tire math w/5sp and ratios

h.curtis · Oct 28, 2003

Not sure if I am doing this right. I am using this site... http://www.4lo.com/4LoCalc.htm

If I am going at 2000 rpms with a tire diameter of 32.7 Axle ratio of 4.10 and since I have over drive I put .75 as transmission ratio and transcase is 1.

Is it correct to put in .75 due to overdrive? It isn't 1 to 1, but I don't know if this is correct. I come up with 63.3 mph at 2000 rpms in OD.

Seems lower RPM's than I thougt it would be with the 4.10's and 33 KO's (32.7).

Am I doing this right?
Curt

woody · Oct 28, 2003

Yep

though those #s don't factor in for 'parasitic losses'

the math for speed @ X RPMs is:

RPM x tire dia (or better yet, working radius x 2) /
axle gear x tranny gear (x OD or low range) x 336

or for crawl ratio: Ax X tcase X Tranny

Beezil · Oct 28, 2003

this will be half-spobi, but yer 5spd is more like .82

h.curtis · Oct 28, 2003

Beezil said:
this will be half-spobi, but yer 5spd is more like .82

OK, that is the main thing. I really didn't know, but I thought I read .75. That makes a big difference in the equation.

Thanks,
Curt

h.curtis · Oct 28, 2003

Re: Yep

woody said:
though those #s don't factor in for 'parasitic losses'

the math for speed @ X RPMs is:

RPM x tire dia (or better yet, working radius x 2) /
axle gear x tranny gear (x OD or low range) x 336

or for crawl ratio: Ax X tcase X Tranny

Thanks for all this. I am just getting a feel for all this before spending the $$ on all this stuff. At first I was looking at 3.73's, and everyone told me no I need more gear. I am now looking at 4.10's with the 5sp and the lower 1st. I know most people think I should have 4.56's, but I think 4.10's would be good with my skinny 10.50's that I will be putting on.

Thanks,
Curt

Eagle · Oct 28, 2003

Beezil said:
this will be half-spobi, but yer 5spd is more like .82

BA 10/5 is 0.72

AX-15 is 0.79

NVG3550 is 0.78

What have you been smoking?

robs · Oct 28, 2003

Got MSExcel?

http://members.intertek.net/~simons5/misc/rpm.xls

BrettM · Oct 28, 2003

Eagle said:
BA 10/5 is 0.72

AX-15 is 0.79

NVG3550 is 0.78

What have you been smoking?

are you sure about this?! I was under the impression my BA 10 was much closer to .80. That sure would be nice if you're right since I'm swapping a .70 AW4 and was worried the OD would be too deep.

Eagle · Oct 28, 2003

mad maXJ said:
are you sure about this?! I was under the impression my BA 10 was much closer to .80. That sure would be nice if you're right since I'm swapping a .70 AW4 and was worried the OD would be too deep.

Source = AMC/Jeep 1988 BA 10/5 FSM, publication # 8980 010 302

Jekyll · Oct 28, 2003

My XJ has a 5 spd, 4.0L, 33x10.50" BFG A/T KO, 4.56 gears. I drive plenty on the hwy at 65-70 mph, rpm is usually around 2500-2800. I'm convinced 4.56 is the correct gear ratio for my setup. I can haul a load, push a headwind, climb hills, pass other vehicles...all without any trouble. And it's great off-road.

Fuel economy isn't too bad, either...I can drive about 300-350 miles per tank(20 gal). I drive about 3000 miles/month.

My speedometer reads within 2% of actual, checked against GPS and hwy markers.

Beezil, that's not half-spobi...it's only about 3%

h.curtis · Oct 28, 2003

Eagle said:
BA 10/5 is 0.72

AX-15 is 0.79

NVG3550 is 0.78

What have you been smoking?

I have a '96, which one do I have? AX-15?

Curt

XJEEPER · Oct 28, 2003

Jekyll said:
My XJ has a 5 spd, 4.0L, 33x10.50" BFG A/T KO, 4.56 gears. I drive plenty on the hwy at 65-70 mph, rpm is usually around 2500-2800. I'm convinced 4.56 is the correct gear ratio for my setup. I can haul a load, push a headwind, climb hills, pass other vehicles...all without any trouble. And it's great off-road.

Fuel economy isn't too bad, either...I can drive about 300-350 miles per tank(20 gal). I drive about 3000 miles/month.

My speedometer reads within 2% of actual, checked against GPS and hwy markers.

Beezil, that's not half-spobi...it's only about 3%

Toss the gear charts and books and listen to your XJ bretheren in the know..........Jekyll has nailed it.

Scott Mac · Oct 28, 2003

h.curtis said:
I have a '96, which one do I have? AX-15?

Curt

If it's a manual tranny, you have the AX-15.

I'm considering running 4.88s with my 33s.

Ivan · Oct 28, 2003

If you're looking for gear calculators, this is one of the better ones.... But first hand experience from those that "have been there" is really your best bet....

Grimms

Ivan

dennisuello · Oct 28, 2003

i've been using this one:
http://www.tommyjeep.com/apps/ratio/index.htm

tire math w/5sp and ratios

h.curtis

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