Holley Mechanical Fuel Pressure Gauge 26-504 | Holley offers two types of mechanical fuel pressure gauges: dry and liquid filled. The dry gauges are offered in 0-15 and 0-30 PSI increments and are budget priced but not short on quality. The liquid filled gauges are also offered in the 0-15 and 0-30 PSI ranges. They contain a liquid glycerin which dampens the needle movement and makes it m ore linear in operation. Liquid gauges are premium quality instruments which will give years of service.