Orginal Tire size 1972 GMC K1500 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Batman · 09-28-2009, 09:58 AM

Since the search function isn't working so hot right now, figured I'd throw up a topic for this one. My speedo is WAY off and I'd like to get it back to where it should be. Right now I have 33x12.5r15 tires on the truck, pretty sure it has the 3.08 gears. Can anyone tell me what the original tire size for these trucks was, so I can figure out exactly where the speedo should be?

70 beast · 09-28-2009, 02:09 PM

http://67-72chevytrucks.com/vboard/s...d.php?t=362722

pretty sure its around 29-30"

CheyLadd · 09-28-2009, 02:57 PM

Are 3.08's common or did you put them on? My 72 k10 swb has the 3.73 and I thought most of the 4x4's had 3.73 or 4.10. I am probably wrong. In some ways that is probably nice for keeping the rpms down on the highway.

I am running 31x10.5's that measure 30 and my speedometer is 4 mph slow at 70 mph according to my gps.

You can also go to a speed/gear calculator site and type in you tire size, gear ratio, etc and it will tell you your speed. Such as http://www.richmondgear.com/101032.html

Batman · 09-28-2009, 03:51 PM

I didn't put the gears in, nor have I bothered to jack up the rear and count revs yet (I'm going to do that before correcting the speedo). I believe I ran across some information/heresay that base gears in these trucks was 3.08, with 3.73 and 4.10 as options. With the 33s on this truck is definitely feels like some very high ratio gearing, when I'm going a GPS indicated 60MPH(speedo is only reading about 40MPH at this point) the engine is only spinning about 1400RPM, from what I could find on TH350 gears and such I believe that left me with somewhere around 3.08:1 in the pumpkin. Anyway, after I figure out the stock tire size and the rear end that should get me to a correction that I need. Assuming I have 3.08:1 and the original tire was a 29"OD that ends up at 12% too slow, so if I got a correction gear for something close to that at least my speedo would be in the right league.

70 beast · 09-28-2009, 09:23 PM

my jimmy had 3.08s stock.

LF_MN_71GMC · 09-29-2009, 11:17 PM

You can run a Stewart Warner speedo adapter inline on the cable. They come in step up ratio's. My truck runs 32x11.50r15 and is usually about 10 low on the speedo with the adapter out.

My adapter ratio is 1.202, which brings it to a manageable 5 mph low. They come in different ratios, you can get one that will step it up over 30% so you should be able to find something that works.

JameslovesJammie · 10-01-2009, 02:36 AM

3.07's are the truck ratios. 3.08 is a car ratio. Different combo.

Outlaw_71 · 10-02-2009, 12:11 AM

haha mines about 15 mph off according to the gps. Ive got 33" DD Mud Country's

09-28-2009, 09:58 AM	#1
Batman Registered User Join Date: May 2007 Location: Central IL, USA Posts: 51	Original Tire size 1972 GMC K1500 Since the search function isn't working so hot right now, figured I'd throw up a topic for this one. My speedo is WAY off and I'd like to get it back to where it should be. Right now I have 33x12.5r15 tires on the truck, pretty sure it has the 3.08 gears. Can anyone tell me what the original tire size for these trucks was, so I can figure out exactly where the speedo should be? __________________ 1972 GMC K1500 350 V-eight TH350 4x4. Last edited by Batman; 09-28-2009 at 01:13 PM.

09-28-2009, 02:09 PM	#2
70 beast Certified Hobbiest Join Date: Jul 2007 Location: Sacramento, Ca Posts: 384	Re: Orginal Tire size 1972 GMC K1500 http://67-72chevytrucks.com/vboard/s...d.php?t=362722 pretty sure its around 29-30" __________________ my jimmy build http://67-72chevytrucks.com/vboard/s...d.php?t=352536

09-28-2009, 02:57 PM	#3
CheyLadd Registered User Join Date: Oct 2006 Location: Glendora, CA Posts: 85	Re: Orginal Tire size 1972 GMC K1500 Are 3.08's common or did you put them on? My 72 k10 swb has the 3.73 and I thought most of the 4x4's had 3.73 or 4.10. I am probably wrong. In some ways that is probably nice for keeping the rpms down on the highway. I am running 31x10.5's that measure 30 and my speedometer is 4 mph slow at 70 mph according to my gps. You can also go to a speed/gear calculator site and type in you tire size, gear ratio, etc and it will tell you your speed. Such as http://www.richmondgear.com/101032.html

09-28-2009, 03:51 PM	#4
Batman Registered User Join Date: May 2007 Location: Central IL, USA Posts: 51	Re: Orginal Tire size 1972 GMC K1500 I didn't put the gears in, nor have I bothered to jack up the rear and count revs yet (I'm going to do that before correcting the speedo). I believe I ran across some information/heresay that base gears in these trucks was 3.08, with 3.73 and 4.10 as options. With the 33s on this truck is definitely feels like some very high ratio gearing, when I'm going a GPS indicated 60MPH(speedo is only reading about 40MPH at this point) the engine is only spinning about 1400RPM, from what I could find on TH350 gears and such I believe that left me with somewhere around 3.08:1 in the pumpkin. Anyway, after I figure out the stock tire size and the rear end that should get me to a correction that I need. Assuming I have 3.08:1 and the original tire was a 29"OD that ends up at 12% too slow, so if I got a correction gear for something close to that at least my speedo would be in the right league. __________________ 1972 GMC K1500 350 V-eight TH350 4x4.

09-28-2009, 09:23 PM	#5
70 beast Certified Hobbiest Join Date: Jul 2007 Location: Sacramento, Ca Posts: 384	Re: Orginal Tire size 1972 GMC K1500 my jimmy had 3.08s stock. __________________ my jimmy build http://67-72chevytrucks.com/vboard/s...d.php?t=352536

09-29-2009, 11:17 PM	#6
LF_MN_71GMC Registered User Join Date: Sep 2009 Location: Isanti, mn Posts: 177	Re: Orginal Tire size 1972 GMC K1500 You can run a Stewart Warner speedo adapter inline on the cable. They come in step up ratio's. My truck runs 32x11.50r15 and is usually about 10 low on the speedo with the adapter out. My adapter ratio is 1.202, which brings it to a manageable 5 mph low. They come in different ratios, you can get one that will step it up over 30% so you should be able to find something that works. Attached Images __________________ 1971 GMC 4x4 half ton np 205 1969 chevrolet camaro 1984 K10; 4" lift; sbc; Build link http://67-72chevytrucks.com/vboard/s...d.php?t=647172

10-01-2009, 02:36 AM	#7
JameslovesJammie God Bless Mr. T Join Date: Apr 2004 Location: Minot, ND Posts: 348	Re: Orginal Tire size 1972 GMC K1500 3.07's are the truck ratios. 3.08 is a car ratio. Different combo. __________________ Like a midget at a urinal...sometimes you must stay on your toes.

10-02-2009, 12:11 AM	#8
Outlaw_71 Registered User Join Date: Mar 2009 Location: Seymour Tennessee Posts: 72	Re: Orginal Tire size 1972 GMC K1500 haha mines about 15 mph off according to the gps. Ive got 33" DD Mud Country's __________________ VIN on glovebox says 1972, previous owner says 1971 C10 4x4 Convert. 7" of lift. New 33" Cepek Mud Country's. Thanks to our Vets past and present



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