Before I launch into this, I'd like to point out that all observations made here are purely that - observations, with no real applied scientific method behind them. However, there may be some merit to them; YMMV (no pun intended). Onwards...
Some background: I never really got much better than 14mpg on average in my '01 Sport (I6, NP232, AW4) in mixed town and freeway driving. A couple of days ago, it was replaced with a 2000 Limited (I6, NP242, AW4, Up-Country and Towing packages). My goal over the past couple of days has been to see if I can improve on that 14mpg figure without resorting to major changes such as different exhausts or remapped computers.
The Limited should weigh more than the Sport given the extra equipment it carries: tranny cooler and rear tow receiver, skidplates, centre diff, tow hooks, a buttload of power toys, fog lights, cruise control, and a roof console. Both vehicles are on 225/70R16 tires with 3.55 final drive ratios. Since the body styles are pretty much interchangeable, we'll assume aerodynamics remain the same. The Limited can run its transmission at lower temperatures than the Sport did leading to slightly improved efficiency, but the Sport had a freer-flowing air filter (Fram Airhog) than the Limited's stock item. In any event, the second assumption in all of this is that the gains relative to each item cancel out the other when comparing both vehicles. If anything, the odds should be stacked in the Sport's favour due to its lower weight.
The two items I used in all of this to try to gain an improvement on the Limited were the cruise control and instant fuel economy readout on the roof console (yup, they're renowned for absolute accuracy). The Sport didn't have either of these items, so I'm relying on my driving habits and gas receipts with noted mileages at each fillup to get that 14mpg figure. The theory is that with judicious use of both the cruise control and economy meter, the 14mpg figure can be bettered.
Filled the Limited on Tuesday with 18.5 gallons of 87-octane Chevron gas from the same station I'd fill the Sport at. The DTE (distance-to-empty) indicator gave an initial range of 283 miles. With a 20-gallon tank, this compares favourably with the 14mpg average figure returned by the Sport.
After two days of mixed freeway and street driving, I'm just under the half-tank mark and am showing 153 miles to empty on the DTE after 183 miles of travel - adding the two together, this totals a tank range of 336 miles, or 53 miles (19%) over the DTE estimate. Average fuel economy is (IIRC) 16.2mpg, an improvement of 2.2mpg overall - or approximately 16% better, which bears out the DTE readings somewhat. With greater weight on the Limited, why is this when overall driving conditions were much the same?
What I figure is this: use of the cruise control is eliminating over-throttling. By this, I'm referring to what happens when you put your foot on the accelerator to maintain a constant speed but are actually introducing more fuel into the combustion chamber than is necessary to maintain speed. To simplify, the cruise control is better at metering fuel than your foot is - so the extra fuel you're stomping into the cylinders is being burnt without producing significantly more power.
Now, here's a caveat to all of this. The cruise control doesn't operate below 35mph - so improving town economy was done solely through my boot and observation of the instant economy indicator. The worst figures came under initial acceleration (typically 4-7mpg, though 2mpg happened a couple of times under hard take-offs), but levelled off around 30mph at approximately 20-24mpg on a flat. Obviously, the only way to control these numbers is by watching the economy indicator and trying to get them as high as possible.
In freeway driving, instant figures were sampled with the cruise on at 65, 70, and 75mph on fairly flat roads in five separate cases. Each stretch of road was driven at the given speeds. Numbers averaged out to 25mpg, 24mpg, and 23mpg respectively.
Lowest observed fuel consumption figure: 1mpg, stomping hard out of a driveway with the wheel turned right. Highest: 96mpg, at 70mph on a downhill stretch of the 14 (this will mean something to locals
). Obviously these are extreme numbers and shouldn't be taken seriously, but there are a couple worth mentioning:
- At 38-45 mph on level surface streets, 36-40mpg was a fairly common ocurrence with cruise on.
- At 70mph on level freeways, 26mpg was regularly seen with around 30mpg being possible - again with cruise on.
What this suggests is that poor XJ fuel economy largely comes down to over-throttling for the given conditions. It's not so much that the engine is fuel-inefficient as the throttle probably isn't sensitive enough in terms of power produced for given pedal travel.
Anyway, apologies for letting this run as long as it has - it just seemed like other people might find this interesting, even if it isn't necessarily telling us anything new. Draw your own conclusions as to its merits or otherwise.
