Google Inc. began shuffling its rankings for mobile websites Tuesday, potentially benefiting sites like job board Indeed.com and real-estate companyZillow Group Inc., and hurting others like retailer Zumiez Inc. and IRS.gov.

Google said it tweaked its algorithm for mobile searches to favor sites that look good on smartphone screens, and penalize sites with content that is too wide for a phone screen and text and links that are too small.

Given Google’s importance in driving traffic to websites, some in the industry dubbed the change “mobilegeddon.” Google updates its algorithm frequently, but this is considered by some experts to be its most significant change in years. In an unprecedented move, Google warned website operators in February that the change was coming, and gave them tips on how to prepare.

“A lot of sites have been racing to beat the clock on this issue,” said Mark Ballard, director of research at Merkle RKG, a search-marketing firm.

The change is also important for Google. A company executive said at a conference last year that searches from mobile devices were poised to surpass searches on personal computers.

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Google’s algorithm favors sites that look good on smartphone screens. PHOTO: CHRIS RATCLIFFE/BLOOMBERG NEWS

But advertisers typically pay less for clicks from phones, because they don’t lead to sales as often. Encouraging developers to tailor sites to look good on smartphones should lead to more sales and consequently higher prices for Google’s mobile ads, said Matt Ackley, chief marketing officer of Marin Software, an advertising technology firm. Advertising accounted for 90% of Google’s $66 billion in revenue in 2014.

Google also wants more users to surf the Web on their phones instead of using mobile apps. Google sells ads that point to websites, but generally cannot direct searches to content inside apps. Some companies, including India e-commerce giant Flipkart, have dumped their mobile websites and told visitors to use their mobile apps instead.

“As people increasingly search on their mobile devices, we want to make sure they can find content that’s not only relevant and timely, but also easy to read and interact with on smaller mobile screens,” a Google spokeswoman said.

In a blog post Tuesday, Google said it had seen a 5 percentage point increase in the proportion of sites that are “mobile friendly” since its February announcement. The spokeswoman declined to specify the percentage of sites that are mobile friendly.

The change isn’t simple for sites to roll out, since they must change all of their website’s pages to optimize them for smartphone screens.

According to a Google testing tool, the websites of Zumiez, technology wholesaler CDW Corp. and the IRS aren’t mobile friendly. Nor is Google’s own Finance site for stock quotes and related information.

United Healthcare Services Inc.’s primary site also failed Google’s test for mobile friendliness. However, the company has an additional site for people it insures that a spokeswoman said is designed for smartphones.

Zumiez, CDW and the IRS didn’t respond to requests for comment. Google didn’t respond to a request for comment on its Finance site.

Failing the test doesn’t mean sites will automatically fall out of Google’s rankings on smartphones, notes Rand Fishkin, founder of Moz, which makes software to help site operators improve their search rankings. Mobile-friendliness is just one of over 200 different “signals” that Google uses for ranking its results, the company said.

Research firm Searchmetrics said Zillow and Indeed saw a boost in their search rankings recently. Searchmetrics said Google may have started testing its new algorithm before Tuesday.

Jeremy Wacksman, vice president of marketing and product management at Zillow, said the real-estate information site has long designed new features with mobile devices primarily in mind, because roughly 70% of its traffic comes from mobile devices. Indeed didn’t return requests for comment.

The government’s preparedness for Google’s change varies. While the IRS website fails Google’s test for “mobile friendliness,” FEMA.gov and Healthcare.gov get passing grades.

Winners and losers from the change won’t be fully clear until after Google finishes rolling out the new update, which it said will happen over the next week.