EPA highway economy estimates are based on an average speed of 48 mph, with a top speed of 60 mph. You can expect to see a 20% decrease in miles per gallon between 55 mph and 75 mph. That means if you get 30 mpg at 55 mph you'll get 24 mpg at 75 mph.
The TPMS read out on the screen is 33 psi cold on all tires. I haven't checked the intake for plastic bags, but that's a good idea. Ever since I went to new tires (Michelin Pilot Sport A/S3+), my mileage has tanked from what it used to be. Tires are same size as well. 255/40/19. Although I moved closer to work when I got the new tires. So I cut out 25 minutes of highway driving by moving closer to work.
On the way to work, engine warms up on the highway. On the way home, engine warms up city driving with a little stop and go.
If temps are below 40 when I drive the car to get lunch (all city driving with a cold engine) I see economy numbers below 15 mpg. A couple of lunch runs can drop my combined economy numbers by 1-2 mpg over a tankful. It has a much greater impact than you think.
Yeah, you increased the size of your contact patch with wider tires. Your mileage will go down...does your PCM know about the new tire size? There's a good chance that your speedo, odometer, and trip computer are all off, so if you're going by the numbers on the trip computer, you might do well to reflash it with the appropriate tire size. But I'm going to say that when you increase the size of the contact patch (i.e. wider tires), expect your mileage to drop...
Contact patch is part of it, but he's also significantly increased aerodynamic drag, especially at 80 mph. My guess is you probably wouldn't notice such a big difference in mileage around town, or if you kept your highway speeds down to 60 mph.
My issue is the horrible final gear in mine(3:31). It's unusable until 75-80 mph if you have any hills around you. I find myself driving in 5th at 3k rpm around 70 mph because of this issue. 6th gear sucks unless you have the pp 3:55.Contact patch is part of it, but he's also significantly increased aerodynamic drag, especially at 80 mph. My guess is you probably wouldn't notice such a big difference in mileage around town, or if you kept your highway speeds down to 60 mph.
I know it seems like less fun, but I've experimented with different speeds over my commute and I've come to some interesting conclusions. It's 35 miles from engine start to shutdown on my commute, with 32.5 miles of that highway/interstate. If I drive 60 mph on the highway portion it takes 32 minutes and 30 seconds. If I drive 80 mph it takes 24 minutes and 20 seconds. The reality is I can never stay at 80 the entire distance because of the left lane bandits; I find I am constantly turning the cruise on and off, on the lookout for the highway patrol and getting frustrated with the idiots who believe they don't have to move over as long as they are going the speed limit. At either end of the highway portion of my commute are a few stop signs and lights; it's a rare day I catch all the lights green, or don't have to wait on crossing traffic at the stop signs.
What this all means is that once you account for the left-lane bandits and traffic lights at either end, the difference between 60 mph and 80 mph during the highway portion of my commute is at most three minutes total travel time. In addition, the time on the highway is often spent weaving around and cursing people who refuse to get out of the left lane. Needless to say, not very relaxing or enjoyable.
Contrast that with a leisurly 60 mph commute. The cruise control almost never comes off, so I can enjoy my morning coffee. I smile and wave at the officers on traffic duty when I pass them. And since I'm driving a car with a relatively powerful engine, I can easily accelerate and match speed with the fast-lane traffic to get around the rare driver who is driving even slower than me.
All in all, it's a much more relaxing and enjoyable experience, and you'll get much better fuel mileage. The icing is the occasional burst of power to accelerate around slower traffic.
I do have a PP car. My personal rule of thumb is not to put the vehicle into 6th gear below 55 MPH. You definitely don't want to do something like accelerate uphill with the car in 6th going 55... (LSPI). 6th is pretty usable in a PP car. I use it all the time for freeway cruising, and for state highways and such (as long as traffic is moving at 55+ MPH).