A lot has to do with expectations.

Its a truck, it drives like a truck.

I have the 255/75/17's, essentially 32's...and I got the 3.73 gearing on a manual tranny.

I went through the gears to get a feel for the rpm ranges, and jotted down a few numbers:

1st gear at 4 k rpm = 21 mph

2nd gear at 4 k rpm = 38 mph

3rd gear at 4.1 K rpm = 59 mph

4th gear at 3 K rpm = 59 mph

5th gear at 2.4 K rpm = 59 mph

6th gear at 2 k rpm = 59 mph

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A quick look at the revolutions per mile for a few tire sizes:

my 32's = 660 rpm

33's = 630 rpm

35's = 600 rpm

37's = 570 rpm

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If you do the math, lets say from 6th gear at 2k rpm cruising at 59 mph (To stay cruising at 59 mph)...the rpm would drop by tire size like so:


At 59 mph, in 6th, compared to stock:

32's = No Drop = Stock = 2,000 rpm at 59 mph

33's = 91 rpm drop to 1,909 rpm at 59 mph

35's = 182 rpm drop to 1,818 rpm at 59 mph

37's = 272 rpm drop to 1,728 rpm at 59 mph

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Dropping to 5th gear from 6th, stock, raised the 2,000 rpm by 400 rpm to 2,400 rpm

IE: Going from 32's to 37's was not enough of a difference to even span one gear range.

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Essentially, going from 32's to 37's Raised the gearing/Dropped the rpm by 13.6 %/in any given gear across the range.

35's came out to a 9.1% change.

33's came out to a 4.5% change.

32's come out to a zero percent change, etc.


So, if I were to do the same math for any of the other speed/rpm combo's above...I could see what my rpm drop for each tire size might be at any given combo.



An obvious solution to gearing issues...downshift if you want to raise your rpm.

Your final drive ratio is the same, as your tires are actually PART of your REAL final drive ratio....

So, it makes no difference if you are going 2,000 rpm at 59 mph in 6th, 5th, or whatever...its still 59 mph at 2,000 rpm as far as the Jeep knows.



The rolling resistance, the taller wind profile, and so forth make more of a difference than your gearing when you get down to it.

So, sure, lower gearing DOES improve power/raise the rpm...but, to me at least, its just not enough of a difference to spend that kind of money re-gearing to get 13.9 % of my rpm back.......when I can downshift for free...........for others....its a priority.

 

OK....... now my head hurts......

 

Essentially, I am running the same set-up (officially 34.36" tires if the tire conversion calculator is correct), and have only done some trail driving (limited occasions in 4-wheel low), and I don't seem to have much issue with power/torque/speed. Although, gas mileage pretty much sucks (and I have many of the bolt-on mods in an attempt to bring it back to respectable --> but I think I'm failing --> but it sounds mean!)

 

I have 3.21's and 33.4' tires, the only time I have a problem is highway and passing (I just shift down lower) or climbing in the mountains (again I just down shift).

I have been offroad and not had a problem in 4 hi or 4 lo, now I don't do blacks or reds but no problems with blues. If it were an auto, I would hate life.

I will eventualy regear to 4.1's but I am in no hurry. It is not much more of a dog then when I got it. When I bought it I new it wasn't a race car.

 

One thing I see over and over again when people talk about a loss in mpg when they get larger tires:

They forget to recalculate their speed, and their miles.

IE:

1) They have 35's, and the speedometer is reading 9.1% low....so, they are cruising along at 65 mph like they always do, but they are really going 71 mph, which uses a lot more gas in a brick shaped projectile.

So - They are driving faster than they think they are...and they lose a few mpg.


2) They hit the gas station, and see that they only went 240 miles instead of 300 miles between fill-ups, and think, WOW, those big tires really KILLED my gas mileage!



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So - if you want to really know your mpg....and compare it to your "Before" mpg....

Say for those 35's -

...remember that you have to drive about 10% slower than what the speedometer says you're going (Makes the math easy that way) to get down to the real speed you wanted.

So - if you WANT to drive at 65 mph - your SPEEDOMETER needs to be reading about 59 mph.

If you want to go 60 mph, your speedometer needs to read about 54 mph, etc.

That will correct for using more gas to go too fast. (BUT - IS very handy for pointing to the wife, etc, and saying, "What, no, look, I'm not speeding...")


and - when you get to the pump and hit the trip odometer to see how many miles you drove, add about 10% to the number of miles driven as well.

So - if it says you went 270 miles, add 27 miles to that, and write down that you went 297 miles.

(The 10% works well because when ADDING to the artificially REDUCED number, it takes an increase of about 10% to be equal to the SUBTRACTION of the 9.1% from the LARGER REAL number)




IE:

If the odometer reads 270 miles traveled, but its off by 9.1%, and you add 9.1% to 270, you get 294.6 miles.

If you really travelled 297 miles, and subtract 9.1%, you get 270 miles.

If you add 10% to the 270 miles instead of 9.1%, you get 297, the true number.



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Summary -

Don't make the mistake of driving faster than you thought, and going further than you thought, and then assuming the tires killed your mpg (As badly).

 

I think i understand...The bottom line is that we didnt get into jeeps to pass people on the highway...or to get better than 20 mpg's...we got into jeeps to get away from it all deep in the woods. The math lesson above was very eye opening and was effective in that it brought me back to reality. maybe I need a simple budget boost and some new fenders instead. And after studying every post in this whole darned site I think I'll just relax and do my mods in stages...maybe gears will come later.


Thanks dudes,

 

TEEJ Began:

Then said a whole lot more.

Great post. Thanks for laying it out so clearly.

SV

 

That chart is NOT accurate. I'm running 4.56 gears, with 33" tires. My RPM's are 2,450 at 60 MPH, and 2,850 at 70 MPH.

I think that the 4.56 gears are a perfect match for 33" tires. I'd go with 4.88's for 35" tires.

My 2 cents

 

Are you measuring this in 5th or 6th? If in 6th, the chart is bang on for your numbers of wheel size and differential ratio, once you allow for the approx 18% factor of the OD. The chart is calculated with a 1:1 final drive ratio. Based on 4.10 stock gears and 32" tires, you could fit 35" tires and have the identical mechanical advantage with your 4.56 gears. By putting 33's on your jeep, you have the equivalent seat of the pants handling as if you put 29" tires on a stock rubi.

You can use the calculations below the table if you wish to corroborate the mumbers in the table; it's pretty straightforward math.

SV

 

I am not sure 32" with a 3.21 is a good benchmark to use. I have the 3.21 with 32" tires and it is a slug. personally, I think Quadratec's chart is probably a better benchmark.

http://www.quadratec.com/jeep_knowle...article-26.htm