New tires and decrease in MPG...Why?

[powayroger]

I just got rid of my OEM tires (Goodyear eagles with 33,000 miles on them) and bought 4 Pirelli P5
http://www.discounttire.com/dtcs/findTir...99&sw=false

I’ve noticed, with my 81 mile RT commute, that I’m using ¼ gallon more a day. How do I know that? I use a scan gauge so I keep tabs of the fuel I used everday.

Now, what in the manufacturing of tires (We’re talking about tires with a high tread wear rating, with the P5 at 760) will cause this kind of decrease in mileage?

Yes, I did read in the reviews that some have experienced a decrease in MPG, but I bought these tires to last.

 

[RxR]

I would think that .324 gallon/mile could be contributed to other factors i.e. climate, driving paterns etc. or as simple increased traction.

 

[GMGuy]

Rubber compound, the amount of flex in the tire, tire pressure. You've got new tires, you may need to adjust tire pressure a bit. Add a pound or two of pressure and see how it behaves. Look at the thread and see how the dirt pattern is on it (I look to see how far the tire is rolling over during turns, and in general if its making a somewhat reasonable even pressure point across the thread. The old, roll through a wet spot and see if the thread pattern is consistant across for instance).

Alex.

 

[shpankey]

that's not a lot of difference. it could be something other than the tires too. headwinds. temperature outside. traffic. how you accelerate. or a combination of many different variables. it will probably work itself out over a larger sample of time.

 

[Quattro Pete]

There are quite a few reasons. First of all, if you're comparing old worn out tires to new ones, then you have to consider that tires with full tread generate more 'squirm' (if there is such a word), so some of the energy is lost on moving the tread around instead of actually moving your vehicle.

And secondly, different tires use different rubber compounds that have different rolling resistance.

I assume you measured it multiple times, and not just once to minimize the effect of different ambient temps, traffic, wind direction, gasoline brand, etc.?

 

[daves87rs]

Maybe you just have to break the tire in a bit.

 

[Brons2]

Some tires have higher rolling resistance than others. Then there are tires that are specifically engineered for low rolling resistance like the Michelin "Energy" tires and Goodyear Assurance Fuel Max.

 

[MNgopher]

That minor difference may not even be the tires at all as others have pointed out. Get a long term average and then it may be statistically significant.

New tires may decrease mileage slightly through a variety of factors. Higher rolling resistance (tread, rubber, squirm, etc...), perhaps a slightly different size (there is some slight variation in size even if it says it is the same size as the old - there is some wiggle room in there), increased weight could be in there (often the case on light truck tires), different air pressure may be optimal, etc...

 

[Solo2driver]

New tires are taller also (more tread)... slower spinning tire adds less miles to the odometer, having the appearance of lower mpg....likewise a shorter tire will spin faster and "add" fractions of miles to the odometer, but may also have the engine running at slightly higher rpm.

 

[Oregoonian]

Should have purchased General Altamax HPs.......

Pirelli P5s do not have very positive user rateings on the TireRack website...check it out.

 

[opus1]

Dumb question but I have to ask (it's my forte'

); are the tires exactly the same size as the ones you replaced? Not a different size but +0, but exactly the same size?

The reason I ask is that when I replaced the tires on my Dakota, even though I got a different size that's advertised as a +0 change, there is enough of a difference that my speedo is off slightly.

 

[powayroger]

Tire rack does not show the P5. Discount does.

I have kept tabs on my mileage since day one of the vehicle. Even when it was new, it was making teh same MPG

 

[BeanCounter]

Rolling mass on your tires can make more difference than anywhere else on the car. I would have expected a small change like this.

 

[powayroger]

same size and everything.

 

[BeanCounter]

Originally Posted By: powayroger
same size and everything.


Exact same weight? Probably not. Road grip, as mentioned by some others, definitely comes into play.
Also, tire sizes aren't always "exact" from the manufacturer.

 

[suspiciousmind]

Your old tires weren't the same size when you took them off as they were when new. Likely they were at least 1/2 inch smaller in diameter due to wear.

Your new tires aren't the same size as your old ones. Your "percieved" mileage will be different.

 

[MNgopher]

Same size does not mean the exact same . It means the tire falls within the range that can be branded as that size with that particular size. It is likely the tire also weighs different due to different construction materials and methodologies between manufacturers.

When looking for tires for my pickup, the same size tire varied in weight by more than 5 pounds for the same size, same weight rating between brands and types of tire - and that's before even changing load ratings. It can make a significant difference.

Pirelli does not appear to have the techincal data for this tire published yet, so can't rally compare there yet.

 

[shpankey]

Originally Posted By: Oregoonian
Should have purchased General Altamax HPs.......

Pirelli P5s do not have very positive user rateings on the TireRack website...check it out.


Yup those are great tires. I like the Continental ExtremeContact DWS as well.

 

[CapriRacer]

Long Version:

http://www.barrystiretech.com/rrandfe.html

Short Version:

1) New tires have more rolling resistance than worn out tires - all other things being equal.

2) There is a technology triangle between rolling resistance, traction - especially wet traction- and treadwear. To get improvement in one area you have to sacrifice another area (or both)

3) OE tires generally go after rolling resistance.

4) You went after treadwear.

 

[shpankey]

Also, just a heads up. Sorry, ignore this unless you're buying tires as I don't want to derail the thread. But for a great bang for your buck tire, the Goodyear Eagle F1 A/S-C cannot be touched. Amazing low cost and have an $80 mail in rebate up until Feb. 28th which puts them at close to $200 cheaper than the Continentals! They have amazing snow traction too, from the user reviews. I wanted the Continentals so bad. They are no doubt the best all season tire out there, period. But I cannot pass up the value of the Eagle F1's.