When ChatGPT and Google AI Overviews answer the same question, only 8.4% of the websites they cite are the same. Across five AI engines, any two share 15.4% of their sources on average, so being cited by one engine says little about the others.
Analysis of 27,665 paired AI answers to 2,984 questions · answers from ChatGPT, Gemini, Perplexity and Google's AI features, September 2026
Key Findings
- 1.ChatGPT and Google AI Overviews share 8.4% of the websites they cite when answering the same question.
- 2.Any two AI engines answering the same question share 15.4% of the websites they cite on average, about one in seven.
- 3.ChatGPT shares at most 10.6% of its cited websites with any other AI engine, and 8.2% with Perplexity, the lowest overlap of any pair.
- 4.Google AI Mode and Google AI Overviews share 37.8% of the websites they cite, the most of any pair of AI engines.
- 5.The website all five AI engines most often cite for the same question is irs.gov, on 14 of 715 questions.
Summary
A marketing manager checks Google and is pleased: the company's guide is one of the sources in the AI Overview for its main product question. She asks ChatGPT the same question and the guide is nowhere in the answer. None of the other sources match either. The two engines answered the same question from two different sets of websites.
That matters because customers do not all use the same AI tool. A brand that measures its AI visibility in one engine, or pools every engine into one number, can think it is covered when it is invisible to a large share of the people asking.
The overlap is small. ChatGPT and Google AI Overviews share only 8.4% of the websites they cite for the same question. Across all ten pairs of five engines, the average is 15.4%, about one site in seven. Even Google's two AI products, which draw on the same search index, share 37.8%, fewer than four in ten.
What we measured
On September 23, 2026 we asked the same 2,984 everyday questions to five AI engines from a US location: ChatGPT, Gemini, Perplexity, Google AI Mode and Google AI Overviews (the AI summary above Google's search results). For every question and every pair of engines, we listed the websites each engine cited and measured how many they had in common. We kept the 27,665 pairs where both engines cited at least one website, then averaged the results for each pair and for all pairs together.
Overlap is the share of websites two engines have in common: the websites both engines cited, divided by all the websites either one cited. An overlap of 100% would mean identical source lists; 0% means no website in common. Websites are counted at the site level, so two different pages on irs.gov count as the same website.
When all five agree, it is an official source
On 715 questions, all five engines cited at least one website in the same round of answers. Full agreement on a single website was rare: the most common one, irs.gov, was cited by all five on 14 of those 715 questions. Apple followed on 11 questions and Microsoft on 9.
Source: Suff Digital analysis of 27,665 paired AI answers to 2,984 questions · answers from ChatGPT, Gemini, Perplexity and Google's AI features, September 2026
The websites every engine agrees on are primary sources: the IRS for tax rules, Apple and Microsoft for questions about their own products, the National Institutes of Health for health questions, Delta for air travel. When all five engines do agree, they agree on the definitive source. Everywhere else, each engine goes its own way.
What this means for marketers
Measure each engine separately. A citation report that pools all AI engines hides the fact that each one draws from a different set of websites. Showing up in Google AI Overviews makes it more likely you show up in AI Mode, but tells you almost nothing about ChatGPT. Check the questions that matter to your business in ChatGPT, Gemini, Perplexity and Google's AI features, and track each result on its own.
Then aim to be the primary source. The websites every engine agrees on are the official, definitive pages on their subject. Make your site the clearest source on questions about your own products, services, prices and data, with the answer stated plainly, and each engine has a reason to cite you. An ai seo company that tracks all five engines separately, as Suff Digital does, shows where you are missing and why.
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<p style="font:14px/1.5 system-ui,sans-serif">Source: <a href="https://www.suffdigital.com/resources/data-studies/ai-engines-source-overlap">ChatGPT and Google's AI Overviews Share Only 8.4% of the Websites They Cite - Suff Digital</a></p>
Cite this study
Suff Digital. (2026). ChatGPT and Google's AI Overviews Share Only 8.4% of the Websites They Cite. https://www.suffdigital.com/resources/data-studies/ai-engines-source-overlap
ChatGPT and Google's AI Overviews Share Only 8.4% of the Websites They Cite - Suff Digital - https://www.suffdigital.com/resources/data-studies/ai-engines-source-overlap