Cars before radials came with bias ply tire construction. These had a factory-specified air pressure. In the case of my ponton it was 26 lbs/insq. Many such owners have converted to radials. Does anyone know of a general information data site where there has been a standard pressure conversion when fitting radials? FYI, I changed from 6.70x13 bias originals at 26 lbs to 185/80R13 radials. By experiment I came to 31 pounds/insq. I have never found any discussion of this subject. Anything "out there?"

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Douglas
1959 M-B 220S cabriolet

Doug - the owner's manual indicates that the original bias ply tires could read about 34psi rear and 33psi front after a fast trip on super highways. Since that's pretty close to tire pressure ratings for today's radial tires, that's about what I'm using for my radials. Seems to work fine.

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Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles

I must have missed the statement in my owner's manual. Interesting that it specifies cold pressure at 26 lbs. Thanks. I'll hjave a look. It does seem that the manual's statement may be a good proxy for today's radial tire pressure.

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Douglas
1959 M-B 220S cabriolet

Doug:

TireRack has some good articles on tires, the readings on the sidewall and what they mean, etc....

I'd start there and get a good understanding of tires, their makeup and reasons for what's what on those tires.

Then, as a general rule, you'll notice that tire pressure readings should be taken when the tire is cool (before the first use of the day) and recorded. If you add air at that time, then as a general rule, the pressures should be equal all around.

However, if you measure, then drive to the nearest air-pump, then you need to perform some math, as the tires will now have had a chance to warm up and the pressures may be a bit different from your first measurement.

Now, for a little math (this is the fun part...)

Let's say all your tires, cool, read approx. 27 p.s.i.. You, generally, like the feel when they're all at 33 p.s.i.. Then, at the pump the RR reads 29 and the others read 28.

Q: Do you add only 4 p.s.i. to the RR and 5 p.s.i. to the other three?

A: Add 6 p.s.i. to all the tires and when the tires are measured later (preferably, the next morning before the first "sun and run"), they should all be the same pressure...aprox. 33 p.s.i..

And, as a general rule for myself, I run all my tires between 10% - 15% lower than the MAXIMUM PRESSURE printed on the sidewall for a fully-loaded tire.

I just got done removing and replacing a set of BF Goodrich Radial T/A Longtrails on my S-10. They're rated as a 50K tire. All four had over 79.1K on them. Granted, the tread was at 2/32" - but no cords, belts, cuts or scuffs showing. As you can guess, I maximize the use of everything I own and I'm a bit anal about the air pressure in the tires themselves.

BTW, this is only the third set I've had on the truck and it has over 214K on it. The first set, I believe, was Goodyear - If I remember correctly, I got 82,000, way better than the rated mileage on that set. Then the second set was a junk-yard pull-off set (truck was crushed by a tree in a storm - nothing was wrong with the tires) and I ran those 'til the cords/belts started showing (about 54K), then this last set of BFs (79.1K).

The other point that I should make here is this - most people don't give this much thought, but I feels it's important to mention it...when you're looking for a set of tires, check the load rating (1,335 lbs./tire) - multiply that by 4 and your answer should be higher than the weight of your vehicle, fully loaded with passengers/luggage and a full tank of fuel.

If it's not, don't buy them. They'll wear out much sooner than they should and if the tire shop is smart, they'll warn you before hand, or it it's crooked, they put 'em on and when you finally have a failure, they'll deny your claim. Buyer beware...

The bottom line?

Brush up on your knowledge of tires - It won't hurt and it will definately help you in your final decision of what to buy.

And, since it's your butt that's riding on those things, don't you want that decision to be the best, if not the correct one?

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M. G. Burg
'10 - Dakota SXT - Daily Ride / ≈ 172.5K
.'76 - 450SLC - 107.024.12 / < .89.20 K
..'77 - 280E - 123.033.12 / > 128.20 K
...'67 - El Camino - 283ci / > 207.00 K
....'75 - Yamaha - 650XS / < 21.00 K
.....'87 - G20 Sportvan / > 206.00 K
......'85 - 4WINNS 160 I.O. / 140hp
.......'74 - Honda CT70 / Real 125
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“I didn’t really say everything I said.”
. . . . . . . . . . . . . . . . . . ~ Yogi Berra ~