Does anyone have guidance on how to set tire pressure for non-original size tires? I'm surprised I don't see anything definitive at tirerack.com. What tire pressure should I run on an 87 300D with 205/55-16s (gas flap specs are for 195/65-15s) and a 97 'burb with 265/75-16s (door jamb specs are for 245/75-16s), assuming I'm happy with the specs for original size tires?

Thanks,
Sixto
87 300D

If the "volume" is the same (bigger wheel size but lower profile to match, etc) then the stock pressure recommendation holds true IIRC.

But on my mother's 300TE w/ 16x7 wheels (same tire size you have), we went with 32F/34R since it's getting colder.

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There is a chart for the tire that specifies the weight load based on the inflation pressure. Match it to the vehicle that way. Doesn't take into account the match to the rim. Tire is not shaped right on the wrong width rim.

The chart holds true, but regardless what size, original or not. Add about 2PSI to make sure you don't get uneven wear. Set tire pressure only when tires are cold.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

Wouldn't that be determined by the manufacturer? I always assumed that's what they stamped on the sidewall, based on max load capacity. Is that an incorrect assumption?

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1980 300TD-China Blue/Blue MBTex-2nd Owner, 107K (Alt Blau) OBK #15
'06 Chevy Tahoe Z71 (for the wife & 4 kids, current mule) '03 Honda Odyssey (son #1's ride, reluctantly) '99 GMC Suburban (255K+ miles, semi-retired mule) 21' SeaRay Seville (summer escape pod)

The sidewall pressure is MAXIMUM inflation pressure.

I read a long time ago, and I believe it's true, that the car manufacturer designates a pressure for best ride and handling, the tire maker designates a pressure(in their books in the days of the tire stores, not what's stamped on the tire) for best tire wear.

I have learned that occasionally running the cold pressure indicated on the door jamb sticker will lead to uneven wear. Adding 2 PSI to that figure will prevent this.

If you load a truck or trailer to the max, or even a car with a trunk overloaded with moonshine, air the tires to the max pressure stamped on the sidewalls to prevent tire damage from internal overheating.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

There are several considerations in setting tire pressure:

1. Heating due to sidewall flex under load.
2. Ride quality.
3. Handling
4. Wear

If you know your axle weights, the Tire and Rim Association Year Book will give you the lowest pressure you can run and be ok with respect to sidewall flex/heating. I had a copy of it saved somewhere, but now I can't find it. Ride quality generally improves with lower pressure and gets worse with increasing pressure. Handling is usually bad with low pressure, gets better with increasing pressure up to a point, and then stops improving or even worsens as you continue to increase pressure. Wear can be uneven with too much or too little pressure. With some experimentation, you can find the tire pressures that work best for you.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

I recently switched from P-Metric to LT tires on my Xterra and placed a request into the tire manufacturer for a recommended tire pressure. They used the methodology you suggested to arrive at an answer.