i got pwnt on a couple of sets on time. i had some 38x14.50's on a 10 inch rim. they measured kind of close, but more like a 37 at 50psi. even so, i was adequately pleased with the tires. they weren't the best off road, but i could never hurt them and they wore like iron on the street, burnouts included. i swear they just laughed at you when you quit they said "that all you got?". when they started wearing low, i started attacking every sharp rock, tree roots, and any bank littered with sidewall killing materials in an attempt to end their life. could NOT do it. finally, towards the end of the day going up slickrock i wedged up good against that one section of roots on the right. i've got a vengance against that for many broken side glasses and ruined sidewalls. well, i dump the clutch in 4 hi and let her eat. slamming the steering wheel left also and going up through the gears. i smell wood buring , roots taking flight, and hordes of insects vacating the premesis ASAP. i see the dagger like roots spintering apart, but some are just disappeariing all together. either way, i've completely cleared the bank in that section. what's left will die from trauma. doubt i'd do it again since land issues have changed so much, but it was a fun way to kill a tire!
the clincher is, i got out to survey the situation and was greeted with at least 10 of the bastards lodged in my sidewall at least 3" deep, one in the tread, and two pieces of auto glass from smashed windows. i always suspected that old tree was posessed, but i know it is now to own my ass so thoroughly.
so, a normal person goes and drags his spare around, bolts up, and moves on. not me. i had a matching spare. i just wasnt using it. it must have been inflated with all my common sense, becuase i sure don't have any! that tire was taking me back, inflated or flopping around off the rim.
the plan was to keep the tire extra inflated, and attack each puncture by itself, much like you would plugging a tire with two "normal" holes. i drag out my rain or shine pvc glue, known for its quick setting and bonding in less than ideal conditions. i also have a length of washer hose, and plenty of tire plugs and glue. i went after the tread offenders first. they just got a few simple black plugs and slathering of rubber glue.
now, here's where it gets tricky. some of these things are so large, even at high pressure when you remove one the tire deflates in seconds and you have no time to reinsert. okay, try a midsize one first. thumb in the hole, and didn't lose much air. so, i go to work. in the meantime, i have remembered this god awful black shit that we use to connect flashings for roof vents to the roof. it smells horrible, practically welds things together immediately, and is highly offensive to your senses. i'm pretty sure it's illegal stuff now for lead or something else toxic, but my dad hoarded it around and still uses it where he wants the absolute best job done.
so, to keep this ridiculous story going i decided the wrap each root/stick with duct tape as a better material for the roof goop to bond between tire and stick. things go great, and the roof goop is doing like it should. i couldn't have ripped that wood from them if i wanted to. finished them up with a bead of silicone around each tire to wood transition and along any of the small cracks leading out. the glass slices were just french fried with tire plugs, lots of rubber cement, and ductape horizontal to the split hoping to keep it closed. then i smeared/coated EVERYTHING with the quik set rain or shine cement from oatey.
anyway, aired it up and the damn thing held air. no way. there were a few spots hissing, but not enough to matter. amazing. so, bolt it back on. take bets on what happens if blows out, cut off the extra wood poking out and hit the trail back towards trail 1 and eventually crawfords. make it back to crawfords un eventfully other than being horribly out of balance. it's dropped from 50psi to 30psi on the ride. not surprising. NOW is time time to kill! into the rock garden we go! it made it about 3 big rocks in, and the sidwall said "no more". it had played connect the dots on 4-5 of the sticks, creating one huge sidewall slice. others wanted to attept yet another repair. this time with lapping it over itself onto a cement seal then wiring it together. the only thing i was working on the rest of the night was a 1/5th of vodka.
i do have a couple of pictures of it (real pics, they'd have to be scanned) i kept one of the sticks and it sits in the carnage pile i have in the attic of my garage. we sacraficed the rest in the campfire that night.
okay, so now that my completely useless and irrelevant story is out of the way, the next set of tires i bought were 40x17's. at street pressure mounted on a vehicle they were 41.5 inches tall. the main point is i bought the same tire from the same company, and they're variations were complete opposite. the radial 38.5 was 37 and the bias was 41.5. nothing a sawzall didn't fix. in their defence, they were two completey different tires. they might have the same name and the tread patter was similar, but different everywhere else.
i'd say just get used to it and plan ahead by checking with other people that have them or the company. it's always been a common fact that actual size runs an inch or so smaller for most. i wouldn't expect that to get any better either, especially for comp tires trying to get an edge but still comply with the rules. look at drag slicks. they have advertised sizes that are a joke compared to the actual size. 10.5w slicks for example.