The equation has more variables than than.

First, how significant a MPG gain can you achieve, second how expensive your tires are.

If decreasing rolling resistance (and traction proportionately btw, as an aside for us with bad weather), yields 3mpg. (dont forget city driving, where you are accelerating / decelerating).

Now my S-Type in real world conditions = 20-21mpg. if I can boost it to 23-24mpg= 100 gallons of fuel savings per year. Multiply that by 3.30 or so (for premium) = 330 a year in decreased fuel costs.

However, my tires cost around 1000 a set. (Staggered 18s). If i wear them out 1/3 quicker - say 22.5k instead of 35k. Cost of tires per year went from 428 per year to 666 per year. So of the 330 of my fuel savings, 238 is consumed by increased tire wear. so savings is around 90 dollars per year. Now if the cost of gas goes up, then the savings will be greater. OR if your S-Type uses cheaper tires like stock 16-17" wheels with greater treadlife, the savings again will be higher. BUT if my fuel economy increase is less that 3mpg avg, then there is no savings at all..

On another note, the tires just struck me. Maybe that's the reason I can't get the mileage the other v6 guys are claiming? Cause I have 245/40/18 front and 275/35/18 rear tires from the S-Type so I'm working the motor harder?

George

 

I think it's appropriate to mention something about over inflating tyres to reduce rolling resistance:
Yes, overinflating any vehicle's tyres above the 'correct' figure will result in less rolling resistance and thereby less fuel consumption, of that there is no doubt. But there is a penalty, and one that's invisible until it rears its ugly head. BRAKING DISTANCE. The amount of tyre actually contacting the road is reduced, as exemplified by the wear of the tyre down the centre of the tread rather than across the whole of the tread. This phenomenon is likely to show up in its extreme form under difficult conditions, e.g. when the road is wet and/or greasy.
Now you have no idea just when you are going to have to depend for your life (or someone else depending on you for their life) for braking distance. It may never happen, and then again it may happen tomorrow. It might involve a child, it might be at 70mph on the motorway when a wagon has had a burst tyre in front of you and is in the process of jack-knifing, it might be in a bad fog, it could be anything.
I'm not one for dishing out admonishments as readers of my contributions will know, but suffice it to say, although it's not often applied in UK, improperly inflated tyres are covered by law, and can attract penalty points on one's licence.
Your tyres are the only contact with the road, something to bear in mind.
Leedsman.

 

Thank you for posting that. I was going to say something as well but now you're the skunk at the picnic ...

You can ruin tires and compromise safety by over inflating them as well. You may also be reducing the load rating of the tires. A blow out and serious crash will obviously negate any fuel savings. Follow the manufacturer's guidelines.
Call up them up and ask what the acceptable range is. They didn't leave MPG on the table for no reason. Some of that "may" be ride quality but you should find out.

Yes George, wider tires are less efficient but grip is probably import to you as well. I don't know if it accounts for all of your mileage issues though. I'd find that hard to believe. There might be some lead foot factor as well no? In the STR I know I'm guilty of that at times but not when the cruise is on.

They use skinny tires for all the straight line high speed runs.

Skunk #2

 

Useful idea - but who to call? Unless you've the actual tires recommended by the car maker (here, jaguar) then they'll be little help, but will the tire maker have an answer?

Anyone know why Jaguar changed the tire pressures dramatically for the STR going from 2003ish to 2005ish? Were front 28, rear 32. Changed to 40 for both.

 

Call the tire manufacturer and the the regional Jag tech support. Yeah I know it's not going to be easy but it can be done. A few years back a friend and I were taking our cars to an open road race in Nevada and we wanted qualified information from Michelin. With a bit of work we got to the right guys.

I wasn't aware of that regarding the 2003 to 2005 change. My tires were under inflated when I got the car. It did ride better that way but I changed it immediately. Hmmmm. Maybe a dealer suggested this to the PO of my car. I would like to know more about that one.

 

Thanks. It handled poorly when I bought it, with the original (low) values (but was probably a softer ride), so I stuck a few psi more in and handling improved vastly. Still lots under the 40/40, however, and I wonder if I've "enough" pressure - though it's hard to define what "enough" means here! Looks like I have some calls to attempt.

 

I know...I know...it SHOULD make a difference...and maybe if I swapped tires/wheels in total I'd have one good mileage S-type and one fuel sucker....but the 05 3.0L has 245/40 ZR18's whilst the 03 3.0L sports 225/55X16's....both relatively new Michelin Pilot A/S sport....no appreciable difference in fuel mileage...if anything..the 05 slightly better...

 

I completely agree with both of you regarding compromising traction when over inflating tires, however I think that the Jag recommendations of 28 and 32 PSI front and rear are a bit TOO low and designed to tune the car towards ride comfort, and possibly add a bit more grip. There's a "safe" operating range for each tire, before we get to true "over inflation" and 34-35 psi is normal for these tires. And on the upper end but within the manufacturers guidelines. It will not compromise structural integrity of the tire.

I'm not saying dump 45 pounds into it and have it ride like the flintstonemobile. Someone with an 05 - can u check your door card for pressure recommendations - I find that 40psi recommendation somewhat suspect, although lower profile tires generally run increased air pressures.

