60 MPH and 3,000 RPM's?

[Georgia Mike]

OK,I know the formula for calculating future gear ratios like this:

MPH X Gear Ratio / True Tire Height X 336 = RPM's

What I need to know is the formula for figuring out my existing GR. I have 31X10.50's (probably a true 29.5"-30" tall) and 3,000 RPM at 60 MPH. Any help,please? smile.gif

[SpruceMoose]

wow! thats really humming! i have the same size tires and i do about 2k at 60. 2.73 axles under there.

sm.

[River Beast]

Use the gear calculator at www.4lo.com (http://www.4lo.com) they are linked at the top of the page.....

[River Beast]

actually the numeric value of 336 is used for 1:1 ratio in manual transmissions and autos with a lockout converter....

if you have a TF727 or TH400 use 355 instead of 336

[Georgia Mike]

Hmm...I was under the impression that it was for a 1 to 1 final drive,like fourth gear manual,or third gear auto.

[River Beast]

a non lockup converter always has slippage.... but yes.... 1:1 ratio in a manual or 3rd in an auto with a lockup converter is 336

355 is what I used for my TH400 and dang near on the money

[derf]

Originally posted by Georgia Mike:
OK,I know the formula for calculating future gear ratios like this:

MPH X Gear Ratio / True Tire Height X 336 = RPM's

What I need to know is the formula for figuring out my existing GR. I have 31X10.50's (probably a true 29.5"-30" tall) and 3,000 RPM at 60 MPH. Any help,please? smile.gif Actually, your forumla may be off. You need a little grouping and some parenthesis to make sure you're doing it right:

RPM = (Gear Ratio x MPH x 336) / Tire Height

Once you get that right, you cn use simple algebra to derive the other formulas...

MPH = (RPM x Tire Height) / (Gear Ratio x 336)

Tire Height = (Gear Ratio x MPH x 336) / RPM

Gear Ratio = (RPM x Tire Height) / (MPH x 336)

I'm betting you have 4.10's in your pumpkins. They were a factory option in the 70's.

[Georgia Mike]

But mines an '83 :D I'm hoping they are 4.10's,as I was told when I bought it they were 3.31's,and 4.10's were what I was wanting. It's got a great big ol' tranny cooler that looks like a factory unit. The hitch is a U-Haul unit,but the vehicle had been rear-ended pretty hard at some point in time(enough to need a small bit of frame-rail repair). Maybe a factory towing package? Reasoning would say that if it was that the factory hitch may have been damaged beyond repair in the wreck. I dunno,maybe just dreaming....... smile.gif

[Crazy_Jeepman]

Originally posted by Georgia Mike:
But mines an '83 :D I'm hoping they are 4.10's,as I was told when I bought it they were 3.31's,and 4.10's were what I was wanting. It's got a great big ol' tranny cooler that looks like a factory unit. The hitch is a U-Haul unit,but the vehicle had been rear-ended pretty hard at some point in time(enough to need a small bit of frame-rail repair). Maybe a factory towing package? Reasoning would say that if it was that the factory hitch may have been damaged beyond repair in the wreck. I dunno,maybe just dreaming....... smile.gif Factory gears would be 2.72 or the 3.31. 4.10 were not factory on any Wag 79 and up I beleive.

[Georgia Mike]

It's a Cherokee. Does that make any difference? Probably not,eh? Anyhow,I'll just go out and jack the rear up and see what it is. Will post back shortly :D

[Crazy_Jeepman]

Front Diff will have a Tag on it with the Ratio stamped in it. It will mosty likely have the 2.72, unless it does have the Tow Package

[Georgia Mike]

No tags to be found anywhere. That's the first thing I looked for :D Anyway,they appear to be 3.73's. Jacked one rear tire up,rotated it two complete revolutions,counted driveshaft revolutions--3 & 3/4 revolutions. Not the 4.10's I was hoping for,but still better than 2.73's smile.gif

[Stuka]

My J10 does 1940rpm's at 65MPH. Which is why it will cruise at 85mph all day long and get 16mpg.

[Georgia Mike]

I'm scared something will blow through the hood at 85 mph :D I think the ol' girl would crap out on me with a continuous 3750 rpm redface.gif ,or at least overheat smile.gif

[Max Power]

whoa, I've got 3.31s and 31in tires and I'm a little over 3000rpm at 85, so you must have some aftermarket stuff there, my '80 tsm shows 3.73 as the lowest gear avail. {only on j-trucks}

[Stuka]

The J20 was the only 80's FSJ with 3.73's. All else had 2.73's or 3.31's.

[HeepofaJeep]

Actually, your forumla may be off. You need a little grouping and some parenthesis to make sure you're doing it right:
Actually, the parenthesis do not make a difference in this case. Just FYI (AHHH, math class must be getting to me!)

[norville]

I had the same prob, so 3.73's huh, sounds right. I awitched to 33's and still have pah-lenty of power and RPM.

norv

[mdill]

For 60 mph go to http://www.tirerack.com/tires/SearchTires.jsp and
search for your tire, look at its specs for the first one a grabbed it
speced out at 680 rev/mile

So take that a X your gear ratio and that is your RPM (drive shaft) at 60 mph.

680 * 3.31 -> 2251 rpm
680 * 4.11 -> 2795 rpm (With a non-lock converter 3000 would be in the ball park)

Check your speedo to see if 60 mph is really right by getting on the freeway
and timing yourself a a constant 60 mph between 4 or 5 mile markers. (you should
be getting one mile marker/ min.)

Mike D.

[Georgia Mike]

I'm pretty sure the speedo gear hasn't been swapped out,as it does seem to read a tad bit slow,maybe 5% or so. I'm pretty confident it's 3.73's. If not,it's 4.10 smile.gif smile.gif smile.gif smile.gif Too bad it's only a tow rig :( :( :( :(

[ May 01, 2003, 02:18 PM: Message edited by: Georgia Mike ]

[Stuka]

If the speedo wasnt changed but lower gears were put in it would read fast. Like read 65mph but your only going 58 or so in reality. Unless bigger tires are also on. In which case it may be close to stock, or a bit under. Although the speedo on my J10 is off. When it says 75 I am actually going 85.

[Georgia Mike]

Well,they did put on 31" BFG AT's. I have no clue as to what the stock size is supposed to be. Maybe 235-75's? Who knows. I can say that I'm happy with the gearing,though. smile.gif