Tire sizes and pressures

[AuroraGirl]

First, I dont know the proper tire size on a 1980 GMC K25 with 4wd and heavy duty suspension(whatever that was), but what was it? In radial sizing. The previous tires on my truck were narrower and taller, but the tires I put on were free and had a lot of tread, so it was an obvious choice on a truck with no working speedometer.

Second, I use 245 75 16 BF goodrich KO2 all terrain TA. Is there a way to know my proper tire pressure that would give full use of tread? My truck weighs about 10,500 in the winter, maybe about 10 or just under 10 in the summer without the plow on. Im assuming. I have no actual idea, just estimating extra steel on top of 8400 on the door. Id just like to know so I can make full use of my tread and not wear tires bad if I were on a road.

 

[Turbo4whl]

Please post a picture of your door sticker. I believe you may have miss read it.

 

[ali_c20]

@ first: original it had 9.5x16.5D (sales brochure) On 16" wheel you could use 235x85R16; 245x75R16; 265x70R16 and 285x65R16 within a range of -0.3 to +0.7% in diameter.

@ second:

https://tirepressure.com/lt245-70r17-tire-pressure

I think the 8400 means the GVW (gross vehicle weight) It's the maximum operating weight/mass of a vehicle as specified by the manufacturer including the vehicle's chassis, body, engine, engine fluids, fuel, accessories, driver, passengers and cargo but excluding that of any trailers. Your curb weight is lower than that, but I don't know the numbers. You could run it over some scales w and w/o the plow to get the right numbers.

 

[idahovette]

Seems we have had this discussion before somewhere in this forum. I thought the K20's usually went around 4500 to 5000 lbs empty?

 

[ali_c20]

Seems we have had this discussion before somewhere in this forum. I thought the K20's usually went around 4500 to 5000 lbs empty?

It's surely in that range, 78 k20 is rated with ~ 4,416 pounds but there's not mentioned which engine.

 

[75gmck25]

I have a '75 K25 that should be similar to your '80, and it weighs about 4800-5000 lbs empty, depending on how many people I have in the cab and whether the fuel tanks are full. Mine is also rated at at max 8400 lbs GVW, and for a 2650 lbs camper load in the bed. My truck originally had 9.50 x 16.5D tires on split rims (no longer a good choice) so I bought 16" wheels at the junkyard and switched to 245/75R16E tires that are about the same height. 245/75R16E and 235/85R16E (slightly taller) are very common sizes for HD pickup trucks, so they usually have the most reasonable prices.

To get the right load rating for max GVW you need to make sure you use E load rating tires (10 ply rated), not standard light truck tires. Also remind the tire shop to use high pressure valve stems, since E rated tires can be inflated up to 80 psi and you may need to go above 65 psi (max for standard valve stems) to get the full GVW load rating. However, whenever I ask a shop to use high pressure stems I get just a blank look back, and usually have to bring the valve stems in myself. This tirerack article explains the differents valve stems (412 vs 600 series).https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=208

I use about 35 psi in the front tires and 60 psi in the rear, since I use the truck mostly for hauling relatively heavy loads (concrete block, etc.). If its just a daily driver I'd probably start out with 35 psi in the rear also, and see how it rides.

Bruce

 

[gotyourgoat]

Are you perhaps reading the towing/loading tag instead of the gross vehicle weight?

 

[AuroraGirl]

Thats very likely. Well, I guess, if 5000 empty is about a stock weight then add 1500 in unloaded weight because of my extra bumpers and plow connectors. Then another 1200 for my sand. Maybe another 500 for my plow. Im really estimating, once its road worthy I can scale it at my local incinerator. But 35 unloaded psi sounds fair, its about what is in the front (38) and there just seems to be a little bit of squat on side walls with the plow. And they are E rated. I do got good valve stems, as well. The rears I have at about 56 psi at least when I put air, and seemed low. But I have all the weight in the back as well. I can send placard, I must have read incorrectly.

 

[idahovette]

I have a '75 K25 that should be similar to your '80, and it weighs about 4800-5000 lbs empty, depending on how many people I have in the cab and whether the fuel tanks are full. Mine is also rated at at max 8400 lbs GVW, and for a 2650 lbs camper load in the bed. My truck originally had 9.50 x 16.5D tires on split rims (no longer a good choice) so I bought 16" wheels at the junkyard and switched to 245/75R16E tires that are about the same height. 245/75R16E and 235/85R16E (slightly taller) are very common sizes for HD pickup trucks, so they usually have the most reasonable prices.

To get the right load rating for max GVW you need to make sure you use E load rating tires (10 ply rated), not standard light truck tires. Also remind the tire shop to use high pressure valve stems, since E rated tires can be inflated up to 80 psi and you may need to go above 65 psi (max for standard valve stems) to get the full GVW load rating. However, whenever I ask a shop to use high pressure stems I get just a blank look back, and usually have to bring the valve stems in myself. This tirerack article explains the differents valve stems (412 vs 600 series).https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=208

I use about 35 psi in the front tires and 60 psi in the rear, since I use the truck mostly for hauling relatively heavy loads (concrete block, etc.). If its just a daily driver I'd probably start out with 35 psi in the rear also, and see how it rides.

Bruce

Can't say as I have ever seen split rim 16.5 wheels. My 75K20 came with 8.75-16.5 mud grips(ordered that way). No spare tire ,just a wheel in the carrier. I still have the original tires still mounted on the original wheels with a lot of tread left. I still use them occasionally in the winter if the snow gets too deep around here. Always keep at least load range "D" usually "E" and at least 50 psi at all 4 corners, it's a truck.

 

[AuroraGirl]

I have a lot of split 16.5 and split 16. No idea if they are original, my grand fatrher had a lot of squares come and go for parts. But I put non split 16s on.

 

[80BrownK10]

First, I dont know the proper tire size on a 1980 GMC K25 with 4wd and heavy duty suspension(whatever that was), but what was it? In radial sizing. The previous tires on my truck were narrower and taller, but the tires I put on were free and had a lot of tread, so it was an obvious choice on a truck with no working speedometer.

Second, I use 245 75 16 BF goodrich KO2 all terrain TA. Is there a way to know my proper tire pressure that would give full use of tread? My truck weighs about 10,500 in the winter, maybe about 10 or just under 10 in the summer without the plow on. Im assuming. I have no actual idea, just estimating extra steel on top of 8400 on the door. Id just like to know so I can make full use of my tread and not wear tires bad if I were on a road.

What are you carrying around to weigh 10,000 pounds, a service body full of tools and 200 gallons of diesel fuel?

As for tread you can look at the max on the tire wall and they you can adjust pressure down till tread fully contacts the ground at whatever load you want to adjust it to carry.

 

[AuroraGirl]

mis reading of door placard, that was my bad

What are you carrying around to weigh 10,000 pounds, a service body full of tools and 200 gallons of diesel fuel?

As for tread you can look at the max on the tire wall and they you can adjust pressure down till tread fully contacts the ground at whatever load you want to adjust it to carry.