 35. With the 33's and all the armor and winch that I have on my jeep now it feels kind of sluggish at times on up hill grades and especially when I'm on the interstate even the slightest uphill grade it makes it a little bit difficult to hold the 70 and 75 miles an hour speed limit.

So I would like to compensate for that and get my ability to maintain interstate speed without having to turn off the overdrive which is too many rev's and of course that would also make it better off-road. But I don't want to go so low that I totally ruin my gas mileage completely. No one ever bought a jeep for excellent gas mileage but there's bad and then there's really bad.

I was originally thinking 4.88 gears but then I thought that as much as I drive on the interstate a LOT in places where the speed limit is 70 or 75 miles an hour that that might be a little too low and 4.55 gears might be a better option.

I was trying to find out what the Rubicon ran originally with that stock tires at 70 miles an hour, like you know, how many RPMs it run and see what your racial put me at least back that high and then maybe a little extra to make up for the added weight my jeep now has.

"CJ" Candice Jolene, "Dashie" 82 CJ-7 [Currently being rebuilt from ground up] and "Rubi-Dash" 04 Rubicon
 
#7 ·
Jerry Bransford said:
Go to the Grimjeeper gearing calculator site and you'll discover it takes your transmission's .69 final Overdrive gear ratio into its calculations. http://www.grimmjeeper.com/gears.html
I tried to load it earlier when he first posted it and it wouldn't load for me but I'll try it in my motel room tonight on my laptop. And thanks for tell me what my final Drive ratio was because I did not know. I've seen your calculation sheets before but I've never seen one that asked what your final drive ratio was so that would clearly be more helpful and hopefully it will load on my laptop since it's not loading on my iPad.

"CJ" Candice Jolene, "Dashie" 82 CJ-7 [Currently being rebuilt from ground up] and "Rubi-Dash" 04 Rubicon
 
#8 ·
Don't even CONSIDER 4.56 for 33's and your particular transmission. Seriously. If it were me I'd likely go 5.13. Go nothing less than 4.88..seriously.

My Rubicon is an absolute dog with 4.88 and 35's and its automatic. I'm constantly having to shut the Overdrive off to stop the engine from lugging on the highway. I'm hoping to regear to 5.38 at some point. You'd have the exact same issue I'm having if you installed 4.56 with 33's. It'd be a dog on the highway like mine is.
 
#9 ·
Part of what prompted my decision that it may be time to upgrade my gears is that I recently took A several week work trip to California and while I was out there two of my tires got damaged. I ended up buying a new set while I was there before coming home and even though the new tires are only 0.15 inches taller than my original tires there a much heavier tire. Even though I could and have put even more weight in my jeep in the luggage then the combined weight of the new tires something about that weight spinning on the tires just feels like it's eating up my power I noticed it immediately while I was out there and then the return trip home across I 40 was really making my jeep struggle with every hill I kept having to turn my overdrive off or it would downshift a lot or both.

"CJ" Candice Jolene, "Dashie" 82 CJ-7 [Currently being rebuilt from ground up] and "Rubi-Dash" 04 Rubicon
 
#10 ·
Jerry Bransford said:
Don't even CONSIDER 4.56 for 33's and your particular transmission. Seriously. If it were me I'd likely go 5.13. Go nothing less than 4.88..seriously. My Rubicon is an absolute dog with 4.88 and 35's, I'm constantly having to shut the Overdrive off to stop the engine from lugging on the highway. I'm hoping to regear to 5.38 at some point. You'd have the exact same issue I'm having if you installed 4.56 with 33's. It'd be a dog on the highway like mine is.
Really? I'm surprised, because mine wasn't a dog with 410s until I recently installed these new heavier tires that seem to just be sapping my power. But even so I don't think I would go so far as to call mine a dog even with 4.10 so I thought 4.55 would be a noticeable improvement. But yes I was originally leaning towards the 4.88 in the first place but then I became a little scared that that would be too much. If I do this I'm looking at upgrading my axles as well, just not a simple read your on my axles but actually putting in a pair of rock jock drop in axles that are already set up and ready to install with ARB lockers so that's a very expensive mistake and I just want to make certain that I get it right with how much I have to drive on the interstate.

May I ask how many RPMs you're running on flatland with 4.88 and 35s in your Rubicon? Is yours also an automatic?

"CJ" Candice Jolene, "Dashie" 82 CJ-7 [Currently being rebuilt from ground up] and "Rubi-Dash" 04 Rubicon
 
#11 ·
I'm back in my motel room now and I did get that Grim Reaper page to load on my laptop but I have no idea like no clue what all the stuff is it wants me to enter I can't even find a button that says to calculator to perform the calculation.

I'm completely lost on this website, is there one that's a little simpler to understand?

"CJ" Candice Jolene, "Dashie" 82 CJ-7 [Currently being rebuilt from ground up] and "Rubi-Dash" 04 Rubicon
 
#12 ·
OK I'm still kind of throwing that is telling me I'll be doing 103 miles an hour at 2000 RPM which is clearly not right.

But after some tinkering I did find one field that tells me that if I put in 488 gears I would be turning 2412 rpm's at 70 miles an hour, does that sound about right?