Some background: I never really got much better than 14mpg on average in my '01 Sport (I6, NP232, AW4) in mixed town and freeway driving. A couple of days ago, it was replaced with a 2000 Limited (I6, NP242, AW4, Up-Country and Towing packages). My goal over the past couple of days has been to see if I can improve on that 14mpg figure without resorting to major changes such as different exhausts or remapped computers.
The Limited should weigh more than the Sport given the extra equipment it carries: tranny cooler and rear tow receiver, skidplates, centre diff, tow hooks, a buttload of power toys, fog lights, cruise control, and a roof console. Both vehicles are on 225/70R16 tires with 3.55 final drive ratios. Since the body styles are pretty much interchangeable, we'll assume aerodynamics remain the same. The Limited can run its transmission at lower temperatures than the Sport did leading to slightly improved efficiency, but the Sport had a freer-flowing air filter (Fram Airhog) than the Limited's stock item. In any event, the second assumption in all of this is that the gains relative to each item cancel out the other when comparing both vehicles. If anything, the odds should be stacked in the Sport's favour due to its lower weight.
The two items I used in all of this to try to gain an improvement on the Limited were the cruise control and instant fuel economy readout on the roof console (yup, they're renowned for absolute accuracy). The Sport didn't have either of these items, so I'm relying on my driving habits and gas receipts with noted mileages at each fillup to get that 14mpg figure. The theory is that with judicious use of both the cruise control and economy meter, the 14mpg figure can be bettered.
Filled the Limited on Tuesday with 18.5 gallons of 87-octane Chevron gas from the same station I'd fill the Sport at. The DTE (distance-to-empty) indicator gave an initial range of 283 miles. With a 20-gallon tank, this compares favourably with the 14mpg average figure returned by the Sport.
After two days of mixed freeway and street driving, I'm just under the half-tank mark and am showing 153 miles to empty on the DTE after 183 miles of travel - adding the two together, this totals a tank range of 336 miles, or 53 miles (19%) over the DTE estimate. Average fuel economy is (IIRC) 16.2mpg, an improvement of 2.2mpg overall - or approximately 16% better, which bears out the DTE readings somewhat. With greater weight on the Limited, why is this when overall driving conditions were much the same?
What I figure is this: use of the cruise control is eliminating over-throttling. By this, I'm referring to what happens when you put your foot on the accelerator to maintain a constant speed but are actually introducing more fuel into the combustion chamber than is necessary to maintain speed. To simplify, the cruise control is better at metering fuel than your foot is - so the extra fuel you're stomping into the cylinders is being burnt without producing significantly more power.
Now, here's a caveat to all of this. The cruise control doesn't operate below 35mph - so improving town economy was done solely through my boot and observation of the instant economy indicator. The worst figures came under initial acceleration (typically 4-7mpg, though 2mpg happened a couple of times under hard take-offs), but levelled off around 30mph at approximately 20-24mpg on a flat. Obviously, the only way to control these numbers is by watching the economy indicator and trying to get them as high as possible.
In freeway driving, instant figures were sampled with the cruise on at 65, 70, and 75mph on fairly flat roads in five separate cases. Each stretch of road was driven at the given speeds. Numbers averaged out to 25mpg, 24mpg, and 23mpg respectively.
Lowest observed fuel consumption figure: 1mpg, stomping hard out of a driveway with the wheel turned right. Highest: 96mpg, at 70mph on a downhill stretch of the 14 (this will mean something to locals
). Obviously these are extreme numbers and shouldn't be taken seriously, but there are a couple worth mentioning:- At 38-45 mph on level surface streets, 36-40mpg was a fairly common ocurrence with cruise on.
- At 70mph on level freeways, 26mpg was regularly seen with around 30mpg being possible - again with cruise on.
What this suggests is that poor XJ fuel economy largely comes down to over-throttling for the given conditions. It's not so much that the engine is fuel-inefficient as the throttle probably isn't sensitive enough in terms of power produced for given pedal travel.
Anyway, apologies for letting this run as long as it has - it just seemed like other people might find this interesting, even if it isn't necessarily telling us anything new. Draw your own conclusions as to its merits or otherwise.
Last edited:
