Your question is a reasonable one, but without any additional information, the question is not entirely meaningful.
What make / model of tires did you have before? What do you have now?
For your 34mpg average, what distance are you averaging across?
How many miles have you driven to arrive at your 29mpg average?
What other environmental variables have changed?
At the extreme, if you went from steel railroad wheels with nearly no rolling resistance to track-oriented summer tires designed for maximum dry grip, then that would likely contribute to a very large loss in mpg. However, if you’re going from one all-season tire to another all-season tire with similar treadwear ratings then I’d say your fuel economy shouldn’t be varying that dramatically. A few percentage points, sure... but not in the 15 - 20% range. So that’s one thing we’d need to know first.
Next, if you are looking at an average of 34mpg across a single tank vs 29 across a second tank, then there are way too many variables to really make the comparison meaningful even if you hadn’t changed your tires out. If, on the other hand, you track your mileage and have 5000 miles logged at 34mpg then you’ll want to look at another 5000 miles with your new tires before really jumping to any conclusions. And even then, the comparison may not be entirely valid because of seasonal differences (temperature, humidity, road conditions, etc.)
To make a long story short, you probably need to look at all the factors going into your fuel economy (or lay them out in a post if you want the general community to help dive into the details more). Only then will it be possible to draw up meaningful theories. Otherwise you’ll get some guesses, but those guesses can apply to any circumstances and won’t really help you truly identify what may or may not be going on in your situation.