I found it with 14/44 (standard is 14/40) and it's great for putting around back roads, but too short for the occasional 65mph stroll I see needed to access some riding areas.
My Q is, who runs what for gearing? I tend to favor long legs on all my street bikes, but I don't want to overdo it here. Any comments? I'm thinking of 16/40

might try 15/40 gearing as well. each tooth more on the counter shaft sprocket has the effect of taking off 3 teeth in the rear.

I use the stock gearing on both of my personal DT400s and it has plenty of leg for a 70 mph cruise on the highway. top speed is a little over 80 mph on the stock one and who knows on the Super DT400 as it has much more revs available.

the stock gearing is also low enough in 1st and 2nd could be used in tighter trails.

I am running either 14/40 or 14/39 on my DT400. Plenty of grunt in the hills and good enough to hit 70+ even with DOT knobbies. They are not really built for high-speed freeway cruising, but can manage if needed.

I observed Saturday the 14/44 equaled 65mph (indicated) and 5800RPM.

Well I just bolted on 16/39. Felt great on the street (just a quick one mile blast). 
I'll have more info later

16/40 on mine. Very good for all around usage. Will safely and comfortably cruise at 65-70 MPH, keeping revs below the danger zone for an air cooled big bore running sustained speed under load. Motor isn't stock by any means but it does still retain plenty of grunt and putt-around ability.

I have 15/40 on my 400C seems to be fine for road use. Happy cruising at 60/65.

I think my DT400E is 16/43, if I do my math correctly 16/40 would increase final drive by 7%
Higher gearing would be nice on the road, but I am curious if you've take these bikes with a higher gearing up long, relatively steep, low-speed off road climbs and how things performed?