Significant mpg drop on new tires?

First post, apparently.
I've got a '10 Jetta TDI Cup which came with 225/40R18 Pirelli P-Zero Nero All Seasons (Tirerack link: http://is.gd/oPHDNU). They have an H speed rating. I was getting between high 33s to near 37 mpg at the pump since I bought the car last April. It had 25k on it, and at 45k, the rears were on the wear bars and the fronts weren't far behind. (I rotated them at 40k.) I swapped them on Feb 16 for Michelin Pilot Sport A/S 3 (same size - Tirerack link: http://is.gd/AZc8Zd) with the Y speed rating because it's all that was in stock in my area... and I might be taking it to the track soon.
Since the moment the Michelins were on the car, my mileage has been terrible, and acceleration has felt ...slower. None of the other handling characteristics are worse - cornering is definitely improved. My dash display now rarely reads over 33 average on my commute where I used to be able to get near 40 if I slightly babied it (and that was driving in Sport mode the whole time). I do understand that the dash display is more of an idiot gauge than a true measurement of mileage, but it's still a good relative measurement of economy.
I've got the Malone 1.5 + DSG tune on it, but the car is otherwise stock. I've had the tune since last June.
(sorry, i'm a spreadsheet whore )
Those last two fillups were 100% driven on the new tires. The car tracks perfectly straight, and none of the wheels feel warm after a long drive (ie. no sticking calipers due to squished brake line). Could I need an alignment anyway? Is the mileage drop purely because I swapped to a higher speed rating tire?

Hard to imagine new tires dropping the mpgs over 10%. Did the tire guys do an alignment? Mis-alignment might do it.

Whats your driving style like?

I thought the newer models did closer to 50mpg?

Any possibility you have an e brake dragging, or perhaps a caliper? Up north here that is a likely scenario. I see you're in Florida so a rusty something shouldnt be the problem. Perhaps when your tires were mounted something was disturbed that would cause one or the other. It just seems odd that it happened with new tires. Jack your car up and spin the rear tires by hand with the ebrake off . Do they both spin the same? Do they spin at all?

minameismud said:

I was getting between high 33s to near 37 mpg at the pump since I bought the car last April. Is the mileage drop purely because I swapped to a higher speed rating tire?

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You have only 2 fillups on the new tires, which makes it difficult to have a reliable comparison. That being said, I think you'll continue to find lower FE with the new tires. They are heavier (the worst place to add weight for FE), and you are comparing to worn tires. New tires will almost always indicate lower FE, and the highest FE will generally be seen on worn-out tires. Also, you are now running tires with a slightly taller diameter, compared to worn tires that had (even when new) a slightly smaller diameter. This will mean the under-reporting and over-reporting of your miles traveled is more significant, skewing your FE numbers. Add to that a different compound for the Pilot Sport, and you'll note a FE drop. Lastly, the only way to get good FE numbers is to compare actual mileage to actual fuel used, the trip computer has too much error for comparisons (even trip to trip).

what are your tire pressures?

I've seen new tires do it on my truck. I went from a store brand AT to a BFG AT and lost 3 mpg. Took the BFGs off after 10k miles and went to a Firestone AT and got the 3mpg back

BTW, when I said your new tires were also heavier and taller in diameter, I was referring to the specs you provided from the links to Tire Rack. This was in addition to the difference between new tires and worn tires.

all new tires will not get the mileage a worn tire will, the rubber is taller at the tread and deflects

wrenchman30 said:

all new tires will not get the mileage a worn tire will, the rubber is taller at the tread and deflects

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One has to wonder how much of the increase of fuel mileage from "break-in" is actually that of less tire mass...

I had similar (3-4 MPG) drop when I switched from Michelin Energy to a cheap Sumitomo tire. When the Sumitomo's wore out, I switched back to the Energy tires and mileage increased.

Also, make sure your tires are inflated. I usually go above the door jamb sticker but a bit below the max shown on the sidewall. Most tire dealers inflate on the low side. Makes a big difference.

cavymeister said:

One has to wonder how much of the increase of fuel mileage from "break-in" is actually that of less tire mass...

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This would be a more reasonable question if the FE increase after engine break-in wasn't well documented over a range of TDI engines, using various tire selections and conditions, over the last 17 years or so.

hskrdu said:

This would be a more reasonable question if the FE increase after engine break-in wasn't well documented over a range of TDI engines, using various tire selections and conditions, over the last 17 years or so.

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How much FE is lost by switching from worn to fresh tires of the same model/efficiency rating?

I've seen FE improvements after 30k or so with gassers, so I have no doubt in my mind that it's true. But my statement was more to question how much total increase was due to break-in versus tire wear.


i.e. if a tire switch makes you lose 2%, and 60,000 miles later you are at 10%, the break-in really only increases you FR by 8%.

cavymeister said:

How much FE is lost by switching from worn to fresh tires of the same model/efficiency rating?

