I was reviewing the 8-speed as used by the V8 F-type. By using 8 speeds instead of 6 speeds, Jaguar was able to move the gear ratios closer, where 6th gear is 1:1 in the 8sp tranny. That appears to be the reason for the F-type to turn-in acceleration times that are significantly better than the more powerful XKR with nearly the same weight and even quicker than the 2014 Vette that has a better power-to-weight ratio but, wider transmission ratios.

So, the 8 speed definitely contributes to "speed" as in acceleration, assuming that the same ratios are used in the XF variations. Yes, it also provides significantly lower freeway cruising RPMs (1500 vs. 2000 in the 6sp) which improves the freeway MPG significantly. I was getting 32MPG+ with a loaner XF 3.0 with the 8sp. Same exact roads and distance, my V8, 6sp XF was getting 26MPG.

I got two 2012 Jaguars with 6sp and wish I would have waited for the 2013s with the 8speed. It is a great improvement, all around.

Albert

 

texavora, I bought my 2010 XFR with the intention of it being my daily driver. The XFR is a great car....just asking for trouble all the time....the throttle response is intoxicating. I love this car. But, I found that I would grab the keys for my Lincoln LS (Jag S Type) when I went for local runs. I ended up getting a 2012 XF Premium (5.0 L NA) which is now my daily driver. The XF Premium is a very capable car, but not an XFR. None the less, I believe the XF 5.0 is a better daily driver. The XFR sits in my garage and is used less often, but I have no intentions of letting it go. It is an special mission car....a super sleeper if there ever was one. This car can provide plenty of stop light surprises. It is a thrill to drive this animal. And, yes, the slight body alterations and 1/2" lower stance vastly improve the eye ball effect on the R. With all that said, I gravitate to the XF keys for most daily drives. Way too bad the 5.0 was replaced by the 3.0 S/C.....I significant downgrade with minimal, if any, fuel economy gain.

 

First I like to say that "your the Man".. The best I could get out of XFSC on a good day with the lightest of right foot was 21 MPG.. I assumed that with the two extra was the reason, I was see 2 to 3 more MPG with my XFR and that at a average speed of 70 MPH. But I didn't mean to say that the two extra gears didn't nothing at all for speed. I believe most company are looking to get more MPG's out the engine thats the reason for the extra gears.. More speed is a side benefit. The tree huggers demand more fuel efficent cars for todays market. I'm waiting to see the day when Jaguars will be electric with no gears.. The day is coming, wait and see.. IMHO..

 

The 5.0 was about 10% more fuel efficient because of the direct injection so the XFR with the 5.0 SC was actually more efficient than the 4.2 SC.

The 3.0 V6 SC replaced the 5.0 NA V8 in North America primarily for the AWD.

The fuel economy advantages of the smaller engines mean nothing in North America except to the US government.

 

Wow! I am truly surprised that the best you could get out of the SC is 21MPG. Now, that I am thinking back, some posters here had reported pure freeway MPGs in their SC cars close to my V8 XF and XJL, which are in the 25-27 MPG ranges.

I have gotten the very same mileages now with both the V6 XF and the V6 XJ, both dealer loaners, over the exact same 40+ miles of freeway stretch. To be entirely accurate, one way from my East SF Bay work location to the dealer both times with both cars I got 32.7 MPG but, in the opposite direction I got 2 MPG less. All driving are at 70MPH. The difference was that there were small hills but, the freeway was ever so slightly uphill from the SF Bay elevations to my destination.

Yes, I know of those coming days. I'm fighting it which is why I purchased the two 2012 cars with the V8 engine. Now that I have repeatedly driven the V6, I have absolutely no complaints about them, decent power, super economy, good throttle response etc. but, after driving them and getting back into my V8 car, I could tell the difference and I like the difference. But, I would love to have the 8-speed tranny in my cars.

Albert

 

[QUOTE=axr6;793905]Wow! I am truly surprised that the best you could get out of the SC is 21MPG. Now, that I am thinking back, some posters here had reported pure freeway MPGs in their SC cars close to my V8 XF and XJL, which are in the 25-27 MPG ranges.

Highway fuel economy is a direct function of the vehicle frontal area x the coefficient of drag (Cd) and also a function of speed to the square power. For example, with the same engine/trans compared to 70 mph fuel consumption is 10% more at 73 mph and 15% more at 75 mph. That would be a respective decline of about 2 and 3 mpg for a typical XF. Far greater going from 65 to 75 mph increase consumption by 33% or about 6 mpg less. When I'm driving 80 and someone else is driving 70, I'm using 30% more fuel. I note about a 2 mpg sacrifice with my XFR vs my 5.0 natural XF.

