if you look at the impedance range between an internal auto tuner, and an 
external one, usually the external one is much higher, that is, it'll tune a 
far higher impedance in ohms than the internal.
Though as was said, I think the newer internal tuners are pretty close now. 
Especially the elecraft internal option that will tune up to a 1400ohm miss 
match.
The ldg z100 for instance does up to 1000 ohms I think.
The tuner in the ts2000 does up to something like 300 ohms.
nothing will beat an old palstar tuner with the massive rollers though lol.
Or a heathkit or whatever.
Even MFJ has some big 2KW manual tuners that will tune damn near anything.

73
Colin, V A6BKX
----- Original Message ----- 
From: "Mark" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, July 19, 2012 11:28 AM
Subject: Re: External tuner vs internal tuner?


> Hi
>     Over the years it has been my experience that external antenna tuners
> can cover a greater mismatch between the antenna and the radio than 
> internal
> tuners. However, it may be that modern built in antenna tuners are able to
> be just as robust as external ones.
>
> God bless,
> Mark
>
> God Loves you!
> You can visit my website at:
> http://christiantraditions.info
> Please visit my blog at:
> http://christian-traditions.blogspot.com
> My Authors Page is:
> httP://www.amazon.com/author/marktorgerson
>
> -----Original Message-----
> From: For blind ham radio operators [mailto:[log in to unmask]]
> On Behalf Of Jim Gammon
> Sent: Thursday, July 19, 2012 10:11 AM
> To: [log in to unmask]
> Subject: External tuner vs internal tuner?
>
> Steve, what advantage is there to using an external auto tuner
> over the internal tuner in the TS-480? I have the 100 w model and
> really think it's a great rig.  That question about the tuner has
> nagged at me though.  I have been wondering if it would be
> worthwhile to get something like an MFJ external tuner, or is
> that simply redundant because of the existing internal one.  I
> have occasionally used the rig running low power when I had
> terrible SWR and still worked stations.  Jim WA6EKS