Not to sound like an idiot, but:

What are the factors that change MPG?

I know weight does, and the engine. But what else?

 

Weight is number one especially in city driving.

In highway driving weight is not as large of a factor on flat roads. Aerodynamics start having an effect around 50-60mph. That's probably why the TL gets such good mileage on the highway but not in the city.

No matter what type of engine is powering the car it's going to require a certain hp to accelerate a certain weight period. Believe it or not, the more throttle you give it, the more efficient it is. Basically, the less vacuum in the intake manifold, the more efficient it is. A small engine will require more throttle to make the same hp so it will burn less fuel.

Many small things in engine design such as a high compression ratio, low friction, efficient combustion chamber, variable valve timing and lift, etc make a difference.

 

I have reset my trip computer while doing city driving as well as highway driving.

Stop and go on the highway is worse than hitting lights in the city. At least in the city you can accelerate up to speed between the lights

I've seen as low as 13 mpg and as high as >50 when coasting downhill on a mountain highway.

I average 18 to 20 mpg during the work week in city only driving. On the weeknds when I throw in some highway driving I'm up to 24 mpg. I have not really done a level highway test over a longer distance yet. I'm wondering if I'll be able to break my coastal mountain highway trip of 27.7 MPG over 143 miles. This was on cruise control at 57 mph.

I have an 07 TL Type-S with 77k on it.

 

i tracked a trip from east TN to east FL, about 570 miles. averaged 73mph, 30mpg. took about 7 hours and 50 min. that includes atlanta traffic and some very spirited driving on I-95

 

A related question:

Does anyone know at what speed is the TL (Type-S; if diff) most efficient? In general, most cars are around ~55-65mph... but of course this can be narrowed down depending on the car (engine specs, drag, torque converter, etc.)

This would be very easy to find out if anyone has the instantaneous mpg display mod... otherwise using the Trip Recorder would work, but a bit more inaccurate unless you can either keep repeating the same run or drive on a fairly consistent road.

 

https://acurazine.com/forums/3g-tl-2004-2008-93/fuel-efficiency-highway-various-speed-6mt-551948/ there it is. found it finally. i will re-post for the extremely lazy

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Cruise control at various speed, drive for about 5 miles to let the MID settle 6MT, at 6th gear

55MPH = 38MPG

65MPH = 34MPG

75MPH = 31MPG

85MPH = 27MPG

95MPH = 24MPG

105MPH = 21MPG

It was a bit difficult to keep the speed at 105 for 5 miles, kept coming up to other slow cars. So for the rest of the way, I set the cruise control at 80MPH, and after 300 miles, it indicated 29MPG.

 

Good info. It's a little surprising. On the way to Vegas at 2AM I was following my friend's Z06 with the cruise set on 105-110mph for 3 hours and my average was 26 or 27mpg. Average mph was around 101mph because we backed off to the speed limit when going through cities.

Maybe it's the taller final drive of the 5at. Mine also starts to get worse mileage below 50mph. Again, maybe beause the 5at brings the rpms too low and out of it's efficiency range.

 

wow, that's pretty amazing MPG. if 6MT was that good i would definitely speed even more than normal

 

I have taken very similar measurements on one of my road trips before. This was on a cool fall day, so windows closed, no A/C. 5AT using cruise control and drove for roughly 10 mile stretches (didn't go that long for the ones above 90mph though) on pretty flat interstate with hardly a car in sight. Very similar numbers to above except at the really high speeds. (I wonder if that could be due to wind differences between my environment and the original poster's environment.) I had also noted the RPMs. Here were mine:

45mph (1500rpm) = 31mpg
50mph (1500rpm) = 36mpg
55mph (1550rpm) = 36mpg
60mph (1700rpm) = 36mpg
65mph (1800rpm) = 35mpg
70mph (2000rpm) = 33mpg
75mph (2150rpm) = 31mpg
80mph (2275rpm) = 28mpg
85mph (2400rpm) = 25mpg
90mph (2550rpm) = 23mpg
95mph (2700rpm) = 21mpg
100mph (2850rpm) = 18mpg
105mph (3000rpm) = 16mpg

 

^ Great info! The previous results weren't 100% helpful since it started at 38 mpg and dropped from there. This shows an incorrect trend line that the lower the mph, the higher the mpg (obviously not true).

Your data shows an increase to a peak and then a decrease. It seems somewhat accurate, but there are a few "blips" in the data which seem odd. The main one being from 45 mph to 50 mph resulting in a 5 mpg increase, which seems awfully high.

Otherwise, good to know that 50-60 is the sweet spot... though painfully slow for most highways around here.

 

Great info guys! I've found the same thing as you did. Between 50-60mph, I get my best gas mileage on the highway/interstate! Might be going a little slower than everyone else but hey who cares!

 

Several people have mentioned tyre pressure. At least for the '04 and '05 TLs, the specification varies between the 6MT and the 5AT. The specification for the 6MT is 35 psi front and 32 psi rear; I'm not sure what it is for the 5AT.

I've found that the two biggest things that affect fuel economy are being able to drive on the highway and being able to maintain a constant speed. The TL loves the highway. I've pulled over 30 mpg for an entire tank of gas even at average speeds of over 70 mph when I've been able to spend almost the whole time on the highway in 6th gear. The other big thing is that even when you're not on the highway, it's best to keep moving if possible. Someone else mentioned how you're getting 0 mpg when you're stopped at a red light or stuck in traffic. Same thing applies to crawling along in the "stop-and-roll" you may encounter during rush hour.

Err on the side of the higher gear if you're borderline between two of them. Generally you save more fuel if you drive at the slowest speed that will allow you to use a higher gear rather than at a higher speed in a lower gear, as long as you're going fast enough to allow you to use the higher gear without lugging the engine.

 

For the Base TL 5AT the tire pressure recommended is 33 PSI Front and 32 PSI Rear! Usually though, the lower you keep air pressure the worse the gas mileage but I'd rather have that then keep it higher and suffer with the ride quality. Typically, I keep it at 34 front/33 rear so that its a little above recommended and not to high that ride quality suffers. The highest I have ever gone is 35 F/ 34 R. Anything over 36 though cold, I've found the ride starts to get worse.

 

OK, I figure that this is the proper place to post my question.

*106,0xx miles on the TL so far.

In the past ~4000 miles I've had the following work done;

- 105k maintenance: 1000 miles ago.
- new tires - balanced, weighted, and aligned: 200 miles ago.
- Sea-foamed Once: 2-3,000 miles ago.

I get about 15-17mpg in the city and 23-26+mpg on the depending on how long I'm actually on the highway for. Now I tend to drive pretty "spirited" (lol). I usually burn through $20 of Shell Premium in 2-2.5 days. I blame it on the fact that I mostly drive in the city and I drive aggressively.

What else can I do to improve my mpg? What PSI should my tires be set at? I'm running 245/40/17 all around. And is there anything else I can do to increase mpg? Perhaps try sea-foam again?

 

^ i believe a good psi would be 32- 33 cold