Car: 1996 Jaguar VP 4 liter Straight Six. Almost 80k miles Replaced front shocks to Bilsteins last week ( Had original Shocks with bushings almost gone and was clunky). Now ride is very quite and feels steady.

Highway driving 24.5 mpg
Street driving 16.5 mpg

I had a chance to make two long freeway trips last two days, at an average speed of 65 to 75 miles per hour, and noticed that the car averaged 24.5 mpg for the last two days.

However, on regular commute of 8 miles per day without freeway driving, was only getting 16.5 mpg.

So far, replaced fuel filter, spark plugs and had mass airflow sensor cleaned. Have used Chevron Techron fuel additive to (hopefully) clean injectors.

What else can I do to improve mileage for regular driving. Does the new shocks have anything to do with mileage? Would cleaning idle air control valve, EGR, Throttle body and pulling out injectors and cleaning them help?

 

Sounds normal/typical to me. Certainly matches my experiences with various cars, including Jags

Cheers
DD

 

If you have not done it already I would check or change the air filter. I had a bad thermostat on mine that was making the engine run too cool and was causing the engine to run too rich. You may also want to check tire pressures.

Replacing the front shocks would have no effect on gas mileage.

 

Normal.

Highway always near double city.

City is stop start, accelerate, slow down, accelerate etc, kind of sucks teh fuel.

Highway on my 3.2ltr, (you guys missed that AWESOME engine), is near 30MPG, Imp of course on the highway, and city is around 18ish.

The V12's were always single figures around town, and mid to high 20's on trips.

 

I half-agree with Doug on it sounding normal/typical....except that 24.5 half. Given your location, I'd guess you are talking US Gallons? Sounds high, I wouldn't change a thing!

However, if you come by that number by resetting the trip computer and reading the avg fueling at the conclusion of the highway run, then yeah, ok, normal. With the right conditions, you may be able to get that number even higher.

8 mile commute is part of your problem. You spend some amount of it in open-loop fueling, as well as greater friction as the engine is coming up to operating temperature - and the oil doesn't seem to get there as soon as the coolant temp gauge does.

If all 8 miles is city/stop-n-go, you probably won't be able to repeat/confirm the following: My commute is a bit over 50 miles, seat-of-the-pants would say 80% freeway, 20% city, but OBDII data says about 50/50 if you look at actual time spent above 50 mph vs. below 50 mph. (I took that data when there was a lot more construction going on that has now completed, and before the Toll lanes had been opened, so maybe a little past 50/50 now) Anyway, the first 40 mi in the morning are freeway - I ease down the driveway, then left down the 40 mph County Road for about 3/4 mi (normally 25-30 mph....lotsa deer out early am) then 5 mi. on 70 mph US 380, where the limit drops to 55 mph for the next 3-4 mi. through Decatur, even though it is still all freeway.

Why all the detailed description? Well, having done this routine since I bought the car in 2010, its given me an opportunity to observe lots of things. Here's what gets back to your 8-mi commute: If I dial up instantaneous fuel on the display, I note that the fuel mileage increases noticeably about midway through Decatur. Yes, I realize instantaneous varies markedly with wind, slope, throttle position and meteorology, not to mention the coast-down from 75 mph to 55 mph! But I'm talking about long-term, day-in/day-out observations across all of those variables, it is a noticeable increase when the car becomes fully warmed up. My coolant normally reaches spec temperature about 2.5 mi into the journey, depending how much idling (pronounced, "diddling") I do regarding backing out of the shop, turning round and pointing down the drive, retrieving forgotten items from the house, and possibly stopping to post mail at the end of the drive. So the increased fuel mileage is not coincident to the coolant reaching operating temperature. In my case, I'm nearly 8 miles on by the time it occurs. More time dependent than distance, but I reckon you do at least a mile or two before you reach a similar thermodynamic state. So that is maybe up to 1/4 of your commute.

From miles driven since last top-up divided by gallons required to top-up? I get a very reliable 19.5-20.5. But if someone says, "Hey, can you make the avg fuel consumption display say, X mpg?" Yep, I can probably do that, unless "X" is over 30, that's a little tougher, and won't stay on the avg display for long.

 

@aholbro1

That is a fine analysis. My one way commute to work is only 3.8 miles. Looks like the engine is barely reaching operating temp. I am a CPA and do keep track of the mileage on a spreadsheet, by tracking miles drives and gas used between fill ups.

However, my post was triggered by why that big a difference in mpg between highway and city driving. My 2007 Lexus RX350 gives about 21/22 mpg in city and gives about 25-26 on Highway even with a trip from Phoenix to LA. That is why I started the post.

So short commute is the issue. Thanks