Is it really the tires?

What tires do you have on the car now? They might be too heavy. Unsprung weight is multiplied by 4.

 

What tires you now have I would speculate is the WHOLE story.

You often see a decrease in MPG with a new set of tires.
If these are not Low Rolling Resistance tires, or tires designed to return higher MPG, compared to whatever you had on them before, you may simply experience a reduction.
But it's probably more likely your MPG will increase as the tires break in.

Check what tires you have, and how they rate as gas efficient tires.
Then just give it some time. I wouldn't panic. A 4 mpg decrease after having new tires installed I think is more often typical of just having new tires installed.

 

I just got Bridgestone Turanza Quiet Trak tires. Stock size for my car. After two tank fulls my mileage has dropped 4 mpg. I can live with it. The tires are definitely much quieter than the tires that came with the car originally.

 

As described the mpg for the last 200 miles went from 42 to 38 when one mile was added. Long idling will do that. You can also dramatically change a 200 mile average mpg by getting up on the freeway and driving at 85 mpg. The relatively short trip miles will allow mpg to vary quickly when idling or speeding.

Here is another mile and mpg related quirk. My car’s speed readout is always about 4% high. I can see the difference when passing radar speeding signs, with a gps and even by monitoring my obd2 port with an app. The car has the correct mph in its computer but displays high. I expect the overall miles on the car is overstated by 4% and it probably causes an erroneous mpg.

From “Car Scanner App” while driving.

 

Have you verified that the old tires and new tires were the same size? It's not unheard of to put on a very similar, but slightly different size. This can directly affect MPG, and can also have indirect effects because of the inaccuracy introduced into the speedometer/odometer/fuel use calculations done by the dashboard computer.

Even for the same nominal size, there can be some variation- a new one that runs large for its size vs. an old one worn to limit.

 

It is most likely the tires....I saw a substantial decrease with new tires on our 2010 liftback and our 2012 v (wagon). Wait until the tires have 'broken in' a few months...and then report back.

Weight, rolling resistance, and the diameter/circumference of the new tires vs. the worn tires all play into it. And the higher the MPG your vehicle is rated at, the more you will notice it. This article really drives home how much of a difference it can make :

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=177

Also note that there are quite a few other 'tire MPG decrease' and 'new tire' threads around here if you search around a bit. Some of them have pretty good data as far as people reporting back, etc.... Some of them are well worth your time.

Hope this helps!

 

What are you running for tire pressure?

The dealer want's to run 36 PSI but I instruct 38 PSI.

for me yearly average.
36 PSI = 39 mpg
38 PSI = 41 mpg
40 PSI = 42 mpg
I know 40 PSI can give me about 3 mpg better, but the ride is better with 38, a good compromise for me.

Side note:
I still have the OEM tires with 50,000 miles and I have noticed about a 2 mpg increase from new.
I believe well scrubbed-in (worn) tires produce better mpgs than new. I still have safe tread for another 8-10K.

 

I was going to ask the same question.
But the OP seemed so dedicated to wanting to believe it was some covertly installed firmware "glitch", that I didn't bother.

All the OP say's is "Coopers".
I'd be interested in EXACTLY what Cooper tires were installed. Cooper makes a lot of tires, designed to promote a lot of different attributes. Tread Wear, traction, Winter, Summer, Touring, All-Season...tires designed specifically to promote better fuel efficiency.

I don't know but sometimes the simplest most direct answer...is the answer.
I'm far more apt to ignore the immediate MPG reading change, and attribute any 4 MPG mileage drop to the change in tires.

 

Did they by any chance perform the Inverter Firmware Update?

 

This is normal. Common to all cars. If it read low, you would be above the speed limit when it says you were at the limit.

 

There are several updates that others have claimed that it hurt their mpg. Make sure you have them because it could matter when your Inverter blows up. These are inverter updates......

 

I run 42 front and 38 back...... defenders.....40 to 42 mpg (with ac on) usually and my v has 254,000
running 0 w 40 mobile 1