As far as my mileage "issues" I don't think tires are the magic answer. Yes I do have a bit of lead foot syndrome, well as much as you can have with the 3.0 v6. I have never used cruise control, because generally speaking I don't see many driving situations where I can really use a consistent steady speed. However I do think the car does have a bit of a baseline mileage penalty going from 225/55/16 tires on all four corners, to 245/40/18 tires in the front and 275/35/18 tires in the rear. Yes there is added grip, and no, I wouldn't go back, but it may be one piece of the puzzle.

Take care,

George

 

Hey George,

The current plan is to take a short highway trip this weekend - approx. 150 miles each way. I'll do a fill-fill calculation and let you know what it is, and how it compares to the computer readout.

Ben

 

It won't be accurate at first unless you are doing the same kind of driving as before the trip. If the return leg is like the outward leg then you'd hope the mpg the car figures for the return leg would match fill-fill quite closely.

 

Hi George,

i recently found what could be the problem with the disappearance of fuel in our STR.
Last week i removed all module, electrical, battery, GPS and CD changer from the trunk to install Dynamat. when i place everything back the fuel gauges did not work, I know i have about 1/3 of tank (from what i remember before the work done)

So i unplug and plug everything again, suddenly the fuel gauge is showing 2/3 of gas in the tank.... ( this only lasted until the next time i restart the engine) then i revert back to 1/3 of the tank.

Do you think this is due to the fuel module?
i was thinking of replacing it with a new one
(this is the example of the module.....http://cgi.ebay.com/ebaymotors/ws/eB...K%3AMEWAX%3AIT )

Do we need to program the module?

thanks

 

Honestly, no I don't think there's anything wrong with it.

Take a look at the following diagrams -

first the fuel sender diagram

The fuel level senders go to the rear electronics module. Regardless of what engine is in the car, 3.0, 4.0, or 4.2 na or SC. They are basically variable resistors, 20ohms = empty, 140ohms = full. My guess is that MAYBE the potentiometers get worn, and their accuracy fades over the hundreds of times we get gas, then empty out the tank. Actual fuel level is sent to the instrument cluster over the SCP (serial corporate protocol level). I suppose we could test the senders for proper resistance level, and ADD or subtract resistance (by placing resistors in series or parallel respectively) to fine tune the senders output.

Now, onto the module you posted.

It's only present on an STR, and it's purpose is to send current to and control the 2nd pump (only), the primary pump is still controlled by the rear electronic module. Only the STR has dual fuel pumps to the engine. All the other S-Types have one pump to the engine and a crossover pump to send fuel from that side of the tank to the side with the pump fueling the motor.

If you put the dash in Engineering test Mode, you can see a digital approximation (0-254) of the remaining fuel of each side of the tank in the instrument cluster.

If you look at the two different fuel systems, you will see the difference. The fuel pump relay on an STR has two fused outputs, one to the Rear Electronics module, one to the Rear Fuel Pump 2 module. The fuel pump relay on an S-Type only has one fused output to the Rear Electronics Module.

So, all it does is control the 2nd fuel pump, it has no bearing on fuel level readout.

Hope this makes sense..

George

 

Don't know if this helps, fuel senders were almost always the potentiometer type, actuated by the float, a bit like a toilet cistern. The resistive 'element' was wiped up and down by a wiper arm as part of the float mechanism. The ones I saw were of the "wirewound" type, where the resistive element was very fine resistance wire amounting to a few hundred ohms. So when tank full, wiper near top of element, arm goes down as the float lowers as petrol is used, wiper down to bottom. The element was wired basically across the +12volt supply from ign. switch. So full tank, arm and wiper up, max. volts. Low tank, arm and wiper down, lowest volts, and pro-rata. The output from the wiper arm with its variable voltage is then fed via a suitable limiting resistor to a common milliamp meter on the dashboard having a 'full-empty' dial instead of milliamps. The value of limiting resistor was chosen to make the meter read nicely on 'full' with a full tank.
The problems were always the tank sender. The fine resistance wire would wear and evenually break somewhere along its track. This usually meant the meter indicated either full or empty and no-where in between, except when the wiper was actually covering the break. The assembly could jam somewhere along its travel again due to wear (no lubrication due to petrol being around), so you might get erratic fuel level indication, or a refusal to indicate correctly due to random sticking of the assy.
If I were designing an indicator nowadays, I'd use a pressure sensor in bottom of the tank. Then you could forget problems as these never go wrong, esp. with the small forces from a petrol tank.
Leedsman.

 

It looks like the Ole Jag is back to getting similar gas mileage as before. In my combined city/highway run around town is around 23.5-24.0 mpg at normal speeds. It appears that my mileage drops appear to be from mainly the cold weather. It was back to 70 degrees today in Central Florida.

 

Quite so FloridaJag, it's a common feature of cold weather. My diesel S-type gets pretty thirsty in cold weather, so much so, I have fitted an engine sump pre-heater (I shall be providing a full assessment shortly in these columns). The cost of my pre-heater is minimal. BTW, sometimes the glowplug system is called a pre-heater, so don't get confused between the two. Glowplugs only heat the combustion chambers. And yes I know there are glowplugs that heat the inlet air, but these are not common, and usually vaporize some diesel fuel at the same time.
Leedsman.