Temp gauge reads low

My temp gauge has always read low and after trying two sending units and replacing the IVR, it still reads low.   I'm using a Standard TS37 sending unit and I suspect it's not in the right range for the gauge. What sending unit is correct for the Rallye Pack gauges? I've checked the wiring, grounds etc but the gauge still sets at the low make when the engine temp is 195°. The oil and fuel are fine so this seems to be just a temp issue. The sender ohms 3500 when cold and 660k when hot. If I put my test light in place of the sender, it reads almost to the bottom of the normal range. I want to fix this while I have the cluster out but the sender seems to be the issue.
I replaced the tach board in the tach yesterday with one from Premium Dash Decals and it works great now. It was off by about 900 rpm before and it's within 50 now which is good for these old tachs.

What a pita problem this was. I found out my ohm meter was lying to me so I replaced that. After trying five different sending units none of them would be in the right range but I did find one that has the gauge reading higher so I'll install a resistor to dial it in to where it needs to be. I also replaced the bulbs with led's which turned out nice. So good luck to anyone who needs to replace their cooling sender!

I thought I was slick. Living in balmy Tucson, I replaced the perfectly good 195 thermostat with a 160. Heater is less of an issue then running hot with the A/C on. Now, it is so cool running ( 30 degrees below stock) that the gauge reads barely on the first "tick" mark on the dash gauge. My inferred tester says all is fine.... 161 degrees. Apparently the gauge is "centered" for 195. Oh well.

Your question initially unless I read it wrong referred to the Rallye Pack gauges. This package was an option and was installed or could be installed after the fact on the car. They did indeed become different over the years.
What year are you referring to as to the car and also as to the Rallye pack units.

This is one of the two I am familiar with and consisted of four gauges, this one a Clock, A vacuum gauge, an oil pressure gauge and an AMP Gauge. It was used with the Spirit based cars and others. The other option consisted of a heat gauge replacing the clock and was used with the Pacer and others. And the sending unit was different than OEM when used in other cars, such as the Hornet for example. When used with the Hornet it came with cosmetic tape which covered the OEM Heat gauge so it could not be seen and looked like part of the dash. So I understand.
Early Javelin just had two gauges, and I would have to look it up in my 1970 TSM to refer to them.
As to the accuracy of any gauge of this era? I really don't think any of them were accurate and no two were exactly alike.

According to my 1974 TSM that is not a RalleyPac gauge package, but a Standard Javelin instrument panel with the optional 140 mph speedometer. Probably OEM on the Javelin AMX. The ralleyPac gauge package for 1974 consists of a two gauge package ( displayed on page 3-45 fig 3-61 and consists of an ammeter and an oil pressure gauge.
The temperature gauge sensor resistance information is on page 3-52 and while it indicates what the OEM gauge should read as a function of sensor resistance, there is no such thing as a RalleyPack Temperature gauge for 1974, thus no separate sending unit.