 |
|
|
08-27-2008, 10:29 AM
|
#1
|
|
Registered User
Join Date: Dec 2005
Location: Louisville, Ky
Posts: 473
|
rpm/gear calculators take actual tire height into account?
I'm trying to figure out if my rpm's are back to stock- my 4.0 seems like it needs to upshift even in 5th gear on the xway. I have 3.73's w/ 31x10.50x15 BGF's. We all know the tire manufacturers don't tell the truth with their sizes- my NEW BFG AT's are about 29 1/2"...nowhere near the advertised 31".
Using the gear calculators, do I plug in 31 or 29.5 for my tires?
|
|
|
|
08-27-2008, 10:35 AM
|
#2
|
|
Registered User
Join Date: Dec 2007
Location: Las Cruces, NM
Posts: 506
|
Do you want an inflated number or the true number? If you want the true number use the true size. You can use the manufacturer's number for a ball park which will usually get you the right gear setup considering that you have a limited selection of gears.
__________________
[COLOR="Red"]1987 Wrangler YJ RED[/COLOR] 4.2 5-speed, NP231, BFG 31x10.5R15 AT/KO on 15x8 gambler wheels.
[COLOR="Yellow"]Caution!!! Under construction!!!!!! Caution!!![/COLOR]
Finishing a 4.0 Cyl.head swap, Just got an AX-15 NP231J combo to replace the busted t-case and craptacular BA10/5. Adding Nutter bypass. Probably more to come before it is back as a DD!
|
|
|
|
|
08-27-2008, 10:52 AM
|
#3
|
|
Member
Join Date: Feb 2007
Location: Huntsville, Alabama
Posts: 522
|
I found this RPM calculator site to be real helpful....to get the acual tire size put in simply mess around with the metric pull down chart to see what come closest for you to 29.5 inches.
For example you can put in a metric size of: 285/65/R15 and that comes out to 29.6 inch tire size (multply 285 by .65 then times by 2 then convert from mm to inches and add the 15 to come up with final tire size)
http://www.grimmjeeper.com/gears.html
This website has two applets side by side so you can compare configurations...i.e. stock size rpm vs modified
__________________
93YJ 4.8L Vortec,AX15,Atlas 4.3:1, 4.56 Gears, Dana 30 w/ Posi-lok,Dana 44 Rear w/ Ox Locker + 99 Explorer Disk Brakes,Tom Woods D. Shafts,1" Body, 2.5" ED RE Lift, 33" BFG KM2 T/A's with MT Classic II Wheels, Herculined Inside, Electric Fan, 20 Gal Tank Conversion,Tuffy Center Console,Warn Front Bumper, Rockhard 4X4 Rear Bumper w/ Tire Carrier,Bushwacker Extended Flat Fender Flares,AMSoil/Redline In Everything and the List Goes On....
AND...
88YJ 4.2L: Interior and mechanical overhaul including engine replacement, nutter bypass, Weber 34 Carb, and work on a whole bunch of other things to get it back to fine running condition without the emissions control...evap cannister kept functional as well as PCV system, and manifold heater.
|
|
|
|
|
08-27-2008, 12:08 PM
|
#4
|
|
Registered User
Join Date: Dec 2007
Location: Deming,New Mexico, New Mexico
Posts: 2,178
|
According to the BFG site a 31X10.50X15 is 30.7"
the 31X9.50X15 is listed at 29.5"
Quote:
Originally Posted by citRon
I'm trying to figure out if my rpm's are back to stock- my 4.0 seems like it needs to upshift even in 5th gear on the xway. I have 3.73's w/ 31x10.50x15 BGF's. We all know the tire manufacturers don't tell the truth with their sizes- my NEW BFG AT's are about 29 1/2"...nowhere near the advertised 31".
Using the gear calculators, do I plug in 31 or 29.5 for my tires?
|
__________________
All other things being equal, the simplest solution is the best.
Ich liebe meinen Jeep!
|
|
|
|
|
08-27-2008, 12:55 PM
|
#5
|
|
Registered User
Join Date: Dec 2005
Location: Louisville, Ky
Posts: 473
|
they must be using that 'new math'!
|
|
|
|
|
08-27-2008, 01:19 PM
|
#6
|
|
Registered User
Join Date: Sep 2007
Location: Metro Detroit
Posts: 3,696
|
Your rim width also plays a roll in how tall your mounted tire is.
A set of advertised 31" tires will be much taller on a 7" rim vs a 10" rim.
__________________
Quote:
Originally Posted by Mean Max
I think I'll keep the YJ. But can I talk you into taking the wife?
|
DISCLAIMER: I (usually) browse the web using Elinks in a (text only) terminal. Which means I do not have image viewing support. So if you get confused by my seemingly inability to comment on your pictures, emoticons, etc; now you know why.
