Originally Posted By: ron350
OK this is for a 1975 Chevy C10 short wheelbase truck that only sees light use. The rims are 6 x 15 and currently have 215/70R15’s that are way too short for the truck. This truck rides rough so I do not want to put a tire on that will make it worse.
OK, a 1975 C10 originally came with either G78-15's, H78-15's, or 7.00-15's all inflated to 32 psi - except for versions that used a 7.00-16 (obviously not applicable here) or LR78-15's inflated to 28 psi front and 32 psi rear (all on a 6" rim)
A couple of points:
1) Inflation pressure affects ride quality more than any other property. You didn't tell us what pressure you are running - and given that the tire size is completely different, that could be the source of the poor ride.
2) For some reason, Pickup Trucks and SUV's are designed with wheel well openings that are huge compared to the tires they put on. I partially suspect that this goes back to "What looks right" given that tires on pickups and SUV's are more marginal in size than passenger cars (Sometimes LOTS more marginal!!) So, please do not go by the size of the fenderwell opening as an indicator of what the right tire size is supposed to be.
3) The difference in ride quality between a 70 series tire and a 75 series tire is small compared to the difference between different tires - even within the same brand.
So let's do some math:
A G78-15 has a load carrying capacity of 1620 # at 32 psi - whioch has to be derated 10% because it is on a light truck = 1472 #
To carry the same load, a P215/70R15 needs to have 35 psi, but a P215/75R15 needs 31 psi.