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Posted by Older in Boulder ('91 V2500) (207.17.136.151) on 01:39:24 05/09/07
In Reply to: Re: stock 3/4 ton wheels: same width/offset through '99? posted by jim
'91 was the last year for the '73 thru '91 body
style, as well as the live front axle (4x4); the
switch to a newer body style + IFS happened four
years after the pickups. True for 1/2 & 3/4.
One bonus for '91's; it's the only year you could
get the old body style + live axle with the 4L80E
lock-up torque convertor (this is when the newer
tranny was introduced).
My seller called with the specs, so here's the
comparison (sorry about the formatting!):
GMC V2500 Suburban GMC Sierra 2500 pickup
old ('91 OEM steel) new ('95 OEM alloy)
-------------------------------------------
tire width 235 255
aspect ratio 85 85
backspace: 4.5" 5.5"
flange-flange: 7.5" 9.0" (total width)
bead-bead: 6.5" 8.0" (rim width)
centerline: 3.75" 4.5"
offset: +.75" +1.0"
(19mm) (25.4mm)
The rim (and tire) are 1.5" wider, with a
little more offset; the web link shows how
to calculate these. Two more useful links
from #1 son's archives:
Wheel offset calculator; this site computes the
effect of any proposed offset/width change:
http://www.1010tires.com/WheelOffsetCalculator.asp
My numbers tell me I'd have 25mm less inboard
clearance (1"), and 13mm more tire width exposed.
Next up: tire size calculator. This one tells
me what will happen if I run the newer width
tire: diameter increase, 31.7" -> 33.1", with
a corresponding -4.2% speedometer error.
http://www.miata.net/garage/tirecalc.html
The offset change was made to accompany the
increase in wheel width; a wider tire is taller
than stock, if the aspect ratio remains constant;
the extra width is split between inboard (+1"
towards the inside) and outboard (+1/2" towards
the outside); there may be reasons to favor
the inboard displacement due to wheel stress,
even if the clearance is less, but I am NOT an
expert, so YMMV.
Since I have no particular need to go wider,
I'll keep looking for stock width rims, and
use oem steels if that's all I can find for
now.
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