First of all there is no matching speed, just DVD recording to lower bitrate and in your case smaller format.
6 hours of video will not fit on any DVD, even if your recorder will work with DVD/DL (8.5GB), it is just too big of a file.
Recorder will adjust bitrate first and if that is not enough than it will reduce format from 720x480 to 1/2D1 (352x240) and start lowering bitrate until fit. That is why your output is lousy.
You can download Bitrate Calculator and see how much of video can be on one DVD, stay with 4000kbs bitrate.
With 6 hours of video on DVD5 you get 1464kbs=very low, on DVD9 you get 3169kbs= could be OK if you are taping EP VHS, which is already lousy video quality.