"CJ" Candice Jolene, "Dashie" 82 CJ-7 [Currently being rebuilt from ground up] and "Rubi-Dash" 04 Rubicon
 
#13 ·
What's the final drive ratio of the Rubicon automatic transmission when the overdrive is turned off?

You're someone knows that and I wanted to compare that on this chart, I found one part of this whole webpage that kind of makes it since it's the bottom most shark on the left side.

"CJ" Candice Jolene, "Dashie" 82 CJ-7 [Currently being rebuilt from ground up] and "Rubi-Dash" 04 Rubicon
 
#15 ·
You only need to enter 3 things. Click on where it says 'enter your own numbers' and scroll down to Dodge 42RLE. Then put in axle ratio, and tire size. It calculates automatically.

Do the same thing on the right side but change the axle ratio to easily compare numbers. Also, you can change the speeds to what you think you will be driving the most.
 
#16 ·
Your RPM may be slightly different than the charts. Not all tires are exactly as marked. I've seen new 33"s that were closer to 32"s. Also, tire wear can affect your actual numbers. Might be an inch difference between new and worn.
 
#17 ·
Jerry Bransford said:
It is 1:1 in 3rd gear when not in Overdrive. My engine is turning over something like 2100 at 70 mph with 35's and 4.88 with the same automatic transmission you have.
2000-2100 it is about what I'm running now with the 410s and 33's at 70 until I turn off the overdrive in which case it jumps up to about 3K

"CJ" Candice Jolene, "Dashie" 82 CJ-7 [Currently being rebuilt from ground up] and "Rubi-Dash" 04 Rubicon
 
#18 ·
1longcj said:
Your RPM may be slightly different than the charts. Not all tires are exactly as marked. I've seen new 33"s that were closer to 32"s. Also, tire wear can affect your actual numbers. Might be an inch difference between new and worn.
my new P285/65 tires according to the manufacturer are really 33.15 inches and right now they're brand new. My old tires or P285/75's and their manufacturer said they were 33.10 but they already had 30,000 miles on them when I get the road hazard on the highway that I couldn't avoid and ruined two of then.

"CJ" Candice Jolene, "Dashie" 82 CJ-7 [Currently being rebuilt from ground up] and "Rubi-Dash" 04 Rubicon
 
#19 ·
Oh see I was trying to find jeep listed under transmissions, I was looking under Dodge.

I did find the NVG to 4104 transfer case listed though, that's correct, right?

"CJ" Candice Jolene, "Dashie" 82 CJ-7 [Currently being rebuilt from ground up] and "Rubi-Dash" 04 Rubicon
 
#23 ·
:brickwall :brickwall :brickwall

See that? That's me. I've explained this so many times it should be a sticky that you click before you can ever post a gear ratio thread or read one.

The gear calculator is only as accurate as what you put in. It does not matter what the letters and numbers on the side of the tire say. The only thing that matters is the rolling radius and that is calculated with a tape measure by measuring from the center of the axle or wheel hub to the ground and multiplying by 2 for a number you can use in the calculator. If you want to use the calculator to find the tire size, go drive a known speed in a given gear and note the RPM. Move the RPM to a number that is easy to define like 2500 etc. and then note the miles per hour.

When you have those numbers, input them into the calculator and keep changing the tire size until the numbers match what you got in the Jeep. That will be your accurate tire size and it has nothing to do with what is on the sidewall.
 
#22 ·
Thanks.

What is a good RPM to run on the interstate at 70 miles an hour so you don't have to turn off the overdrive every time you come to every little incline but is still very drivable?

It looks from this like 488 would be about perfect for 33 inch tires, but just in the house admittedly small chance that I ever decide to go to 35s I was also playing with the 513 ratio. But when I go to the 513 ratio I noticed that the third gear in the event that my transmission kickdown and overdrive for Hill is going to have it screaming or at least it looks like it would be, because you know it's still going to downshift sometimes on hills.

"CJ" Candice Jolene, "Dashie" 82 CJ-7 [Currently being rebuilt from ground up] and "Rubi-Dash" 04 Rubicon
 
#24 ·
It looks from this like 488 would be about perfect for 33 inch tires, but just in the house admittedly small chance that I ever decide to go to 35s I was also playing with the 513 ratio. But when I go to the 513 ratio I noticed that the third gear in the event that my transmission kickdown and overdrive for Hill is going to have it screaming or at least it looks like it would be, because you know it's still going to downshift sometimes on hills.
You don't normally ever hear someone complain about being overgeared. And 'screaming' is relative; the tach. goes to 6K.
 
#25 ·
The only thing that matters is the rolling radius and that is calculated with a tape measure by measuring from the center of the axle or wheel hub to the ground and multiplying by 2 for a number you can use in the calculator.
Does the (smaller) rolling radius x2, vs. total tire diameter, actually give you a more accurate calculation? Ultimately, it's the derived circumference that is used in the gear ratio calculator.
 
#27 ·
We can't even learn the basics and you want to do it the hard way? Yes, it is the RPM or revolutions per mile that are ultimately the most accurate. Since it is difficult to find that info for each tire at a given pressure and load, we have to do it another way that is just about as accurate and far easier to determine. If you measure your tire and use the rolling radius to find the diameter or use the calculator with the known quantities to determine the tire size, it will be at least as accurate as the odometer and speedometer.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top