IFTA is based on mileage and gallons of fuel used, not weight.
I was talking about the requirements for it, not the form used to file the quarterly claim.
If you are not for commerce then what they go by when determining whether you need it or not is the weight rating of the truck (as in GVWR). If you are driving a truck with a GVWR of 26,001 pounds or more (or with tandem rear axles regardless of GVWR) and crossing state lines (i.e. driving outside the vehicle's state of registration) then many states say you have to have IFTA (either a decal or trip permits) and IRP when driving in a state other then the state the truck is registered in. Not all states require it though if you are hauling your own toys and not for hire. Anyhow, my comment was about IFTA requirements for personal use vehicles. And for that they go by weight, not fuel usage.
In TN, IFTA isn't required if you are not commercial.
The trick part is if he gets tagged in TN then TN won't care if he has apportioned plates/IRP or IFTA or not. It's only when he drives in to a neighboring state outside of TN that the possible need for apportioned plates and IFTA kicks in. It's the surrounding states that might ticket him for not having it, not TN as the truck is registered there. For driving in TN he won't need either. What adds to the confusion is most states make all heavy trucks over 10,000 pounds get commercial tags, whether you're commercial or not. Just because a truck has commercial plates does not mean all other commercial rules apply if it is for personal use. The original poster needs to check with TN to see if there are any exemptions from some of the commercial requirements if the truck is for personal use as TN may only require a DOT number on the IRP form if the truck is truly used for commercial purposes. But it is possible for a state to require a DOT number anyhow, even if he's not in commerce. That's why I keep saying he needs to call and ask the powers that be in TN.
I run commercial trucks so there may be some loopholes that I don't know about when you aren't hauling in commerce. If you are buying and selling these trucks it could be considered commerce though.
The way I read the initial post was that he would be picking up stuff for his own collection/use and have based my responses on that interpretation. If he is not for hire then he fits in to a gray area where some of the commercial stuff applies but not all of it. And the best way for him to find out is to call the state and get the regulation number that applies to commercial vehicles and read the regs directly. Internet forums are not the best place to get legal advice, especially when it's your backside in the hot seat should the info be wrong.
Anyhow, if I am wrong in regards to the original post and that he plans on selling the stuff he gets in operation of a business then what you said is correct about needing IRP and DOT numbers. My comments only apply to personal, not for hire use of big trucks.