Dear Car Talk:

I have a 2012 Prius V and was getting approximately 50-51 mpg. At 47,000 miles I had the tires replaced with Yokohama tires, and my mileage immediately dropped to 40-42 with the exact same driving habits.

I asked the dealership to look into it as I had never experienced this with a tire change in my two prior Priuses. They told me the diagnostic was negative and this was a known effect of new tires.

If this was true, my mileage should have been lower when I first bought my car with new tires and gotten better with time as they wore down.

This doesn't make any sense to me. The increased surface area and friction cannot account for such a decrease, and the car now feels sluggish, like it's always in low gear. It just doesn't want to speed up. -- Anahat



The question is whether you got the right Yokohama tires, Anahat. Cars like the Prius, which are driven by people who know their mileage down to the second decimal place, use special, low-rolling-resistance tires. Low-rolling-resistance tires have specially formulated rubber that reduces rolling friction, and therefore, increases mileage.

The downside is that you give up a bit of traction (which relies on friction). But since most Prius V drivers aren't going out for joy rides on twisty mountain roads, pushing .96 g of lateral acceleration while yelling "Yee haw!" most of them happily take the mileage over the traction.

What I'm wondering is whether your dealership gave you a tire with higher rolling resistance than your originals. If you got a standard Yokohama tire, because that's what the dealer had in stock, it could be a tire that would be fine when replacing a similar tire on a Camry, but could noticeably reduce the mileage on your Prius.

Twenty percent is a big drop in mileage, but I suppose it's possible, especially if they're also a bit underinflated compared to your old set.

If you go to tirerack.com, enter your car's information and then search under "Original Equipment," you can find out which tires came with your car from the factory. Usually, there are several different tire models that manufacturers use on any given car, and that's the case with the 2012 Prius V.

In looking it up (we chose the higher trim level; the lowest trim level came with cheaper tires), we found that the 2012 Prius V came with either Michelin Energy Saver A/S, Bridgestone Turanza EL400-02, or Yokohama BluEarth S34 tires.

Have a look. If the Yokohamas your dealer sold you are not equivalent to your original tires, you should ask him to take them off and sell you a set of original equipment Prius V tires as replacements. And if he doesn't have them, he can order them for you.

Let us know what you find, and if you are able to return yourself to good standing in your local Prius club. We've seen the terrible public shamings they give people who get mileage in the 40s!

Read more about Best Tires for the Toyota Prius here.