Here's another post for our forum "library".
On my last July trip through the Rockies, I noticed Actia dash gauge readings of well over 200 degrees on the toughest grades. I was concerned over the reading and that the 205 degree "hi temp" idiot light (per the Cummins ISL manual) didn't come on. The factory did advise me there may be significant errors in gauge readings.
I installed the Silverleaf VMSpc system and discovered on a recent trip that at 200 degrees on the gauge, I was actually running 185. This explains why a rough gauge reading of ~215 (hot day, big grade) was not preceded by a hi temp alarm, which I've never tripped.
However, there is a not a linear error (i.e. at any given reading adjust ~ -7%).
2004 Alpine 34 FDDS
Anaheim Hills, CA