My front tires have 30k miles on them, with a 65k warranty. My rear tires are new with less than 5k miles on them and a 80k warranty. Does this large difference in treadwear still on the tires affect my mpg? I take 10 hour round trips at least 3-4 times a year, and my usual mpg for these trips is between 23.5 and 25 mpg. This last time, it was just over 22 mpg. Any thoughts would be appreciated.
If it could be measured you would have to use NASA based instruments to do that . Your driving patterns and speed are what you should be concerned about.
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Did you notice any significant differences in environmental conditions during your last trip? Things like temperature, winds, pavement conditions (wet, snow) can have an effect when all else is essentially equal (same driver, same car, etc).
Beyond that, I would suspect other efficiency impacts like; lower tire pressure or needing spark plug replacement for example over tire wear differences…
Do you have cold temperature where you live? Cold temperature,winter tires, underinflated tires will suck mpg.
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Beyond the things already mentioned, different makes/models of tires have slight differences in diameter, which would add further inaccuracy to the result.
However, for a given make & model of tire you could calculate the MPG variation that will ensue with wear. Simply measure the tread depth of a new one, subtract 2/32 (the minimum tread legal to drive on), multiply by 2 (that’ll give you the total diameter change with wear), multiply that by 3.1416 (to get the circumference), divide that into 63,360 (the number of inches in a mile)…
oh, never mind. I’m getting myself confused… 
Tires were inflated slightly above recommendation before trip. Destinations was cold, and car sat outside.
Have made this trip numerous times in December and never saw this much of a change. Tire pressure was fine. Car runs beautifully, which is why I thought it could be the treadware difference.
The new tires could have higher rolling resistance, dropping your mpgs slightly (given the cold weather). I wouldn’t expect just tread wear difference to cause it.
Was the car loaded with passengers,luggage etc?
Tires do vary in rolling resistance. Maybe your new rear tires have more than the ones they replaced. They are also probably heavier, because not much rubber has worn away yet. And winter formulations of gasoline tend to provide lower MPG, and winter (colder) air is denser than hot air, so more wind resistance. These are all small factors but they can all add up in the same direction.
Is it cold where you live? According to the EPA, winter blend fuel has about less energy per unit volume than the three-season blends. If you lost about 1/20th of your MPG in the past month or so it is most likely due to fuel and Boyles law (tire pressure).
Might be time for a tune up. I suspect the tire difference might affect handling more than mileage. Just as an aside though, I wouldn’t judge the life of the tire based on the mileage warranty and would prefer to have all four the same. You have two harder tires and two softer tires. How’s that gonna work in the winter?
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Mickey,
It’s not clear what you are asking, so I’ll try to cover all the bases. I apologize if this is a bit long.
First, in tires, there is a technological triangle involving treadwear, traction (especially wet traction), and rolling resistance. Improvements in one area needs to come by sacrificing one or both of the other areas.
So a tire with very good treadwear ratings will very likely get poorer fuel economy than one with low treadwear ratings. This is one of the reasons why OE tires (the ones that come on new cars from the factory) have a bad reputation. They were designed to the vehicle manufacturers spec which call for low rolling resistance and the treadwear and traction were sacrificed to get it.
But also, new tires get poorer fuel economy than worn tires, That’s because the mass being moved as the tire rolls (mostly the tread rubber) is what consumes energy (the technical term is hysteresis) - and since worn tires have less of it, they get better fuel economy.
For anyone interested, I have a web page on the subject: Barry’s Tire Tech: Rolling Resistance and Fuel Economy
Thanks. Best and most logical response I’ve had yet.
Car was loaded to the gills in both directions. Somehow my wife brings back home just as much as she takes up to her kids. And I drive a Grand Marquis, which had the biggest trunk in the business.
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Yes Capriracer is the resident tire and suspension expert/professional. Engineer type, not tire store manager. Sometimes you get conflicting information here, nothing against anyone, but its hard to know the qualifications of the answers.
No christmas tree strapped to the roof 
Very interesting thoughts. So it sounds like my rear tires (more treadwear) are fighting my front tires (less treadwear) when it comes to mpg. Now, based on this hypothesis, if I were to replace my front tires now with new ones matching my rear tires, then what you are saying is that my mpg would drop until I have placed sufficient wear on all 4 tires. Is that correct?
Mickey,
Your tires are not fighting one another. They’re just consuming fuel at different rates.
Your front tires are likely getting better fuel economy than the rears because they are more worn and have the worse treadwear rating. Replacing those with a pair of tires identical to the rears is only going to make the fuel economy situation worse, and it just might be, you’ll never get the fuel economy you once had.
It’s those long trips that really emphasize the difference in RR of tires, since most of the fuel is consumed by either aerodynamic drag, and driveline inefficiencies (tires being part of that!).
If you can find RR test results for both brands of tires - tirerack.com and Consumer Reports both test for and report on that factor - that would be useful in this discussion.