According to this chart, M8008 ST Radial
Look under spec's
Maxxis does make a 205/75R15 D range tire that is the same OD as the 205/75R15 C range tire. Both are listed as 27.1" OD
That for sure is something to verify as the low ridding Sunlines do not have excess fender clearance.
The D's are also 65psi tires. I'm making an assumption that your rims are rated for 65psi? Yes/No?
If you do get the upgrade request high pressure valve stems. They really do not cost much if anything more then the standard stems but are a better stem. They look like this. See the long brass stem verses the standard all rubber on the outside.
I'm also assuming you know to request that all tires are ideally 6 months or less old and no older then 1 year. You can see the dates on the DOT code. They have no problems selling you 2 or 3 year old tires as new if you do not ask. You may have to wait a few days but it's worth the wait.
Now to upgrade to D's or not. Taking your camper to the scale and getting an actual axle weight is a good thing to do. Then you know what load your putting on the tires and how much safety cushion you have. The good news is Sunline would build the camper using the axles, springs and tires to be able to hold the entire GVWR of the TT. Not all TT makers do this. The tongue is held by the truck so it you are staying at or under the TT GVWR you should not run out of tire capacity providing your evenly loaded enough left to right and your towing close to level.
Now do you add the extra safety cushion of D range? If you rims can take the pressure/load and the OD checks out then in your case it comes down to the extra dollars of the C to a D. The C's have a total load capacity of 7,280# and the D's 8,600. So your gaining an additional 1,320# on all 4 tires or 330# per tire. If your camper loaded come in at say 6,500# and the tongue is 13% or 845# as a estimate then that leaves 5,655# on the TT tires. On the C's you have now that would be 1,625# total or 406# extra per tire if left to right loading was equal which it is not. Going to a D will bring you up from 406# to 736# safety cushion per tire close to almost double. On paper it looks like a good upgrade. Then the question is will a tire pressure monitor system on the C range gain you a similar safety upgrade? Good question.
OK what is the price difference between C and D? After looking at it on the weights the difference in price may be easier justified or not.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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