What is the ratio of the tuners in the flex 6400M? i.e. will it tune above 3 to 1?

Can it match a 3:1 or higher SWR?  Depends.  It should match a 3:1 SWR (and beyond) for resistive loads.  Here are the specs:

Integrated Antenna Tuner Specifications

• Matching Range 80m - 10m: 8.3 Ohms - 300 Ohms
• Matching Range 160 and 6m: 16.7 Ohms - 150 Ohms

As you can see the tuner is capable of matching resistive loads of at least 3:1 (and higher on most bands).  But for “real-world” loads that are not purely resistive, it would depend on the nature of the reactive component.  For example, my LDG autotuner is “rated” at 10:1 SWR, but for my 160M dipole it will not match a portion of the band although the SWR is under 3:1.

Howard

Personally, I’d go with the internal tuner option, if only for its integration into SmartSDR.  If you kept the LDG inline (normally set on Bypass), you’d have the option of switching to it in case the internal tuner had a problem with certain frequency ranges.   Besides, the internal tuner only adds $300 to the cost of the radio, and might make it easier to sell in case you decide to trade up later. 

Howard

I'd have to agree with Howard on this one.  I had an LDG tuner with my mobile rig (FT 991) and it was unable to tune consistently on 80m on my vertical.  The internal tuner on the Flex 6500 handles it with no problems at all.