Help w/ my math??

I read in a 4wheel drive magazine that the formula to figure out you gearing should be new tire size over old tire size....then multiply stock gears...and the result is the new gear ratio you should go w/ for the new tires....when i did this i did 33 over 31, and multiplied my answer to get 3.77....but i know everyone and the sticky says for a 33 to go w/ a 4.56. What am i doing wrong to figure this out? Or for a daily driver and occasional off roader is 4.56 going to be overkill? Thanks for any info.

I'll have to dig out my maths handbook later (I've got to go run some errands shortly...) but - more than anything else - if you're running your rig on the street, you want to gear for "optimal cruise." Unless, that is, you actually like making OPEC richer...

"Optimal Cruise" for the 4.0 is 2500-2800rpm, verified experimentally (not under "experimental" conditions - just under what I do on the roads.) I quit using fifth gear in my 88, and picked up 3-4mpg as a result.

As I'd said, I'll have to dig up my maths handbook - because you're probably interested both in the results of your tyre change AND in what your gearing will net you.

While you're waiting for that, go to your favourite tyre manufacturer's website (whomever that may be) and look up the ACTUAL rolling diameter of the tyres you're thinking of. It will be slightly different from the NOMINAL rolling diameter, and that could play a part...

5-90

Because a stock XJ size tire is closer to a 27, or 28 not a 31.

33 over 27 X 3.55 = 4.33 Closest you can get 4.56.

myself said:

Because a stock XJ size tire is closer to a 27, or 28 not a 31.

33 over 27 X 3.55 = 4.33 Closest you can get 4.56.

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Actually it's the same to 4.10 from 4.33 as it is to 4.56, but rolling resistance and other factors come into play making 4.56 the better choice.

Ohhh thats where i went wrong, magazine said "New tire diameter, over Old tire diameter" and i have 31's on it assuming that is my "old" tire diameter, ok so i came out to the same thing now, is a 4.56 really better for on the road gas mileage than the 4.10? Thanks again for clearing this up for me. 5-90 i'm looking into tires now, i have the trxus mt in a 31 and i'm bent on what to go w/ now in the 33, i want something good for (separately) rock, mud, and snow conditions. Any suggestions on that greatly appreciated.

my vote goes TrXsus MT's (however they spell their name)

great tire... nice sidewall tread, good wear, low road noise, and just plain great offroad.

you'll enjoy 4.56s and 33s I'm looking forward to my 35" street tires wearing out so i can put some 33s back on with my 4.56s

JeepXJ93 said:

Ohhh thats where i went wrong, magazine said "New tire diameter, over Old tire diameter" and i have 31's on it assuming that is my "old" tire diameter, ok so i came out to the same thing now, is a 4.56 really better for on the road gas mileage than the 4.10? Thanks again for clearing this up for me. 5-90 i'm looking into tires now, i have the trxus mt in a 31 and i'm bent on what to go w/ now in the 33, i want something good for (separately) rock, mud, and snow conditions. Any suggestions on that greatly appreciated.

Click to expand...

"What gearing should I use for the street" has a lot to do with tyre size. Let me dig out my maths book...

OK, got it! Put on your thinking cap, and get out your notebooks...

First and foremost, remember that gear ratios are multiplicative, not additive. This means you multiply transmission gear ratio by axle gear ratio to get an "overall" gear ratio. This is important - don't do it, and you're going to get wrong answers... Generally, it's easier to figure "cruise" RPM one gear short of overdrive, which should be 1.00:1. However, do not guess - this is exact science. It would take me some digging, but I probably have gear ratios for most common transmissions, if you can't find them anywhere else (on the "someday list" for coding into HTML.)

The basic formula is:
RPM = (mph x overall gear ratio x 336)/(tyre diameter); with tyre diameter in inches. This can be re-arranged algebraically into:

Gear Ratio = (rpm x tyre diameter)/(mph x 336). Note that this is still overall gear ratio - if you plan to cruise in fourth, you're good with something close to this number. If you plan to use overdrive, then you'll have to divide this number by your overdrive gear ratio to select your axle ratio.

Typically, cruise speed can be taken as 65 or 70mph. And, select anything from 2600-3000rpm for cruising engine speed, with preference toward 2800rpm. So...

GR = (2800 x 30.8)/(65 x 336) (I'm using BFG 31's here...)
GR = 86240 / 21840
GR = 3.94 - so you can split the difference and go with either 3.73 (for primary street use) or 4.10 (if your focus is off-road.)

Granted, this assumes that you're going to cruise in fourth gear. Since (using another example) the Aisin AX-15 uses an overdrive of 0.79:1 (21%,) your gear ratio "core number" would end up being 3.94 / .79, or 4.98:1 (meaning either 4.88 or 5.13 gears to cruise in overdrive.) You may be better off not using fifth gear.

Since the AW4 uses an overdrive ratio of 0.75:1 (25%,) you'd want to change that to 3.94 / .75, or 5.25:1 (meaning 5.13 gearing.) If you have an AW4, start thinking about getting either a D44 or Ford 9" for the rear...

5-90

Whoaaa....thanks for all that info 5-90...i'm going to sit here and re-read it a few times to make sure i understand it as you put it. As for my rear, i actually bought a D-44 from one of the guys on this forum...its got stock 3.55's in it right now but i was planning on rebuilding it before i put it in, and thats when i'd gear it to the 4.56's. Thanks again for all that input.

5-90 tends to overinform just a little.

Yes, 4.56 with 33's is just about perfect. I've driven rigs with 4.10's and 33's, and while thats better than 3.55's, I would never actually go out of my way to end up with that setup. I ran 4.56's with 32's for a long time and loved it. 4.88's with 35's leaves me wishing I had just a hair deeper gears, but its doable.

come on people.
it's the first post up in the faq.
http://www.naxja.org/forum/showthread.php?p=960773#post960773

Thanks for that link, funny no matter how many times i use the search option and advanced search "gear ratios" "gearing" etc etc that thread doesnt come up for pages and pages. Sorry

cal said:

5-90 tends to overinform just a little.

Yes, 4.56 with 33's is just about perfect. I've driven rigs with 4.10's and 33's, and while thats better than 3.55's, I would never actually go out of my way to end up with that setup. I ran 4.56's with 32's for a long time and loved it. 4.88's with 35's leaves me wishing I had just a hair deeper gears, but its doable.

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Yeah, I do have that habit - but I enjoy teaching, and math just don't explain itself, y'know?

5-90

5-90 said:

I quit using fifth gear in my 88, and picked up 3-4mpg as a result.

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I have read this time and time again, and always wonder...

What MPG were you getting before?

Are we talking 15 to 19 or 20 to 24?

Just curious as I always got 20 + in my stocker using fifth -and find it hard to believe that dropping fifth would have helped me out so much...

Zebaru said:

I have read this time and time again, and always wonder...

What MPG were you getting before?

Are we talking 15 to 19 or 20 to 24?

Just curious as I always got 20 + in my stocker using fifth -and find it hard to believe that dropping fifth would have helped me out so much...

Click to expand...

As I recall, the jump was something like 18 to 21/22. It's dropped a bit since - but I need to renew the engine management sensors, and haven't had the readies to do so yet...

I do try to stick to mostly freeway/expressway driving - free-flowing traffic, as much as possible in the Ghey Area, anyhow...

5-90