I needed to take both Jag's on a trip from Pennsylvania to Virginia last week.
We departed PA at the same time and took Rte 81 South. We met 219 miles later at a Sheetz for coffee and gas.
The STR arrived first averaging 70 miles per hour and used 9.05 gallons
24.2 MPG. The S 4.2 arrived 15 minutes later averaging 63 miles per hour. We turned North in the S and the STR continued South. Filling up the S 434 miles later in PA with 16 gallons. 27.12 MPG. average speed was 69 MPH for the entire tripin the S
Best guess...Supercharger costs about 3 MPG on the highway.

 

I can't see the SC doing that. More likely that the 4.2 NA has VVT but that era STR doesn't. They're different compression ratios, too, which I guess must make a difference though I don't know how much.

 

Either way, for the power both of these cars can generate, the highway mpg is outstanding....

 

7MPH might make up part of the difference too. Tire pressure and the rolling resistance of the tires (R has much larger units) are surely a factor....were the cars equally loaded (weight makes a huge difference!).....

 

That's right in line with the EPA numbers. I am always grateful to be driving a 7 year old car that gets the same gas mileage as it did when it was new. Not many vehicles you can make that claim with.

 

I can repeatedly do 30-31 mpg with my 2003 4.2 V8 S-Type in highway crusing...truly amazing.

Only 20-21 on the 2001 XKR 4.0 though.

 

Both vehicles have similar Continental tires and TP of 32 PSI. Loading was about the same, I drove the STR with one passenger to VA and then returned in the S with one passenger. I did'nt romp on the gas either way, but did not try to hyper mile either, just normal highway driving with not much traffic. I tried to keep the two cars as equal as possible without being extremely experimentally accurate. I thought 3 MPG for an added 100HP Supercharger isn't too bad. (That and another $13K sticker price!) The six speed transmission really helps with the MPG clicking less than 2,000 RPM at 70 MPH

 

At 55 mph I bet the STR will do the same or better mpg than the N/A 4.2; due to more power moving a mass more efficiently.

 

The cars weigh almost exactly the same (and are the same as regards drag etc, apart from the marginally higher rolling resistance of the fatter STR tires), so will need the same power to maintain speed. Which generates that power more efficiently? (Apparently the NA.)

 

Our S-type gets better gas mileage than our STR. Of course the S-type is a 4.0 compared to the 4.2 on the STR but the S-type averages 4-6 mpg better in all conditions.

What was interesting though was when we had the '10 XF 5.0 supercharged it got almost exactly the same mileage as our '09 XF 4.2. Both of the XF cars got substantially better gas mileage than the STR.

That new 5.0 engine is really impressive.

 

I can understand the GDI (gas direct injection) 5.0 doing well but can't figure out the XF 4.2 unless it's NA? In which case it's much the same as the S-Type 4.2 NA.

 

Exactly...which brings us back to: "I bet the STR will have the same if not better mpg at 55 mph than the N/A due to moving the mass more efficiently". Many Ford Mustang Cobra owners report suprisingly good mpg's, equivalent to or surpassing the N/A models...when not on the gas that is.

 

I think that's wrong both in practice and in theory.

 

10+ trips to St Louis (350 miles each way) driving 80-90 mph, my STR averages almost 26 mpg.
One thing I do on the highway to get good mileage is I never use the cruise control. It's too abrupt applying power on hills and kills the mileage.

 

can anyone hold 55 on the hwy?

 

I like my mpg's in city driving....my average speed is between 35-40mph w/ 21-24mpg.......no true highway trips.....I dont beat on her too much.....I daily drive it, but I lay into her every once in a while to let her know I still love her.....

great lil daily driver to date!

 

I'm lucky to see 20mph in my daily slog. A few WOT romps compensate for any light throttle cruising. A few highway trips have returned about 25 mpg. I'm happy with that. Both of those numbers are better than what I would get with my old X type 2.5.

 

Has anyone tried nitrogen in their tires yet?
For a more consistent tire pressure?

 

hahahaha, good wind-up!!

I have nearly 80% in mine already

 

I was thinking about it but the price the place was charging was for free fills anytime they went low. I was moving so much, to pay that kinda price for free fill ups for life when I'll only live near the place for about a year so I didnt do it. They could only be filled at there location. They did have a nice program though if you were gonna live near them a while. If your tire went flat they said you would let the tire empty and then fill it with air, bring it to them whenever you can next and they would purge out all the air and refill with nitrogen, anytime as many times as you need. I have an air compressor so I check and monitor my tire pressure pretty regularly. Once I actually settle somewhere for a while I might look into it again.