Re: [ihc] RPM, Gearing formula needed

["Ed Sohm" <idaemes@xxxxxxxxxx>]

Rocky,

Beware of some of the web calculators.  They don't always tell you their
formula.  Some factor in a percentage of slip for an auto transmission or
somesuch and don't let you know that.  I also don't believe in measuring a
static loaded radius and multiplying by 2, then by Pi.  If you can lay your
hands on the tires, measure the circumference (IMO).
  Lets compare the calculators listed.

http://www.tmcom.com/~tsm1/scout/faq/ScoutRatios.html
says 33" tires, 2279 rpms, 3.73 r&ps = 60mph

http://www.justinternationals.org/calculater.htm
agrees with the first

http://www.binderbulletin.org/techlib/wizards/rpmwiz.asp
says 33s, 3.73s, and 2425rpms = 60mph

Bob Blair's
RPM = (336/Tire Diameter) x MPH x Axle ratio x Trans ratio
where:
         Tire Diameter is in inches
         Trans ratio is expressed as 1 in high or .8 (.7, whatever) in OD
RPM = (336/33) X 60 X 3.73 X 1
yields 2278 RPM

My reference book says 5280 feet in a mile.  5280 X 12 would be 63360 inches
in a mile.
My newer BFG Muds (33" nominal) measured 102 inches circumference.  63360 /
102 = 621.2 revolutions in a mile.
621.2 x 3.73 ring and pinion 2317 revolutions of the driveshaft per mile
2317 x 1 (direct) gives me 2317 engine revolutions per mile.
60 mph  divided by 60 minutes per hour = 1 mile per minute
2317 revolutions per mile X 1 mile per minute = 2317 rpms
Unhandy? Yep.


Different answers because Tom,  Mark, and Bob all assumed that a tire is
exactly the size it says it is, Bill assumed that the tire is 6% shorter
than it says it is.  I assumed that a tire with a tire with a circumference
of 102" will cover 102" on every revolution.  Who is correct in the real
world?  <shrug>

146 rpm spread between the calculations listed.  You can probably live with
that, though understand that 6% of 44 inches is 2.64 inches which is 8.3
inches of circumference.  The spread between Tom's et.al. and Bill's will
get larger with larger tires.  My RPMs will be even lower than Tom's et. al.
if the tires are actually taller than their nominal size.

Good luck,

Edward J Sohm
Battle Creek, IA
'71 800B
'74 200 Travelette Camper Special (converted to 4X4)
+ 10 or 12 more












----- Original Message ----- 
From: "ROCKY LEAR" <rockylear@domain.elided>
To: <ihc-digest@domain.elided>
Sent: Monday, September 08, 2003 6:06 PM
Subject: [ihc] RPM, Gearing formula needed


> I am trying to figure out my gearing questions. Maybe someone here can
help. I know that there used to be a formula for calculating oversized tires
that used to be in some of the 4x4 mags but darn if I can find it now. I
want a truck that will run 80 mph without too much winding. Here is what I
have:
>
> Two gearing options I have are 4:88 and 6:17
>
> 44" tall tires, actual measurement may be closer to 45"
>
> An OD out of a 71 t-all...what is that model number any way? T34?
>
> Is there any way to determine what my RPM will be at 80 mph in OD?
>
> I am simply guessing from past experience that the tires alone will put
the 4.88's in the equivalent of a 3.50-3.70 range.
>
> Is ther any way to figure out this?
>
> A free 6 pack of your choice to anyone who can give me definite answers!
>
> Rocky