The gauges on my 1988 V12 read consistently low. The fuel gauge never shows more than 3/4 full, even when the tank is overflowing and the oil pressure gauge also seems to read consistently low. Any ideas anyone?

My 4.0 XJS's oil pressure has always been at least 50psi at 1500rpm and above even after long runs on hot days. Recently, during long runs, if the egine is allowed to idle (eg at traffic lights) on resuming driving the oil pressure refuses to go above 25psi until over 3000 revs and falls back to 25 ...