The table which they provide (for reducing pressure) is linked from the Marathon page in the "tire selector", and it provides separate tables for "ST" and "LT" tires.
The "LT" tire Table doesn't list anything below 35 PSI, and the max load for a load range "D" in a "single" configuration is only 2095 lbs. The "ST" table, in contrast, starts at only 15 PSI (with max load of 1060 lbs) and goes up to 2540 (at 65 PSI).
I'm in agreement with you about being "not too aggressive with Tire Pressure reductions" - Owners of TM's equipped with 14" Marathon tires have a long, sad record of tire trouble when running at or near the ultimate limits of those tires. Goodyear's "ST" table values for size P225/75R15-D are:
Code:
Inflation Tire Load
Pressure Rating
(PSI) (Lbs)
15 1060
20 1260
25 1430
30 1600
35 1760
40 1880
45 2020
50 2150 (@ 50 nominal PSI limit for Class "C")
55 2270
60 2380
65 2540 (@ 65 nominal PSI limit for Class "D")
If I wanted the maximum amount of sidewall tire "flex", then I would run close to the limit - and on return trips (with fresh water empty, and with black/gray tanks dumped at an RV park, and no wine bottles left) this would be accomplished by using only 35-40 PSI. Instead, I run at 50 PSI ("max load" 2150 lbs), and add on my 10 PSI for high-speed Interstates above that (60 PSI, 2380 "max load").lbs lbs. My actual loading is 70-80% of the Max value.
Your suggestion, use roughly the same PSI on the "E" rated as the "D" rated, makes really good sense - I'll start at 65 PSI on pair of "80 PSI" E-rated tires, and if damping isn't enough, take out another 5 lbs or so. Probably red-line at 55 PSI minimum.