If you're still confused click here to see how I view the forum.
|
|
|
|
|
08-27-2008, 01:27 PM
|
#7
|
|
Registered User
Join Date: Jul 2008
Location: Tennessee
Posts: 62
|
There is a calculator just for that on www.bfgoodrich.com 
__________________
2005 Willys Edition
2.75" front, 2.25" rear
33x12.50 BFG KM2's
15x10 matte black 8's
|
|
|
|
|
08-28-2008, 06:19 AM
|
#8
|
|
Registered User
Join Date: Dec 2005
Location: Louisville, Ky
Posts: 473
|
Quote:
Originally Posted by Brandon_YJ
Your rim width also plays a roll in how tall your mounted tire is.
A set of advertised 31" tires will be much taller on a 7" rim vs a 10" rim.
|
mine are on 7" stock rims!
|
|
|
|
|
08-28-2008, 06:45 AM
|
#9
|
|
Registered User
Join Date: Sep 2007
Location: Metro Detroit
Posts: 3,696
|
Quote:
Originally Posted by citRon
mine are on 7" stock rims!
|
On a 7" wheel you should have more than 29.5.
How much tread do you have left on those tires?
I have the same tires on the same wheels with a little more than 50% tread left and mine measured (I measured them yesterday actually after reading this thread) dead on 30" with 29psi.
__________________
Quote:
Originally Posted by Mean Max
I think I'll keep the YJ. But can I talk you into taking the wife?
|
DISCLAIMER: I (usually) browse the web using Elinks in a (text only) terminal. Which means I do not have image viewing support. So if you get confused by my seemingly inability to comment on your pictures, emoticons, etc; now you know why.
If you're still confused click here to see how I view the forum.
|
|
|
|
|
08-28-2008, 05:21 PM
|
#10
|
|
Registered User
Join Date: Dec 2005
Location: Louisville, Ky
Posts: 473
|
Quote:
Originally Posted by Brandon_YJ
On a 7" wheel you should have more than 29.5.
How much tread do you have left on those tires?
I have the same tires on the same wheels with a little more than 50% tread left and mine measured (I measured them yesterday actually after reading this thread) dead on 30" with 29psi.
|
BRAND SPANKIN' NEW- ~ 100 miles on them:
[IMG]  [/IMG]
|
|
|
|
|
08-28-2008, 06:34 PM
|
#11
|
|
Registered User
Join Date: Dec 2007
Location: Deming,New Mexico, New Mexico
Posts: 2,178
|
I could be wrong (I usually am) but I would think the measurement that the manufacturer gives is at max psi with no weight on the tire.
Quote:
Originally Posted by citRon
BRAND SPANKIN' NEW- ~ 100 miles on them:
[IMG] [/IMG] |
__________________
All other things being equal, the simplest solution is the best.
Ich liebe meinen Jeep!
|
|
|
|
|
08-28-2008, 07:03 PM
|
#12
|
|
Registered User
Join Date: Dec 2005
Location: Louisville, Ky
Posts: 473
|
...and I forgot to mention -32 psi, no heavy tire carrier, etc
|
|
|
|
|
08-28-2008, 07:22 PM
|
#13
|
|
Join a 4x4 Club
Join Date: Jan 2005
Location: Albuquerque, NM
Posts: 5,796
|
Quote:
Originally Posted by citRon
I'm trying to figure out if my rpm's are back to stock...
|
Thats easy. Does the stock speedo gear give you an accurate speed/odometer reading?
__________________
'93 YJ, '02 GM 4.3 Vortec V6, SOA, dual ARB's, 8.8+D30(w/WarnHubs), 4.88 gears, 35" Trxus M/T's, AX15, 15x7 wheels, NP231+4:1+SYE+2LO, York OBA, Warn M8000, no top or doors Apr-Oct.
Member and Web guy: New Mexico 4 Wheelers My Truck Page on NM4W Site
|
|
|
|
|
08-28-2008, 10:13 PM
|
#14
|
|
Registered User
Join Date: Jul 2008
Location: kansas
Posts: 114
|
Take the tire/wheel OFF the jeep, lay the tire down on the floor. Measure the distance all the way around the tire. Divide that in half, and you will have the height of the tire. You cant measure the height with a load on the tire...
|
|
|
|
|
08-28-2008, 11:31 PM
|
#15
|
|
Registered User
Join Date: Sep 2007
Location: Metro Detroit
Posts: 3,696
|
Quote:
Originally Posted by tawdr77
Take the tire/wheel OFF the jeep, lay the tire down on the floor. Measure the distance all the way around the tire. Divide that in half, and you will have the height of the tire. You cant measure the height with a load on the tire...
|
You meant divide the circumference by Pi (3.14159), right?
EDIT: Wasn't meaning to sound condescending. But I've actually typed "divide the circumference in half" before when I meant "divide by Pi", so I'm assuming you had the same brainfart I've had in the past. 
__________________
Quote:
Originally Posted by Mean Max
I think I'll keep the YJ. But can I talk you into taking the wife?
|
DISCLAIMER: I (usually) browse the web using Elinks in a (text only) terminal. Which means I do not have image viewing support. So if you get confused by my seemingly inability to comment on your pictures, emoticons, etc; now you know why.
If you're still confused click here to see how I view the forum.
Last edited by Brandon_YJ; 08-29-2008 at 12:46 AM..
|
|
|
|
|
|
>
rpm/gear calculators take actual tire height into account?
