Tire size confusion.

REK1 · Nov 12, 2024

I have this 1981 K30 dually and it currently has 235/85R/16 tires one it. When I search that tire size multiple vendors state will not fit my specific vehicle. The rear tires seem to have ample spacing as I have had them off already and no type of spacer between the rims. Anyone with experience on a K30 dually and proper tire setup?
Appreciate any and all info you folks may have. Also any good recommendations on a all terrane 6 tire set?

DoubleDingo · Nov 12, 2024

Most likely came with 9.5-16.5 tires, and someone switched to 16 inch rims and 235/85/16 tires. Nothing wrong except the computers. Keep rocking the tire size on there

Robert Bare · Nov 12, 2024

What DD said. Some came with that large, depends on how the truck was ordered, but the size you have is one of the most common today, I have seen. And, yes, todays online and tire shop size charts are terrible for older vehicles.
Way back my dad had a 74 SRW camper special with 16.5 split rims, what a nightmare.

TotalyHucked · Nov 12, 2024

Just ignore what the websites say (and even the salesman at the counter) about it "not fitting" your vehicle. Even with newer stuff, they're wrong alot of the time. Put whatever tire size you want on there

REK1 · Nov 12, 2024

Thanks for the feedback guys.

fast 99 · Nov 12, 2024

We had several C and K 30's-year range was 74-85. We did run 235's on a couple trucks but with either low air pressure or high weight sidewalls would rub. This is a totally bad thing in DOT's mind. We never got red flagged [parked], but that was a distinct possibility. State Patrol was a little forgiving on it but yours may not.

Some trucks were original 16.5 and converted to 16 for ease of maintenance. 215 tires sidewalls were plenty far apart. Problem we ran into was broken belts, usually before they were run out. This may not be a problem in your case. We ran near or over max GVW consistently.

Grit dog · Nov 12, 2024

235 85s are bout perfect I think with no lift and no spacers.
255 85s if you want a taller wider tire but idk how much space between the duals will be left without spacers. You’ll lose about 3/4” going 255s.

REK1 · Nov 12, 2024

fast 99 said:
We had several C and K 30's-year range was 74-85. We did run 235's on a couple trucks but with either low air pressure or high weight sidewalls would rub. This is a totally bad thing in DOT's mind. We never got red flagged [parked], but that was a distinct possibility. State Patrol was a little forgiving on it but yours may not.

Some trucks were original 16.5 and converted to 16 for ease of maintenance. 215 tires sidewalls were plenty far apart. Problem we ran into was broken belts, usually before they were run out. This may not be a problem in your case. We ran near or over max GVW consistently.

Yeah, I most likely will never see that kind of weight. (never say never ) right? I just recently got this truck back. My dad sold it back I think in the mid 90's there about. Its exactly how he had it other than an extra 5K on the odometer. two previous owners after my pops never even did a title change. Was still in my dads name up to 2015. Anyhow I will just stay with the same size it is currently sporting. Just a more aggressive tread.

Thanks again Square Bodies...

REK1 · Nov 12, 2024

Grit dog said:
235 85s are bout perfect I think with no lift and no spacers.
255 85s if you want a taller wider tire but idk how much space between the duals will be left without spacers. You’ll lose about 3/4” going 255s.

Taller sounds beterer considering them 4:55 gears. Like you mentioned I don't want to sandwich them rears for sure.

Grit dog · Nov 12, 2024

4.56 is tough duty without OD or huge tires if you want to do more than haul heavy or drive around town.
Even 4.10s and 35s suck above 60 mph with a 3 speed trans.
I’d say 255s are ok if you have a little clearance between the duals. I’d take anything over 1/4” and be fine with it unless hauling heavy frequently or long distance.
JMO.

bucket · Nov 12, 2024

I believe it's @Dooley that runs the 255/85 on his trucks and it looks/works well. Maybe I'm remembering wrong though.

JBswth · Tuesday at 3:42 AM

Robert Bare said:
What DD said. Some came with that large, depends on how the truck was ordered, but the size you have is one of the most common today, I have seen. And, yes, todays online and tire shop size charts are terrible for older vehicles.
Way back my dad had a 74 SRW camper special with 16.5 split rims, what a nightmare.

My 73 has 16.5, regular rims. Last year, only one tire shop was willing to order them for me. Probably will have to switch to 16s nest time I need new tires.

J. B.

84GMCSierra · Tuesday at 6:47 AM

My 84 came with 235 85R16's on it yet I find vendors telling me they won't sell me them that it's supposed to have the 215 85R16. My door placard says otherwise(235 85R16)

75gmck25 · Tuesday at 8:42 AM

After GM switched from 16.5” to 16” tires, the standard size for HD trucks was usually 235/85R16E or 245/75R16E. Your glove box sticker should list the tire size for your truck.

My ‘75 8400 GVW truck had 9.50x16.5/D tires when delivered, and they were 31” in diameter. The 245/75R16 is about 30.96 diameter and the 235/85R16 is about 32.25.

Note - you need to use an E (10ply) rated tire with SRW to have the load rating of about 3000 lbs needed for an HD 3/4 ton or a 1 ton. An LT C or D load-rated tire will fit, but the load rating is too low.
One exception - Some duallies only had D load-rated tires, which makes sense since you have four tires to carry the load.

Grit dog · Tuesday at 9:59 AM

75gmck25 said:
After GM switched from 16.5” to 16” tires, the standard size for HD trucks was usually 235/85R16E or 245/75R16E. Your glove box sticker should list the tire size for your truck.

My ‘75 8400 GVW truck had 9.50x16.5/D tires when delivered, and they were 31” in diameter. The 245/75R16 is about 30.96 diameter and the 235/85R16 is about 32.25.

Note - you need to use an E (10ply) rated tire with SRW to have the load rating of about 3000 lbs needed for an HD 3/4 ton or a 1 ton. An LT C or D load-rated tire will fit, but the load rating is too low.
One exception - Some duallies only had D load-rated tires, which makes sense since you have four tires to carry the load.

Not necessarily true.
First depending on size, D loads are good for likely the payload and gvw with room to spare.
Secondly, many squarebodys are not working trucks. Example our K20 would be just fine on far lighter rated tires than it has. Actually would help the ride and be able to maintain greater pressure if it had a smaller lighter duty tire.
Third ply ratings and largely even letter ratings P, C, XL, D,E, F etc have or are becoming a thing of the past. Just use the load index. There’s no subjectivity in it.

Tire size confusion.

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