Speed is all important. Comparing highway fuel economy on an anecdotal basis can be very misleading.

Around town fuel economy is much more a function of vehicle weight and how frequently the car stops and starts. At a steady state of 35 mph a typical XF would get very high economy, maybe 40+ mpg. But no one drives like that. The brakes eat up the fuel economy.

My around town driving is limited to very repeatable driving circuits. My 5.0 XF gets 17.1-17.2 mpg...consistently. If I drive my XFR the same way I get about 15.8-16.1 mpg....unfortunately it is often less due to that lovely throttle response. Interestingly, several days of driving a 3.0 S/C 4 WD loaner resulted with 17.2-17.3 mpg. I'll keep the 5.0 natural.

Exact fuel economy comparisons are problematic for the above and other reasons. I have recently read an article in one of the car magazines that the factory MPG computers in cars all read higher than actual....3-6% high. They are programed to lie so that you feel better, but that's another story.

With all that said, I'm not a fuel economy expert.

 

[QUOTE=Steamer22;793982]Yes, all valid points. That is why in my above comparison I ran the same models, the exact freeway sections, same traffic conditions, same speeds, just to get the most accurate comparisons.

As to how much the factory fuel display is "optimistic"?. I had run 3 tests (actual fill-up gallons divided into trip mileage) vs. factory software MPG display and found that both my XJL and XF were about 0.5 MPG optimistic with the average fill of 15 gallon fuel. That would be 2 percent off from my typical, mostly freeway driving MPGs of 25-26. The worst tester-proven deviation I have come across was a 10 percent optimistic read from a first generation Toyota Prius vs. the actual fill values. (45 actual MPG vs. 50 displayed). I guess, given the flexible fuel tank of that car, Toyota felt that they could "fudge" the numbers without the threats of many people discovering it. But, could not trick the pro-testers.

Now, to get the best MPG, we'd have to follow the conclusions of "MythBusters", the TV series. They realized a 30+ percent instant MPG improvement if they followed a big trailer-truck at the distance of exactly 10 feet; not closer, not further. Don't try this on the roads folks, it might end badly...ha..ha..ha...

Albert

 

The article I read provided fuel filling instructions that provide the most consistent refills as follows: fill the tank up at full speed until the nozzle automatically stops, wait 10 seconds, and then slowly fill until it automatically stops again, and repeat the last step 2 more times for a total of 3 slow fills.

Glad to hear that Jag's reading are more accurate than industry standards.

BTW, drafting at more than 10 feet can provide more effect than might be surmised. While traveling on lonely interstates I have followed cars which are running with cruise control. Following at 2-3 car lengths provides slightly better mileage than at 6 or more car lengths. It easy to try.

 

I just filled up and I use Shell 91 octane with a splash of Marvel Mystery Oil of 6oz/10 gallons..

And this last tank was almost all city driving at 40 MPH or less and I got on the trip numbers; 20.9 MPG..and I get these kind of numbers consistently..I am thrilled that this NA, big *** V8, with almost 400 HP is running this kind of mileage

By comparison...Prior to this car, I had a new 2011 Lincoln MKZ with a 3.5L V6, and it it rarely got better than 16 MPG with same kind of driving in city..

 

I just did a 320 mile round trip from Houston to San Antonio. With cruise control set at 78, the mileage was 21.8 on the way out and 21.6 on the way back (2012 XFSC).

 

Your getting close to what I got in my XFSC. As soon as I get alittle more milage on my XFR. I'm gong to do the average from a full take of gas, just to see what I'm actually getting vs what the computer is telling me. I know they played with Averages on my wifes Audi. Her computer was telling her she was getting 28.7 miles but the math with her 18 gallon tank showed she was only getting 26.4 miles at 70 MPH, from Philly to Baltimore MD.

