I'm trying to build my dream truck. And I want 40 inch tires, but I don't know how big of a lift I need to go. I have a 1996 Dodge Ram 1500 Ext. Cab. I'm wondering if a 5 inch lift would be enough.

 

Big heavy truck, half-ton axles, and 40 inch tires are not a good combination. You will be eating front suspension parts on a regular basis. If you off-road it, you will go thru them even faster. 35" tires is about as large as I would recommend, and a 5" lift to go with them. Get one of the long-arm lift kits, and do a shackle flip for the rear. It'll ride better, drive better, off-road better, and get horrid gas mileage. But, it'll get bad gas mileage anyway.

And don't forget to regear your axles for the larger tires. 4.56 gears would be good.

 

It depends what you plan to do with your dream truck. How much articulation will it need? In general running more wheel offset and stiffer suspension get you to taller and wider tires for less money. Running more wheel offset is hard on suspension components but that tradeoff may be worth it to you. Stiff suspension is less effective for uneven ground and that may also be a tradeoff you are willing to make. For example: if you only plan to use the truck on weekends to send it into deep mud holes and never plan to go rock crawling, maybe care less about spending more on replacing worn suspension components, are willing to give up some ride quality, you could probably run big wide forties without much lift as long as your suspension is stiff and you offset them way out there. That wont work well in the rocks but may be all you need for getting through the mud.

 

My 99 2500 came with 3" body lift and 4.5" suspension (7" total) with 38x15.50s. I am sure with an adjustable arm to push the axel forward and a high clearance bumper 40s would have fit on that much lift. I removed the body lift and went to 35s, setup in my sig, which would fit 37s with adjustable arms. Definitely needed more than 4.10 gears for 38s even with the big ol' v10 in it. If I were to go that big I would do 4.88s, especially with a 360. Don't forget to upgrade your brakes as well. Hey you is right though, 35's are pushing it for factory axels if this truck is actually going to be wheeled hard. I would start looking at dana 60s for 40" tires minimum, even then for a full size truck that's getting to the limit of factory 60s. Our trucks weigh a lot. Even little jeeps can break 1-ton axels on that large of a tire when pushed.