The stock gauge in my 70 Charger scales up to 80 psi. If I install a sending unit with the 100 stamped on the side, will my gauge still be fairly accurate? I think that the 100 on the sender indicates it should be used with a gauge with max scale of 100.

Thoughts?

Me thinks you may not be correct on that stamping,(maybe it is a 100 ohm sender ) contact the manufacturer to get the straight skinny on that part

Good question. My machine shop guy is a Ford man, and he remembers Ford had two sending units, one said 80, and the other had 100 on it. I'd like to know myself. New sending units today don't have that on them.

I have been struggling with this for a few months, trying to find a correct 80 psi sending unit. Your gauges will not read correct with a 100 psi unit. I'm sure there is some sort of conversion you could do mathematically to calculate the actual reading of the gauge, but that's beyond me.

I was thinking something like this - 100/80 = 1.25. At 60psi divide by 1.25 = 48 psi. So, your 80 psi calibrated gauges with a 100 psi unit reading 60 psi, is actually 48 psi due to the calibration difference. Again, I'm not sure my math is right.

You can get a manual gauge & plumb it in with a T fitting, so your sender & the manual gauge are both in operation at the same time to compare them.

Good idea. Also neither the sending units nor the gauges are calibrated/accurate. A mechanical gauge to check is a good idea. Also, the electric gauges are slooooow. I have a fully functioning factory gauge on my Challenger and also run a mechanical to get faster response. If I loose OP briefly on the road course, I want to know!

I have an electric Marshall gauge that reads faster than a mechanical Autometer gauge.

Gotta remember the older Ford vehicles had dash gauges that read...L(low)Normal(which was most of the gauge sweep)and H(High)Nobody really knew what any of that meant but they had full "instrumentation". Like most electrical gauges....highly inaccurate.

Most of the Mopars I've owned have similarly marked gauges, oil pressure and temp, and amp gauges with the high end and the low end with or without numbers, a center mark, or a "normal" range. You have no idea what any of the marks are suppose to mean. I don't hold much faith in any factory gauges. Gene

If U want total pinpoint accuracy, U better install a quality aftermarket guage..
The OE guage is fine for approximating the amount of oil pressure, will indicate the typical fluctuations when the oil is cold or hot as well as increasing with RPM..
In my 70 RR, I have both the ralley dash oil pressure guage and a mechanical guage mounted directly to the engine, not piped inside.. This gives U the best of both, OE guage for basic tracking while driving and EZ access to the mechanical guage for more accurate pressure readings/troubleshooting/monitoring...

Just my $0.02...