I have owned this vehicle since December. When we bought it, we thought it was getting about 26-28 mpg. It has been down at 21 for the last few months. We are also getting some engine codes (Cat code P0420 that could be Cat or engine not burning gas completely, or oil getting into exhaust...)

I ran some RXP through it (3 tanks) and it jumped up just a bit... it got about 24mpg on the last two tanks.

What should I really get if all is well. It had 76000 miles when I bout it and it now has 91,000 miles.

Thanks for your input. What is the real expected gas mileage on the 3.0L if it is driven reasonably mildly?

 

i dont know i have a 03 3.0L v6 and i am getting 16.8-17.2 avg. on my trip computer it has 89k miles on it. no tune up yet. all have done to it is put a k&n cone filter on it. was getting better mpg a year or two ago. when i picked i up with 32k on it i was getting 21-23 avg. i hope when i put new plugs in that it will jump up back to were it was. all my miles are 50/50 city-hwy

 

Hi Bobby181,

I have a 2000 3.0L S-Type that I am the original owner of. I used to average about 23-25 mpg per tasnkfill when I first got the car. For last 18 months I have become more interested in maximizing the fuel economy of the S-type and now consistently average over 28 mpg/ tankfill with a personal record of 36+mpg/tankfill during a road trip to Vegas this spring.
It sounds like you need to get those codes squared away first to get back to your previous level. ALso make sure your tires are at least properly inflated and that you don't have a whell dragging (brakes, whell bearing issues)
Since becoming interested in hypermiling I have modified my driving as well as done some minor tweaks to my car. I have posted my aero mods previously on this forum ( grille block, front tire deflectors and passenger mirror removal) and I have increased the tire pressure to 42 psi all around. I recently added a WAI ( warm air intake ) and am stilling considering adding more undercar streamlining to reduce aerodymanic drag.
While these mods have been beneficial, the biggest gains are from modifying driving behaviour ( maximize coasting, avoid braking, reduce speed, anticipate stops and slowdowns, minimize idling, etc. ). We are fortunate to have excellent instrumentation in our cars and I have found the trip computer's instanteous MPG and MPG average indicators to be pretty accurate and definitely effective ( less than 1 mpg optimistic). All this has lead to my current 28mpg/tankfill status for mixed commuter driving with high tankfills of over 36 mpg. I'm confident I'll soon be getting over 30 mpg/ tankfill shortly.

Let me know if I can be of further assistance.
Best wishes VMV

 

Particularly while trying to figure this out I'd put in a genuine Jag air filter, clean the MAF (do a search) and be really careful to check there are no intake air leaks.

I'd have changed the oil to the right type and used a genuine Jag oil filter. Likewise for the fuel filter, most likely.

If I could, I'd find another owner of the same car and try to compare them in detail: visually (engine etc), driving the same route (for mpg, rpm, etc) and preferably also OBD readings for the obvious items (google is great).

(With an STR this is a tough plan as there are so few of them.)

Obviously, get hold of JTIS and spend time studying it.

(In case Jaguar are reading: please emulate Ford's [sadly only occasional] policy and release information freely, but still under copyright if you wish.)