i have a buddy that just upgraded his fifth wheel and his tongue weight is now 2400 lbs, he thinks he will be pushing the limits of the 2500(9200) rear axle weight even tho he will be around 21000 lbs total weight, he is running 265 michelan ltx tires and wants to stay legal, as for his truck weight it is stock other than a generator located in the box, would a 3500 make it a legal carrier, thanks

Which 3500 ? Dually or Single Rear Wheel?

I suspect your friends suspicions are correct.

My case: with the truck listed in my signature (2500HD / Extended Cab / Short Bed), with a 5th Wheel hitch installed in bed, two adults in the truck, a dog in the back seat and a small tool box in the bed, the truck weighed in at 7200 pounds (on a CAT Scale). That leaves 2000 pounds available for the 5th Wheel Pin. Now attach my 5th Wheel trailer with a pin weight of about 2200 pounds. That puts my truck about 200 pounds over the 9200 pound GVWR. The truck measured in at 94XX pounds (don't remember exactly - the weigh slips are at work). The weight on the rear axle measured 5100 pounds, so it was still under the 6080 pound rear axle rating limit (this was measured at a CAT scale also).

For your friend's case, he will be OK on the rear axle rating (6080#), but will most likely be over on the trucks GVWR of 9200 pounds.

With the 3500 SRW truck, your friend could conceiveably be pushing the limit also. Just depends on how he loaded the truck up. The 3500 dually should be OK.

A lot of determining GVW's is in regard to meeting EPA regulations starting back when Ford brought out the F-150 to avoid catalytic convertors . DOT looks at GVW to determine if commercial drivers need a CDL or DOT physical . Manufacturers may void a warranty if their GVW is exceeded . You should stay close to the GVW but exceeding it slightly won't hurt . Pay close attention to load balance and tire ratings .