I have a B20E with a Megasquirt retrofit so I am able to read the front Djet sensor on my laptop. Normally in the summer when the car is moving the front sensor reads around 84 - 87 C and the back sensor reads 1 - 2 C warmer. The back sensor will rise about 8 - 10 C higher if stuck idling at a long stoplight on a hot day. As a result I think that the back sensor and gauge are reading within reason.

Woke up this morning to a nasty surprise of a blanket of white wet sticky stuff outside making for packed ice on the roads which the 142 doesn't do. As a result I couldn't confirm the front and back of head temperatures while driving. I did do an idling test and the front of head temperature came up to around 82 C and the back of head temperature was around 87 C. Turning the heater valve and fan on to full caused the back temperature to drop a couple of degrees; but, there was no significant change in the front temperature (bounced from 81 - 82 C). I have an electric rad fan and the fan never started up.

Since I think my back gauge is reasonably accurate and if anything tends to run a little hotter than the front of head temperature, I think the 70 C is an accurate temperature. The service manual says that the thermostat should start opening at 81 C and should not be fully open until 90 C. With a coolant temperature of 70 C the thermostat should be fully closed with no / minimal water flowing through the rad. At highway speeds with a completely closed thermostat I would expect the engine to be generating enough heat to drive the temperature above 70 C.