The unit is inches of mercury (in/hg, or just "inches" for short), not PSI. Your understanding is correct, I'm not aware of any engine that's not supposed to idle at 20-25in/hg (but I'm definitely no expert).
Have you checked for vacuum leaks?
Any symptoms? Misfiring? Bad fuel economy or power?
i have had a few problems with my truck after i made a straight across trade for a 2001 f-250 that needed wheel bearings and registration. thought trading a totally clean ranger would be fair but i ran into troubles. problem #1 was it got a steady 15mpg, smelled like ***, ran rough and felt like a mis fire
cam synchronizer went out, replaced that and it helped run a little better.
took it to smog and it was far from passing, pulled the cats and they were 100% clear. nothing in them. replaced the cats and passed smog, cleared all codes and it now idles smooth, doesnt smell like ***, passes smog.
its a 3.0L v6 with a small 2" torsion lift with 32" bfg all terrain tires. im averaging around 230miles per tank (16 gallon tank) at 15mpg
it is now reading 14-15 on the vacuum guage. the motor is rated about 150hp which is about 40 more hp than my old ranger which was 11 yrs older and also the same motor and it feels the same speed as my old 93. i feel like 15mpg is a little low and the power is also a little low.
not 100% sure if the truck was calibrated with the tires. all i know is that in a 45, im at 45 and so are all the other cars. i calculate my miles by the trip meter and by the gallon. the whole 9. it averages RIGHT at 15mpg. during the time that my fuel pump sending unit was broken, i had to drive by the mile on the guage. i got 6 gallons and went 93 miles, on the 94th, i ran outa gas completely (before i fixed the cats and cam synchronizer)
With 32" tires and only 150hp, anything under a 4.10 rear end is going to feel like your wheels are cast from a chunk of lead. Honestly, all things considered, with 32s, you should be fairly happy with 15mpg. You aren't likely to do much better than that, especially since it only got 20 stock with that 28.5" wheel/tire combination. Even that "small" 2 inch lift is going to hurt your FE quite a bit, especially since all you have to push you around is that 3 liter.
If you want more power or better fuel economy, drop it back down and put on stock size tires. You can also get more power by re-gearing, and if you are lucky, another mile or two to the gallon if it is REALLY running too small of gears for your lift and tires. Of course, if the guy who put on the tires regeared, then checking the door tag or even the axle tag is going to do you ABSOLUTELY no good. It may be helpful to know what RPM you are at a cruising speed of 55 and 65, if you can tell me those, I'll figure out your gearing for you.
THC is right, the 3.0 is every bit as thirsty as the 4.0, despite the fact it does not deliver the power of the 4.0. Honestly though, given what I normally see on other people's rangers or Explorers, you are sitting right about where you should be with that lift and those tires for fuel economy.
ive heard the ranger has terrible mpg. so the flicker between 14-15 is normal and i should not be worried? the truck runs pretty good now that i have replaced the cats, fuel pump, and cam synchronizer. it makes sense now that my 03 lifted with big wheels feels the same power as my 93 with stock wheels
The 4.0 handles going bigger a bit better than the 3.0 because of all that extra torque it makes, but really, if you want a little more power and fuel economy out of a lifted ranger with big tires, what you really want to do is either drop in a diesel, or a v8.