Just a couple of things I have observed, perhaps some others can chime in.

Greenish yellow opaque thermostat 200 - 215 degs

Next two are with a Dorman fan and Hayden 2991 Fan clutch

Gray Soft spring (Atlantic British 2 years ago) 188.6 to 194 when moving peaked at 202 at lights. Hit 212 under load and very slow speed on a significant long steep off road hill climb at 104 F. This thermostat was very consistent, with little variability the soft spring seems to help with that. This thermostat failed as part of my head gasket issue, spring seems to be "stuck" and not opening fully as tested in boiling water it needed a poke with a screw drive to fully open. This may have been some corrosion but it I could no longer trust it.

Bearmach Black 78C current 186- 190 moving above 2000 rpm, 194 between 1200 and 1400 rpm This thermostat is more variable than the gray, moving between 186 and 194 I suspect due to the stronger spring. However it takes longer for the Disco to to reach 188.6 and as the fan kicks in the temp drops rapidly

I have an inline thermostat which will go in, however as I was troubleshooting a cooling issue I did not want to change everything at once.

 

I'm seeing similar temps with the gray OEM soft spring - 186-188 on the highway, 190-194 while moving through stoplights, and moving up to about 200 if I'm sitting in traffic with the AC on. The fan locks up and it starts to pull air and control the temps rising. This is also with the Dorman fan and severe duty Hayden clutch.

Interestingly, my Ultragauge never actually shows 192.2. It always jumps from 190.4 to 194, and then back from 194 straight to 190.4. 89C = 192.2, and I just never see that number. Seems like kind of a weird quirk.

 

@boston4 same with my bluetooth and cell phone never 192.

 

Thanks for relaying your study to us. Gotta say nothing beats the inline thermostat mod with a heavy duty fan clutch. I was running 184-185 pretty much permanently in all weather conditions. Granted I had a new radiator and heatercore also.

Damn I miss my truck.

 

I have experienced the same. I've used the stock thermostat, 180 degree, and finally the 172 bearmach. I've also used the stock fan clutch, a heavy duty type (don't remember part number off top of my head), and finally the severe duty 2991. I only used the stock fan with the stock clutch before I switched to the Spectra CF15001 on the other 2.

I was thinking of making a thread to report my experiences through the different setups, but the information is about the same.

If there is one thing everyone should know, it is:

IF YOU ARE STICKING WITH THE STOCK TYPE SETUP, DON'T WASTE YOUR TIME AND MONEY ON ANYTHING OTHER THAN THE BEARMACH 78C STAT AND THE 2991 CLUTCH.

Today while out and about it was 95 degrees here in the south and humid as hell. My cooling system even had trouble keeping up today, as this was the first time I have seen it go over 195 since I switched to my latest setup. Today I maxed out at about 205. This was during a long idle sitting in the sun and the temps went right back down after driving about a minute or so. I was still very impressed compared to other stuff I have tried. I believe I could control the temps a little better with a coolant mix containing less anti freeze still, and I may try just 10% anti freeze and the rest water and water wetter at some point, but for now I am pleased.

I have actually seen temps below 180 when at interstate speeds as well.