Luckly I don't have a trailer and no intention to get one.

Why would that be? I can't say my engine runs any hotter after this mod :confused:
Actually, last summer I was in a mega traffic jam for hours and hours and the temperature was like 30+ degrees Celsius, the engine temp never got above halfway the gauge.
Also I transported many heavy loads (my GV is a cargo version) and found no problems with temp.

We are 2.8 CRD with 265 lb-ft of torque not the 6.7-litre Cummings diesel and pull no where near 930 lb-ft of torque plus of course. England has the highest minimum temperatures of the three and has a 13.5°C per annum, so no where near desert temp or a fraction of 930 lb-ft of torque you USA lads need.

Clearly non of you ever made a read out with Scan Tool while driving , and notis the trany temperature in lockup on the interstate with a 3200lbs trailer on the hook. Actualy a RAM cummings automatic is not advice to run in lockup with heavy trailer."stat in the owners manual" Why ? course the trany oil pump is running to slow in high gear causes slow flow through radiator = to high trany oil temp . Imagine how the seals in trany like this and why the trany blow up .
You can't even see +/- 30deg.C on dash temp gauge and it's not linear. and now +10 years later the headgasket will give up and Thermostat is stuck wide open.
Try boil the thermostate in to hot water +100deg.C and see what happens.

a Traffic jam will not course a CRD to over heat , even with a stuck thermostat you got electric fan on the radiator to keep it cool. The thermostat got a bypass and this is enough to keep it cool in idle. The problem is under load or High speed in summertime the stuck thermostat restrict the flow even i open possition and now the renault thermostat do all the work .
I do drive with this inline thermostat and it works ,was just feed up changing the thermostat every 80000km . just keep in minde when drive up hill or tow what you might reach a point engine temp is to high . (and you can't see it on the gauge).

If thermostat is working right the electric fan will never kick in under normal condition. ambiant temp. below 30deg.C. These CRD's are cool running engines and will never overheat if thermostat is working.

- not true

Most of the UK in normal circumstances will never need the ATF+4 transmission fluid heat dissipating through anything more than the transmission case itself.If however you are pulling a big trailer or camper then consider a transmission cooler upgrade / heat exchanger. If I was yucking near to 1000 lb-ft I'd certainly consider it, anyway they're not expensive, not that difficult to fit.

Just do add, I already can see that the fuel consumption is gone down a touch by having the constant stable temperature.

I see what you mean by the obstruction by the old stat, but I don't really understand why my gauge isn't showing the actual situation :confused:
I rarely get over the half of the temp gauge, and even when the gauge isnt very accurate I dont think I am in any danger zone ?

I have a manual gearbox so no problem with the transmission here.

I had a Mitsubishi Challenger for a while, the advice I got was to fit a digital temperature gauge as it only had 3 positions.
It read cold, half or overheating. Apparently the temp could be above half and causing damage, but still not reading as overheating.
I towed 1500-1600kg to Cornwall last year, and plan to do the same this year. On a couple of the long hills, the temperature gauge started to rise. It went back to half way quickly enough once the summit had been reached.
My first guess for the rise would be the gearbox kicking in to a lower gear and sitting at 3000-3500 rpm for a prolonged period. Second guess would be a partially blocked radiator. Would never have considered the Renault thermostat.
I got the auto fluid and filter changed at 110k ish, now I'm towing a bit more, I'll do it again at 155k ish.

re. Tow in high gear . It's only a good thing to kick down course then you get rpm on the oil pump in the trany and get more oil through the cooler.

Just don't hogg in high gear to much , now sure how your trailer speed restiction is but around here it's 80km/h and it to low for a heavy trailer in high gear.