Now that my odometer is working 100%, finally got my first read on mpg. 12.5 mpg city driving. Lot of stop and go traffic. 88 HE V12 with super enhanced AJ6 engineering ECU. Is this normal?
Greg,
Based on our experience with my wifeās 1990 XJ-S convertible (5.3L V12 with Marelli ignition) 12.5 MPG is roughly about what we experience, although maybe a bit higher.
We got our car in 2005 with 62K miles on it and it now has 131K miles on it. We keep a book in the car to record each time we add gas as well as some of the service and repair actions. We donāt calculate MPG each time we add gas, or an average over time, just actual MPG ( not just what the trip computer calculates) from time to time. Looking at those calculations over the past 14 years and 69K miles the low readings were around 10 MPG and the high ones around 17 MPG, (in US gallons not Imperial gallons). Most of the calculations appear clustered in the 13.5 - 15.5 MPG range.
Paul
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My 1982 girl had appalling mileage initially but after fitting an upgraded ECU (Melbourne 2015) things got better. She now measures 13.5L/100klm (17.5 miles/USgal ) open road and 15.0 L/100klm (15.5miles/USgal) in town. Having said that my idea of city driving in the likes of Wagga Wagga probably doesnāt compare to the congestion of most.
The other factor that may be at play here the fact that many of these cars donāt get enough exercise. Much can be said for the Italian tune up.
Trev
Yes itās normal.
A big factor is also how long of a drive you do each time, when the engine is cold itās really badā¦ In winter you will see that it will be even thirstier.
In our windy country roads here I get about 14 to 15 L/100Km.
Aristides
Thanks, good to know. Just wanted to make sure engine was running decently.
Yeah, my miles were two commutes to work, only 12 miles each drive with a lot of stop and go. So itās probably the worst case scenario. Iāll take it. I was afraid Iād only get 10mpg. 
Just checked my MPG again after 4 days of city driving. Now Iām down to 10.5 mpg.
BUT as a test, I am now running the vacuum advance off the butterfly throttle area instead of the regulator set up that advances idle timing, so idle timing is more retarded. Iāve also noticed at long idles, my temp is about 5F higher.
Iāll try going back to my 1988 HE vacuum advance system with regulator and dump valve and see what happens next week.
Yeah, thatās not good. An H.E. should get better than that.
But the whole point of my experiment was that I knew gas mileage would suffer if I had no vacuum advance at idle, right? Just I didnāt know by how much.
Iāll hook the regulator/dump valve back up this weekend and report gas mileage next Friday. If anybody cares 
11-ish for city driving is not too bad.
12 mpg in town is on the EPA label, which IMHO is optimistic.
My car is 12 city, 16 hwy (granted, the differential is 3.54 but I have 4-speed electronic box with lock-up TC). Both numbers can be achieved, but if the driver has a heavy footā¦a whole different story.
EDIT: My hwy average:
http://www.jag-lovers.org/snaps/snap_view.php3?id=1469410491
I played around with my ā88 vacuum advance; I have an aftermarket advance unit and let me preface that I do not trust my timing marks. Testing the capsule with a vacuum pump showed the capsule was fully advanced by 15ā (which is correct spec). My idle vacuum was 17ā in stock configuration.
I removed the vacuum delay and dump valve, capped the throttle body port, and ran straight manifold vacuum to the capsule. I set the base timing using the vacuum gauge; advance to max vacuum with capsule connected and backed off 1ā from that reading.
This gave me a smoother idle and better throttle response, and no spark knock on premium fuel. I do run mid grade a lot, and with that get occasional light spark knock (which I avoid by lighter acceleration). I have been averaging 13mpg around town, and can see17-18 on freeway steady speed.
So after a couple months, it seems I canāt get above 10.5 mpg city driving (thatās with about 1/3rd stop and go traffic)
A few things Iāve done which could be hurting - Iāve adjusted transmission so it holds 1st gear and 2nd gear much longer than stock, Iām running a SuperEnhanced ECU (which may run richer than normal), and Iāve got lower than stock thermostats (180F), so the car generally runs between 180-190F.
And I floor it for several seconds at least once per day.
For giggles put in a full tank of NON ETHANOL VR unleaded racing fuelā¦ā¦ā¦you will go to 19.9 in less than 20 miles on the open roadā¦the he head was a vast improvement but the gas is junk on your old motorā¦
TRY IT YOULL LIKE IT!
GTJOEY1314
H.E. 5.3, US gallons? Thatās not good. Have you tried an ITU?
Not yet. But like I noted above, Iāve got a few adjustments that may lose 2 mpg?
Nah. You should be getting around 16.
16 heavy traffic city driving!? Really? That seems kinda high. I thought it was 12 city 18 highway
I always got around 16.
What city? Or is that combined mileage?
Will an ITU make that big of a difference? If so, then what is going on?
And if you were to guess, whatās the #1 reason for an XJS V12 to get such poor gas mileage?
My guess would be timing issues, not enough advance. Sometimes due to timers that retard it during warmup.
It always was really combined mileage, just because I live a ways outta town.
Thatās what I get to, country roads though and short distances.