New gear calculator/tire size question

[saburai]

Hi Gang!

I found this very nice program linked on NAXJA:

http://www.grimmjeeper.com/gears.html

I'm attempting to do a comparative analysis of ratios vs. tire size but I don't know (in inches) the stock tire size for my '99 XJ...

Many thanks :victory:

 

[proto]

Yup that's a good one! Let me share a couple more calculators with you, in case you haven't seen them already.

Tire Rack says your Chero came equipped with 225/75-15s:

http://www.tirerack.com/tires/Selec...oModel=Cherokee+SE&autoYear=1999&autoModClar=

And, 1010Tires has another fun calculator to play with:

http://www.1010tires.com/tiresizecalculator.asp

which lets you compare up to five different tire sizes at once. Very handy for making those critical "fun vs practical" choices.

 

[saburai]

Advice...

Yup that's a good one! Let me share a couple more calculators with you, in case you haven't seen them already.

Tire Rack says your Chero came equipped with 225/75-15s:

http://www.tirerack.com/tires/Selec...oModel=Cherokee+SE&autoYear=1999&autoModClar=

And, 1010Tires has another fun calculator to play with:

http://www.1010tires.com/tiresizecalculator.asp

which lets you compare up to five different tire sizes at once. Very handy for making those critical "fun vs practical" choices.

Thanks for the links!

Using the calculator I posted, my '99 sport on stock 225/75-15's with AX15 and NP2321 with a final drive of 3.07 gives 2017 rpm at 70 MPH, with my 31x10.5 tires, it gives me 1840 RPM.

When driving on the hi way with the 31's in fifth at 65-75 MPH it gives me 1709 - 1972 RPM.
I don't have the truck with me at the moment, but this sounds about right. In this rev range it is not peppy to say the least!
The truck feels much happier at 2300-2500 RPM 70-75 mph in fourth.

It has been suggested to me that "the 4.0 likes to rev" and on the hi way it will have more power and return better MPG at 3000 RPM than at 2000 RPM.
If this is indeed true then it seems that I would be better with some thing in the neighborhood of a 4.56 final drive giving 2700-2900 RPM on the hi way.

Thoughts, comments and real world experiences would be most welcome!

 

[Root Moose]

Bigger tires are heavier... It takes more torque to accelerate them as quickly as the smaller tires.

If you calculate out that you need something like say 4.20:1 ratio to keep a stock ratio don't go down to 4.10, always go up which means with our trucks 4.56 since that is the next ratio available for both the front and rear axle. Also, this gives you that extra little bit of extra torque to deal with the weight of the tires.

When you calculate your cruise speed the difference between say 4.10s and 4.56s will be ~200 RPM. Is that really a big deal? No. And, like suggested, it won't hurt our engines. Will it make a difference "hyper-miling" at cruise? Possibly, and I wouldn't lay money on whether it was a good thing or bad.

I did the math this morning of how much weight I'm adding to my XJ. ~700lbs. Even if I'm out to lunch with that number, lets say 2/3 of that is correct, we are still talking ~450lbs. Go with the lower ratio to compensate for that too.

Tall trucks have crap aerodynamics. Go with the lower ratio to overcome that too.

I've heard people say our engines like to rev. I suppose that is a true to a degree. It's no 9000 RPM Honda engine but I've never noticed any extra vibration up to around ~4k RPM or so. Beyond that engine speed it is an annoying engine. Hey, it's an antique cam in block engine so it does well for what it is. I personally wouldn't want to be cruising at 3000+ RPM.

IIRC with 4.56 gears and 265/75R16 I'm going to be turning ~2650 RPM (to lazy to calc/verify) at cruise. I'll eventually be going to 255/85-16 as my next tire size. That'll drop a couple dozen RPM at cruise as well.

Once I get this thing done I expect I'll play with it highway speed to determine what is the best cruise speed versus comfort level versus mileage. My gut says it's going to be around 110-115 km/h. A balance between feeling like you are actually going somewhere, not watching the gas gauge drop too quickly and noise levels.

FWIW, my 2001 Sport came with 205/75-15 tires. OEM crappy GY Wranglers.

Anyway, back to welding the TNT stiffeners on.

HTH

 

[saburai]

Thanks Chris for the as usual, informative response:elkgrin:

My plan is pretty similar to yours, weight wise anyway: Winch bumper, rear tire carrier bumper, skids, steering & brake upgrades, aux fuel tank, H2o tank and more than likely a Autohome Extreme RTT:sombrero:

With 31's and a 4.56 I'll be at 2500-2900 RPM in 65-75 MPH range. sounds pretty doable...

 

[Root Moose]

Are you suggesting I talk too much?

Yeah, get 4.56. You will be upsizing eventually.

 

[saburai]

Are you suggesting I talk too much?

Yeah, get 4.56. You will be upsizing eventually.

No, not at all. I enjoy your rambling posts You must type fast...

Do you think the 4.56 is too much for 31's?

 

[Root Moose]

LOL

I don't think it is too much, but on the other side of non-optimal lets say.

Like I said, we are talking a few hundred RPM and well within the limits of the engine so I wouldn't stress about it at all.

 

[wADVr]

I had always though keeping the RPMs lower, closer to stock was they best way to get mileage... wrong! My last XJ came with 4.10s and 33s~ total dog, no power, crappy mileage. I regeared to 4.56s and same sized 33s~ probably the best modification ever, great power and great mileage. I have taken a few trips loaded down to over 5k lbs and still got 18-20 mpg. I wouldnt want to overgear much more than that as it seemed right on the edge of efficiancy but have *heard* several people say 4.88s and 33s work great. I will let you know next week if this is a myth... i know the examples I typed above are 33s and not 31s but the same logic applies. If you think 33s are in the future 4.56s would be the best choice.

