Mileage is going downhill

My wife's 2010 Jetta TDI sedan with the DSG has been having problems with MPG's. The original tires were Michelin's HX MXM4's. We replaced them at around 53,866 with Pirelli P7 Cinturato All Season Plus tires. The car has about 63k+ miles on it now. With the Michelin's we did a couple of road trips doing 70mph and the car always seemed to get 43+mpg, but with the P7's we can't seem to get better than 39mpg.


would the new tires make our mpg's drop 3-4mpg? The only other thing I can think of is my wife complained about the car shifting weird around 54k miles, so when we had the car service at 57,700+ miles (was going on a road trip and needed it done beforehand) for the 60k service, they said it was the fuel filter, and replaced it. I never really paid attention to see if it effected the mpg, because we never really went on long drives before this. Even if it was the fuel filter, they supposedly put in a new one.

Other than that what else could be wrong with the car? We fill up at the same places. i even tried filling up at other places to see if that helps, but nope. Could there possibly be a dpf issue? I'm hoping it's just the tires and nothing major, but that still will suck b/c I thought the P7's were a good tire.

New tyres (even LRR) will give lower mpg than old tyres, for two reasons.
a) the circumference is higher if unworn (so the odo will be underreading compared to the old tyres)
b) rolling resistance is higher when new compared to fully worn, because the bigger the blocks, the more they squirm around. They are at their most efficient when illegally worn down all the way.

Also, make sure that they are *always* up to at least the recommended pressure (measured when cold, not hot). Dropping below the recommended will cause your mpg to drop significantly.

What Mike said. Also, assuming the car is functioning properly, there are many factors that can decrease mpg, including weather, traffic, route taken, driving style, etc. At this time of the year, most stations will have winterized fuel. Not sure when that happens in FL, but I'd assume it does since the temperature can get below freezing and Floridians also take trips up north, so they can't have fuel that would gel in falling temps. I'd keep an eye on it for now over a few tanks.

RebelTDI said:

What Mike said. Also, assuming the car is functioning properly, there are many factors that can decrease mpg, including weather, traffic, route taken, driving style, etc. At this time of the year, most stations will have winterized fuel. Not sure when that happens in FL, but I'd assume it does since the temperature can get below freezing and Floridians also take trips up north, so they can't have fuel that would gel in falling temps. I'd keep an eye on it for now over a few tanks.

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I notice an mpg hit +-10% come winter which I attribute to winterized fuel.

fruitcakesa said:

I notice an mpg hit +-10% come winter which I attribute to winterized fuel.

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Partially winterized fuel. Partially the fact that the air is more dense. Partially the fact that the system runs more fuel to increase the idle to get the car up to temp more quickly. Partially the fact that the diesel is not as efficient when cold. All cars take a hit in the winter, even those which don't use winterized diesel fuel.

I'm lucky to get 35mpg right now, with temps in the 20's in Michigan. But my old gasser Passat would have been lucky to get 21.

Just fueled up and got 38.6 mpg which is closer to my winter average

yea i noticed i consistently get high 600s every tank. Filled up yesterday at 610. Winter is here...

I guess the tires and winter fuel could be the reason for my lower mileage. I never thought about winter fuel. I'll keep that in mind and just keep an eye on my mileage. Thanks everyone!

The "winter fuel" is a very small part of reduced mileage in the colder weather. You get less mileage because its colder and rolling resistance is higher from tires, axle joints, wheel bearings, tranny oils, longer engine warmup times, etc, etc....
The fuel is almost insignificant difference in energy content. Burn the same fuel in warm weather and you won't notice the mileage difference.

I was spoiled by getting my Jetta in April. I had a nice consistent run of 50+ mpg, but I've taken a big hit with cooler weather. I monitor the coolant temp with a Scan Gauge, and it takes a long time to get up to ideal temperature when it's below freezing. It probably wouldn't be so bad on a longish commute, but if you do a lot of shorter trips like me, the problem is magnified.

PressEnter[] said:

I was spoiled by getting my Jetta in April. I had a nice consistent run of 50+ mpg, but I've taken a big hit with cooler weather. I monitor the coolant temp with a Scan Gauge, and it takes a long time to get up to ideal temperature when it's below freezing. It probably wouldn't be so bad on a longish commute, but if you do a lot of shorter trips like me, the problem is magnified.

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Yes, another good point about cold temps and distance. It takes longer to warm up not just the engine, but all the other things that turn.
A hill that I normally gain speed coasting down in warm weather when it's -0*F (and colder) makes for a real speed decrease instead. Many things will never really warm up when its cold and contribute to a lot more resistance.
Short trips will really make mileage plummit in the cold. The first several miles on my commute in cold weather make for a poor looking trip average, but toward the end its coming back around.
The only bonus of the colder weather is injection timing stays a bit more advanced, which helps counteract the other losses a bit.

..winterized D2, increased rolling resistance, et al......

Filled up this eve - same Chevron, same pump (even same price)......

510 miles/12 gallons of fuel = 42.5.......

Appreciate y'alls input in understanding my 5-8% FE reduction........

I certainly am not complaining at this point.

ez

This is my first winter with the JSW. Big drop in MPG last tank, but I'm now on snow tires, -10C, and driving through a few inches of snow in residential areas. My last 10 tanks averaged around 42MPG. Last tank was 37. Might be under 35MPG once the really cold (-30C) weather hits Edmonton. Not complaining either, just making an observation.

....cold weather........

......is a big deal, isn't it? At 42.5 last tank, I should be smiling..........

Really don't dig the cold (I left Navy active duty upon receiving orders to Alaska).......


ez

It's funny, my mileage actually improves in the winter here in Texas, since it has been about 102 F for the entire summer, and the temperatures are finally down to the seventies on a regular basis. I only bought my 02 Golf about 2-3 months ago, but my mileage is consistently higher now than it was when I first got it in the heat of summer.

Very true!

caseyfriday said:

it's funny, my mileage actually improves in the winter here in texas, since it has been about 102 f for the entire summer, and the temperatures are finally down to the seventies on a regular basis. I only bought my 02 golf about 2-3 months ago, but my mileage is consistently higher now than it was when i first got it in the heat of summer.

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Winter fuel in Florida?
The OP's city is:
Location: Auburndale, FL

I don't think cold temps have much to do with this down there....

A friend saw a significant drop in fuel mileage when he got new tires on his Prius. Swapped those out for another brand and the mileage came back up. My mileage is usually best from September -November and April-May. No AC needed helps a good bit.

I don't think winter fuel is an issue for the OP, but my mileage started dropping with temps in the 40s and 50s.

First cold weather fill up yesterday on the 03 - 42.5 down 5 from average. Sigh