I've seen FE improvements after 30k or so with gassers, so I have no doubt in my mind that it's true. But my statement was more to question how much total increase was due to break-in versus tire wear.


i.e. if a tire switch makes you lose 2%, and 60,000 miles later you are at 10%, the break-in really only increases you FR by 8%.

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Understood. It's a good question. My point was that, in almost all scenarios (regardless of tires), TDI owners tend to report increasing FE after an extended break-in (the duration seems to vary a bit by engine code). This is true even of those who swap out OE tires with something else (countering the "worn tire increasing FE" argument, and where they should be reporting a decrease in FE). As an example, owners whose engine is past break-in, but who replace their original tires with new, continue to report FE averages higher than a new engine with new tires.

I think exact numbers would be difficult to pin down, even with the number crunching, spreadsheet happy crew we have at Fred's. Part of that is b/c of how hard it would be to really pin down FE factors: highway vs city, average ambient temps, condition and upkeep of car, type of road surface, altitude and elevation changes on average commute, tire PSI (a huge factor), and the varying driver's right foot. My observations are based on reading about TDIs, here and elsewhere, since I picked up a Passat TDI brochure in 1997, and following the FE threads. This makes my observations largely anecdotal, and unreliable. What I do know, is that if I mounted new GY's now (which were the OE tire) I would get better avg FE, despite the fact that they are new.

My FE and fuel used has been tracked to the thousandths since day one. My mileage has continued to improve, despite wider tires, taller tires, and new tires. My 11th year saw my highest yearly average (over 48mpg). Despite this, I still can't nail down definite numbers for tires, since it's so hard to account for all the factors mentioned above.

I experienced nearly the same thing. My fuel economy seems to be coming back. I think the change in diameter of the tires, plus the difference in rolling resistance for the newest, outer layer of treads contributes to this.

my thread:
http://forums.tdiclub.com/showthread.php?t=434723

wrenchman30 said:

all new tires will not get the mileage a worn tire will, the rubber is taller at the tread and deflects

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I have to disagree from my own experience. On the JSW, I took the Hankooks off around 40K miles and replaced them with Bridgestone Ecopias. I saw an increase of 3-4 mpg when I looked at mileage over a 10K spread. On the Golf, we replaced the contis with Michelins around 25K due to a busted side wall. The difference in mileage was better but by only 1 or 2 mpg.

Something I just read makes a lot of sense as well. It was really a kind-of 'well duh' moment.

As your tires wear, your speedo is going to read faster. So it will seem like you're getting better FE. When you put new tires on, it will read slower. Therefore, it will appear as though you are getting fewer miles for the same gallon of diesel.


Here's an example by the numbers. We are assuming that we are talking about 10 gallons of diesel.

2% tire life:
Speed: 53.6 MPH
Miles: 536 miles
Fuel mileage 53.6 MPG

Actual
Speed: 50 MPH
Speed: 500 miles
Miles: 50 MPH
Fuel mileage 50 MPG

100% tread life
Speed: 48.9 MPH
Miles: 489 MPH
Fuel mileage 48.9 MPG


Basically your ACTUAL fuel mileage hasn't changed (assuming the same rolling resistance, driving habits, etc), just the perceived fuel mileage.

cavy- that's what I (tried) to explain above, in my first post:

hskrdu said:

Also, you are now running tires with a slightly taller diameter, compared to worn tires that had (even when new) a slightly smaller diameter. This will mean the under-reporting and over-reporting of your miles traveled is more significant, skewing your FE numbers.

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It's not really about the Speedo, it's about the Odo. As the taller (newer) tire takes fewer rotations to cover the same distance, miles are underreported. That being said, the diameter of new tires isn't always greater, even among the same size tires, the actual height varies, as does rotations per mile (places such as Tire Rack do provide these specs).

Leave it to the TDIClub to correct for it: Owners recorded miles by GPS as opposed to Odo, to correct for changes in miles reported by new and worn tires, and (venting and filling the brim), were able to provide extremely accurate FE numbers. This is partially how the "mileage will improve after break-in" mantra was substantiated.

To further speak to your post: Rolling resistance does change, and it generally reaches it's peak (i.e., least rolling resistance) when tires are worn, adding another variable to the calculation of FE & tires.

Some of these comparisons require, as kj said, same model tire vs same model. I think we all know you can take off worn high performance tires and improve FE with new tires that are LRR and designed for economy.

well us say a worn out tire is 2/32 of tread and a new one is 13/32, the difference in feet will be very minimal at the end of the mile and will probably be less than a mph on the speedometer, and I highly doubt it would matter to tire revs more than one.