Tom Selleck as Indiana Jones?

Banditty: he is known [and also as other things] for being [smokey and the] Bandit. not all words on the internet have to be words LOL. damn. i am thinking of burt Reynolds who looks very much like tom, lets be honest.

being tall and athletic means nothing.

playing dde means nothing. in this.

i don't think he would have been a good indiana jones. and with him taking indiana jones, and magnum it would have been a lot. i agree with you that his voice can change from calm and soft to badass but indi was more bad ass than not.

he would have had to shave between each class.

Ford has this Dry, cynical crass which makes it seem as if Indy has been doing this for awhile, Selleck lacks that charm.

I wholeheartedly agree with you about tom lacking that charm. Tom Selleck has charm and so do many of his characters but this is a different kind of charm. Harrison had/still has that.

And I wouldn't be able to buy Tom as a clean shaven college professor in the 30's. Nope.

You also have to consider the stunts needed done and who to get to make stunt Indy look like real Indy. Tom with being Superman height and very wide shouldered, along with the quite h.a.i.r.y. chest... Who the hell were they going to find to remotely fit that bill?

I think the movie still would have been great. Replacing Ford seems ridiculous now, but in 1981 he wasn't the Legendary Harrison Ford, he was just an actor with a very successful Sci-Fi franchise on his resume. Selleck proved on Magnum he could do both serious crime fiction and light comedy so he would have been fine. We would probably been without one of the greatest scenes ever though. The part when Indy shot the Swordsman in the street was scripted as a full fight, but Ford said "Why should I fight him when I could just shoot him?" (rumored to have happened because Ford had a bad stomach flu and didn't want to film an athletic fight scene that day.) Would Selleck have suggested the change? If not one of the funniest scenes in movie history could have been just another generic Hollywood fight.

Could Tom Selleck be as good in the role as Harrison Ford?

No. He just doesn't have the charisma or screen presence of Harrison Ford and those are the main reasons why Harrison Ford is so effective as the character. I also just don't buy Tom Selleck as an action hero.

The fact that I love Harrison Ford as an actor and am basically indifferent to Tom Selleck as an actor plays a huge part in my opinion though. 

Very well said. 

He has a wimpy voice and something of a "Brady Bunch son" aspect to him.

It's weird though because Ford is much closer in appearance to Charlton Heston and it was the Secret of the Incas that inspired Indiana Jones--in a way Raiders is a partial remake of that film. When you see Heston from behind you can imagine Ford there instead.
But the production art for Raiders makes IJ look like an angry Tom Selleck.

I think he would have been fine to be honest. I've always liked Harrison Ford, but Selleck would have been quite good. I think Ford captures the sarcasm of the character a little better than Selleck could have but outside of that it would have worked. Selleck was good in High Road to China and don't forget Quigley Down Under. Not exactly the same kind of film, but as an adventure hero he could have nailed it.