First - All my responses are constructive, no criticism here. I just don't convey my thinking clearly so sometimes it sounds like criticism, but it is not.
Second - How much data do you have to support the 28 mpg city number? Just asking as I don't know. Sounds like you may be estimating this tank by tank or by the A/Avg number.
Third - The 0.01 lbs/min versus the 0.02 lbs/min ... there's just not enough granularity on those numbers to be able to actually distinguish that those are actually different numbers. Or different enough to know they're different. Difficult to describe my thinking. It's like one number is 0.01495 and the other is 00.01505 (and gets rounded to 0.02 because that's all the tool can display). So I consider that whole issue as a non-issue. Your mpg numbers seem good on these two cars so, I recommend you put those numbers out of your mind.
Fourth - If the problem on your 2015 ES were something like a brake dragging, it would effect your highway mpg as well. If there is a true difference, it would be isolated to the dynamics to do with city driving. Since your highyway mpg on that car seems to be inline, I think I would base my final assessment on said vehicle to be based on overall average mpg of that vehicle versus the others. And I would not just (try to) isolate out the city mpg.
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View my fuel log 2020 Mirage ES 1.2 manual: 42.5 mpg (US) ... 18.1 km/L ... 5.5 L/100 km ... 51.0 mpg (Imp)