Looking beyond impression share: identifying more reliable metrics for scaling your Performance Max campaigns.

Are you looking to scale Performance Max campaigns, but unsure what metrics to look at to judge the success of adding more budget?
As a dealer, you may be used to looking at Impression Share in your Search campaigns to see if you’re reaching everyone in your area searching for “car dealers near me.” Unfortunately for Performance Max, this metric isn’t as reliable because this campaign type can serve in various placements outside of Search (including Shopping, Display, YouTube, Maps) and doesn’t rely on keywords. This essentially gives us no way to accurately measure all possible impressions.
So, how do you determine when to add more budget to your PMAX campaign? It’s important to first think of what your goal is. If the ultimate goal of the campaign is to generate leads, then conversion rate & CPA should be the main KPIs used to evaluate success.
How to determine how much budget to add.
At the campaign level, a “limited by budget” status may appear for campaigns that Google deems underfunded, and it will provide a recommended budget and expected lead lift. While we review these metrics to determine what is possible, your search team ultimately takes into account your overall performance and goals before providing a budget recommendation.
While we don’t use impression share to judge market share with PMAX campaigns, we can look at the impression share lost to budget. This metric gives us an idea of how much budget we need to add to a campaign in order to have the campaign fully funded.
For example, the below screenshot of this column in Google Ads suggests 24 - 44% more budget be added for three campaigns. An important note when looking at the impression share lost to budget is that this only accounts for the search placement, while your Performance Max campaign is also serving Vehicle Ads and additional Google placements. This metric is a good guide of where to start, but it may take more budget to raise both the search & shopping presence of your campaign.

These budget recommendations are also specific to your campaign's current location & audience targeting. If your dealership is also considering expanding your audience, your paid search team can use the additional tools mentioned above to recommend a budget in this scenario. Current campaigns should be funded fully before expanding.
When scaling campaigns, we do expect to see some increase in CPAs, but the volume of leads should still be steadily increasing. We recommend increasing the budget by 20 - 25% at a time. As an additional best practice, incremental budget should first be used to ensure your search campaigns are fully funded before adding budget to your PMAX campaign.
How to evaluate success at scale.
These tools all give you a starting point to scale from. When an incremental budget is added, we recommend waiting at least one full month before judging success.
A few key metrics should be monitored to ensure campaigns remain efficient & not overfunded after this point. If CPAs rise by more than 50% and remain high, this can be a sign to lower the budget to remain efficient. The goal is to get more leads, and not to pay more for the same leads you were receiving before.
Your search manager can also take a look at the channel reporting tool available for PMAX and see if your channel split has changed dramatically since adding budget. If, after a month, the new budget is going towards Search & Shopping placements, that is a good indication that you are still reaching high-value customers.
If the percentage of the budget going to channels like Display, Discover & Gmail has risen, this isn’t necessarily bad, but it's best practice to check whether those channels are converting for you. If spend is higher on placements without incremental leads, this is a sign you may want to dial back to your original budget levels.
Performance Max scaling best practices.
- Ensure the goals you are measuring success by are set up correctly
- Use impression share lost to budget as an indicator to adjust budgets
- Increase the budget by 20 - 25% at a time
- Review Google Channel Placements to gauge if campaigns are overfunded
- Allow one month for campaigns to optimize towards their new spends before evaluating performance
Bottom line.
There are now many tools available in Performance Max campaigns to judge the success of the campaign, if it’s fully funded & what opportunities may be available for your dealership. When your goal is leads for your dealership, it’s important to consider the efficiency of your leads and not just look at how often your ads are showing up for relevant sources.
The best way to determine success is to evaluate performance based on the historical campaign performance, for an accurate look at how your additional spend performed. Lead volume, conversion rate & cost-per-lead will all be important factors.
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