 

I have an XFR-S. I compare it to my '09 C63 AMG, a 7-speed with Benz's AMG Quick Speedshift Double Declutching something or other etc. That really is a good transmission that makes nice decisions in sport mode. The "engine braking" from downshifting as you head into a corner/come off the gas is more noticeable in the Benz; I like that. I like the transmission in the XFR-S OK. In a true sports car, I think the 8th gear is sort of a waste (maybe a 7th gear is too). The XFR-S does have one little niggling annoyance. Its downshift is a little jerky. That is to say, sometimes at speed and I come off the accelerator and the car thinks, "hey time to downshift," it jerks my head forward a tad (from deceleration). I think that's a little engineering problem, a minor rough edge that should've been noticed and eliminated. Perhaps that lets you know you're driving a petulant automobile though

 

I think that Mercedes box is a planetary transmission with a computer controlled wet clutch in place of the torque converter. Works just like a regular torque converter automatic with dry plate lockup clutch, like the ZF.

Mercedes uses a DCT (double clutch transmission) only in its newest A and B series as far as I know.

 

Maybe the 8 spd trans does improve acceleration of an R, but it does not seem to make sense to me based on my experience with my XFR. With the correct performance buttons set, when I WOT to 60 mph it feels like the traction control is intervening all the way.....indicating it is traction limited. That's just my impression. I would believe that there is a very small acceleration improvement due to the faster shift. I think the 8 spd is mostly about fuel economy (and performance for the less powered 3.0 S/C and 2.0 Turbo's. As for me, with an R, I would probably prefer the 6 spd with the faster shifting of the 8 spd. The 6 spd does enough shifting for me....the 8 spd is too much shifting.....somewhat unnatural to me as I was reared and imprinted long ago with 4 spds. Yes, I would like to have a taller O/D 7 th gear used only for highway cruising.

 

8 spd has taller 7 and 8 gears but that still means the bottom six gears are closer.

The acceleration is improved because the engine can return to the fat part if the torque curve on upshift.

Generally speaking more gear ratios always gives quicker acceleration as long as the shifts are quick enough and no drive is lost while shifting. Torque converter and DCT transmissions exhibit nearly zero slip, with DCT sometimes actually accelerating during the shift event due to the way the upshift clutch engages.

6 or perhaps 7 ratios are all a human driver can deal with. Even then, with very powerful cars acceleration will be quicker in automatic mode.

 

Steamer, Jagular is pretty much correct. more gears lets the engine stay in the highest power band of the engine. imagine if your gear change for brevities sake drops a 1000rpm. looking at the torque curve that might be a loss of 100ftpounds of torque. Now what if you dropped only 500rpm with additional CLOSER gear ratios. you would loose 50ftpounds and the car would accelerate faster as it stays in the power band more often. This is another premis behind a gear vender over drive unit that with gear splitting make a standard 4spd auto a 8 speed, and a real old school 3 spd box a 6 speed.

 

Brutal, OK, you are correct about staying in the sweet spot on the power curve, and about having better acceleration. I did a little analysis that convinced myself more specifically on this matter. It goes as follows:

I was able to find data on the ZF 8 spd trans ratios. Couldn't find it for the 6 speed so I did some measurements with my XFR. Could not find power/torque curve for the R engine. The CTS-V LS6 engine data was available and I guesstimated that the shape of that curve was close to the R engine. Yes, plenty of estimating and interpolation with this evaluation. Assuming shift points of 6,900 RPM for the Jag, with the 6 spd the 1-2 shift occurs at 41 mph and drops to 3,900 rpm, the 2-3 shifts at 70 mph and drops to 4,500 rpm, the 3-4 shifts at 106 mph and drops to 4,500 RPM. With the 8 spd the 1-2 shifts at 37 mph and drops to 4,600 rpm, the 2-3 shifts at 55 mph and drops to 4,500 rpm, the 3-4 shifts at 82 mph and drops to 4,600 RPM, the 5-5 shifts at 104 mph and drops to 5,500 RPM. (a table would have been much nicer).

The 1-2 shift is the biggie. With the 6 spd drops the engine to about 70% of it's full power. The same shift with the 8 spd only drops the engine power to about 88% of it's full power. That is significant. On the other hand, at 60 mph the 6 spd is in the sweet spot of the power curve, the 8 spd is not. No doubt, both cars are traction limited all the way in 1st, but the 8 spd has a much closer and better 1-2 shift ratio. Correct you and Jagular. Thought the difference is may be small with the R engine, it surely is more so with the 3.0 S/C. With all that being said, I prefer the fewer shifts of the 6 spd in normal driving situations despite the marginal deficit in performance under WOT conditions.

 

The frequent shifting of the 8 spd is more akin to how a manual transmission is used by an expert driver. It takes a bit of getting used to. Without the computerized shift and the lockup clutch the 8 spd would be slower than the 6 spd in my estimation.