Recently got new tires for the 12-inch wheels on my 1972 KE20 sedan. Since it probably came with bias ply tires I can't use the tire pressures listed on the car. So I usually ask the tire salesman what they recommend. This time they recommended 32 to 35 PSI, maybe 30 PSI at the minimum.

 

The car rode like the tires were wood. I'm not picky about ride comfort - but this was teeth jarring. I wonder if these people really know what they're talking about.

 

I'm running steel belted radials size 155/80R12, Load and speed rating 77T. The tires were Federals SS657.

 

The question:

Is there a place (or way) to find out what tire pressure is really best for these old cars, using radial tires? Without any guesswork?

 

Thanks

Did you look on the tyres themselves for a recommended inflation?

Have a look at similar weight cars that are slightly more modern. Ke70 recommended 26 all round I think. I'd start there and see what it feels like.

I always run 2 psi harder in the front. Typically 32 / 30 or 30 / 28. Always have done in over 30 years of driving whether its a TA22 with performance yokohama tyres, or the xtrail with whatever the round black things are. And you're right, tyre construction and differences in sizing when compared to 1972 render original specs meaningless.

 

The tyre needs to be able to flex in the sidewall, but I think even manufacturer guidelines were arbitrary at best. Under inflation or over inflation will result in premature uneven wear.

 

At my usual settings I've always felt I had a good balance of road holding, comfort and good tyre life.

 

Track days of course are a different matter........

Thanks!

 

I've since replaced the Federals with Veento G-2's. I thought maybe the Federals just had stiff sidewalls. Still size 155/80R12, load and speed rating 77T. They are slightly better but still too rough. The shop recommended 30-32 PSI, maybe 28. I suspect tire shops recommend a higher pressure so someone who never checks their tires won't sue them for having a blow out.

 

My old tires were the same size and did great at 32 PSI. Could it be the load and speed rating? The old tires had a load/speed rating of 76S, the new ones have 77T. In the US a load rating of 76 means a tire will support up to 882 Lbs, and a 77 up to 908 Lbs. An S rating means its good up to 112 mph, T up to 118 mph. I've read the speed rating in particular can affect ride roughness. Unfortunately all the 155/80R12's I find are 77T's.

 

Looking at newer cars with similar weight is an idea.

Then there's trying lower pressures until I get a decent ride - then monitor tread wear for under-inflation.

An aspect ratio of 80 I would have thought was getting into truck tyre ranges?

 

A pretty small truck I grant you, but still

Aspect of 80 does seem a bit high. But original recommended size was 6.00-12. The modern equivalent comes up 155-80R12.

 

What tire pressures and sizes are you fellows running? (Thanks parrot for your input!)

80 seems high until you realise it's probably a 4.5" wide wheel...