This is really kind of outside my area of expertise, but the idea of darksiding a 400 has piqued my interest. I looked at Roy's link, but the thing that bothers me most about those tires is the 'SN' type for the tire. I couldn't find a tire type to match the 'SN' designation.
I did find another link for a P165/70R13 passenger car tire here. It's obviously a foreign manufacturer or otherwise they have the dumbest marketing department in the world for naming a tire 'Achilles' (a warrior who was destroyed by a puncture wound to his 'tire').
By my calculations, the Bridgestone Hoop 150/70x13 has a diameter of 21.3". The 165/70R13 would have a diameter of 22.1". The tire on my 2011 400 is worn down to the wear bars and I can just barely fit a .70" diameter rod between the tire and the back of the engine casing. The calculated radius difference between the 165/70r13 and 150/70x13 tires is .41". I don't know how much of my tire's diameter is missing though. But the larger diameter (3.9%) means the tire will spin more slowly which should improve your speedometer accuracy.
The OEM Hoop is 5.9" wide and the 165/70R13 is 6.5" wide. It would be a tight fit, but it seems like it would work.
I used a Nexen CP621 on a 650 and was very pleased with it. It performed better than the Potenza and the Formoza FD1 I have used. Unfortunately, I can not find another CP621. I do not believe Nexen is producing them anymore. I did find one CP641, but I have not mounted it yet to see how it performs. I will mount it later in Oct.
However, a 400 maybe a totally different situation.
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