 

[saburai]

I had always though keeping the RPMs lower, closer to stock was they best way to get mileage... wrong! My last XJ came with 4.10s and 33s~ total dog, no power, crappy mileage. I regeared to 4.56s and same sized 33s~ probably the best modification ever, great power and great mileage. I have taken a few trips loaded down to over 5k lbs and still got 18-20 mpg. I wouldnt want to overgear much more than that as it seemed right on the edge of efficiancy but have *heard* several people say 4.88s and 33s work great. I will let you know next week if this is a myth... i know the examples I typed above are 33s and not 31s but the same logic applies. If you think 33s are in the future 4.56s would be the best choice.

Thanks for the input!

O.K. last night I hitched a ride from the City out to Long Island and picked up my truck. It was just a few miles from where my truck was parked to my friends house where I'm sleeping, but I did manage to shift less and keep the revs higher.
I kept it around 2500 - 3200, much over three grand, the engine just seems much too busy to cruise in a proper truck like fashion.
Not so bad just under three though...

Now I know 33's would look mighty tuff and all that, but I really don't think I want to raise the CG that much. I don't think that the disadvantages out weigh the scant few inches of extra clearance to be gained. If I were to go up it would be no larger than a 32 or a metric equivalent right around 32.
So the next question is, is the difference in handling between 31's and 33's that much? I know every one's opinion is different as to what constitutes "good" handling - I'm from a sports car back round, I drive on the fast side and I'm pretty anal about safety so - I'm pretty picky:Wow1:

Please keep us posted on the 4.88/33 combo. It might shed some additional light on this for me.
When do you think you are going to do it?

I wish someone made a 4.33 gear set for my application...

So Chris, do you mean sub-optimal or more like nominal? :sombrero:

 

[wADVr]

Well, I am picking the rig up on friday night and have a 1200 mile drive home over the weekend to determine my opinion. I have made this trip in my old jeep so i should have a good comparison.

A really good set up IMO would be 265/75R16s (32") on rubicon Moab wheels (16x8 w/ 5" backspacing) and 4.10s. Just enough back spacing to keep the tires off the control arms yet keep the handling in check. Anything bigger and handling starts going down the tube rapidly along with the necessity of more lift. I forget the 5th gear ratio of the manual tranny but 4.10s would be about perfect for the slightly small metric 32s. If i were not into getting myself into trouble over deep snow I would run this and be content.

edit: 5th gear in the AX15 is .79 so @ 70 mph your RPMs with 32s (31.6 for the metrics) will be 2410~perfect IMO. 31s would be 2473 only slightly more but not too much different.

i didnt look at the links but the formula I have been using for years to figure out RPM @ any speed is:

MPH X axle ratio X tranny gear ratio X 336 / actual tire diameter in inches = RPM

example:
70mph X 4.56 X .75 (AW4 4th gear) X 336 / 32.8 = 2452RPM @ 70 MPH

 

[wADVr]

just an update for those interested:

After driving 1400 or so miles back from Denver Colorado via I70 to Moab Utah I am not sold on 4.88s and 33" tires. Several hours of 2750-3000rpm did not do well for my gas mileage. While it might have prolonged the need to downshift on several hills it still needed to do so. Acceleration however is great but I dont think its bad with 4.56s and for me not that worth the higher cruise rpm and loss of mileage. The jeep weighed in at 4380lbs with 3/4 tank of gas so about 1klbs less than the last time i made the trip with 4.56s when I got considerably higher mileage. This trip home yeilded 14.5mpg while my last trip mentioned was around 18mpg.

Obviously I am very mileage conscious and my opinion is based on this. It is possible that something else could be contributing to the mileage I am experiancing. I havent found any problems but the vehicle is still new to me. If I find something or my results change I will update. Hope this helps.

 

[saburai]

Hope MPG improves

just an update for those interested:

After driving 1400 or so miles back from Denver Colorado via I70 to Moab Utah I am not sold on 4.88s and 33" tires. Several hours of 2750-3000rpm did not do well for my gas mileage. While it might have prolonged the need to downshift on several hills it still needed to do so. Acceleration however is great but I dont think its bad with 4.56s and for me not that worth the higher cruise rpm and loss of mileage. The jeep weighed in at 4380lbs with 3/4 tank of gas so about 1klbs less than the last time i made the trip with 4.56s when I got considerably higher mileage. This trip home yeilded 14.5mpg while my last trip mentioned was around 18mpg.

Obviously I am very mileage conscious and my opinion is based on this. It is possible that something else could be contributing to the mileage I am experiancing. I havent found any problems but the vehicle is still new to me. If I find something or my results change I will update. Hope this helps.

Thanks for the report.
What rpm were you turning with the 4.56 rear?

 

[wADVr]

My old XJ was a 1990 which had a slightly taller 4th gear at .72:1 which brings the calculated RPM to a little under 2400rpm @ 70mph. Might have been a little bit higher but I really cant remember so sure. I just remember being able to go 85mph without noticing the engine reving too high. This trip I was hesitant to push it past 70-75 for long. Another factor i neglected to state (didnt really realize) was that the tires tread is almost gone on the rear which would bring the rpms up a bit. I am sure a good half inch of the calculated diameter is gone.