x10 on the a Cummins stat. Don't run anything else. I've had a napa fail on me, not a fun issue.
I run a 180 stat and haven't noticed any mileage dif myself. More then anything I wanted to keep things cool after my over heating issues with the napa stat.
Once up to temp It runs 180 to 190 all the time. Never cooler never hotter. Lots of hills here so I feel it has a good temp all the time.

 

For summer time I ordered a 180* 24v stat for my 12v by accident. It didn't fit well and it leaked. Proper 12v stat fixed that up. For winter, I wanted a 190* but 6 auto places I phoned (including dealer) said there is no option for 190* on the 12v. I did find one on the net but it was $75. Soooo..... I just picked up a 195* STANT thermostat for an early 70's 440 big block. I drilled one 1/16" hole in the flange and installed it. Out here driving empty at -5c it won't even get to 190* with a cardboard in front of the rads and one in between the air-air and engine rad, but it does run warmer than 180*. If I'm loaded or pushing a good headwind, engine runs 190*, nice and warm and no temp surge - ever.

 

Interesting. Cant answer that one. Why are you after the 24v thermostat anyway? Cant find the temp you want for a 12v?

 

12 valve stat is Cummins p/n 3928499,
24 valve is 3935811
At least the number is different.

 

The 24V called for a 190F thermostat.

I would not use any but a Cummins part on this one.