Gauge normally reads 1 tick above 1/4 scale with the engine operating around 180 degrees, thats just how the originals work.
Heres some resistance values for three senders 6 dollar TU5 Wells sender from Autozone, 25 dollar "correctly calibrated" Lectric Limited model 01513321, and an original GM 1513321 sender (for a Pontiac Firebird - one with the slide on connector- Camaros use a 1513462 with a screw on lug).
80 degrees: Wells at 650 ohms, LL at 573 ohms, GM at 549
90 degrees: Wells at 526 ohms, LL at 445 ohms, GM at 524
100 degrees: Wells at 429 ohms, LL at 365 ohms, GM at 409
110 degrees: Wells at 328 ohms, LL at 266 ohms, GM at 365
120 degrees: Wells at 283 ohms, LL at 227 ohms, GM at 323
130 degrees: Wells at 250 ohms, LL at 200 ohms, GM at 272
140 degrees: Wells at 216 ohms, LL at 170 ohms, GM at 227
150 degrees: Wells at 198 ohms, LL at 155 ohms, GM at 195
160 degrees: Wells at 173 ohms, LL at 133 ohms, GM at 175
170 degrees: Wells at 157 ohms, LL at 119 ohms, GM at 157
180 degrees: Wells at 135 ohms, LL at 101 ohms, GM at 140
190 degrees: Wells at 120 ohms, LL at 89 ohms, GM at 125
200 degrees: Wells at 109 ohms, LL at 79 ohms, GM at 109
212 degrees: Wells at 104 ohms, LL at 75 ohms, GM at 102
If you have a gauge with an external resistor you can change it from the stock 86 ihm resistor to a 91 ohm resistor and that will move the gauge up to 1 tick below half, at 180 degrees. I've done that, I use a potentiometer with two short leads on it, I adjust it to what i want the gauge to read and then lock it down in a ball of electrical tape so it can't change. If your gauge has the resistor built into it, then you have to live with it reading low, or get a sender that has a lower resistance at each of the temps above.
