1966 ralley gauges- water temp markings
 

Maybe this was answered before, but I couldn't find it. The water temperature pod has the following markings- 100 180 245. There is an unmarked hash between 180 and 245. What number does that signify? 200? 210?

I've often wonder this also....but if anyone would know, I'd think Pete Serio would. Last I knew you could reach him at www.precisionpontiac.net

((245-180)/2)+180=212.5F.
Signifies she's getting a little too hot.

That was my original thought, but the unmarked hash is not halfway between the 180 and 245. Its significantly closer to the 180. Did I mention I had a 2015 BMW 533i. It had a temp gauge with markings on each hash. That car would run constantly 245 all day long.

factory gauges have never been known for accuracy.
Padgett and I made manifolds for several different senders to "test"
under same temp,pressure etc.
I have a notebook somewhere that shows the variance.
Padgett had a way to correct with resistors caps etc but my take was to pick
the sender that was most accurate with that gauge.
BTW final voltage does matter,so make sure its close to 13V.

I'm not at home to look at my 66 GTO, so I grabbed a screenshot from one on Ebay. It looks like it's about halfway in the pic- but that's not really important.
A rough ballpark is about all I'd trust the factory gauges to anyway.
If you were so inclined, you could just plug in your perceived percentage of the range (65) between 180 & 245 and add it to 180 and call it that.

Temperature gauge, the 3/4 line?
 

The temperature gauge: the spacing on the lines is non-linear. I am sure GM did that on purpose. Just FYI) The fuel and oil pressure gauges both read linear by design; +-2%.

My tests tell me that the 3/4 line on a 65 or 66 GTO with rally gauges stands for 210/212. In other words boiling.

There is a 15# pressure cap so the coolant does not escape at 212.


In case anybody wonders the 1/4 line on those 2 years (gauge) is right around 140/150. By no coincidence the 1/2 way line (180) exactly matches the degree thermostat the V8 engines came with from the factory.

Thanks for that information. With a 15 psi cap at 3 degrees per psi, that's 45 degrees increase in boiling point, so boiling point is more or less at 255, or on the other side of the 245 mark on the gauges