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?

Re: Orginal Tire size 1972 GMC K1500
 

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

pretty sure its around 29-30"

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

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.

Re: Orginal Tire size 1972 GMC K1500
 

my jimmy had 3.08s stock.

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.

Re: Orginal Tire size 1972 GMC K1500
 

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

Re: Orginal Tire size 1972 GMC K1500
 

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