Posted this on the Identify Plant forum (where someone recommended Cuban Shaddock) but was recommended here:

We moved into a house in Northern California about 4 months ago and found a tree with some type of citrus. We at first thought it was grapefruit until they grew much larger. We picked a yellow one, but the pith was about double the thickness of the actual fruit and it was extremely sour. We thought it might have grown weird since no one had been watering it for awhile. I've been watering and fertilizing it for about 2 months and there is much more fruit and one grew much larger than the previous fruits.

This one is a tiny bit over 9 inches wide. The pith is still large, but not overwhelming. The taste is very sour - almost as sour as a lemon. We were hoping pomelo, but there is no sweetness at all.

I then thought ponderosa lemon, but are they always this large? They do not get yellow until they are about 7 inches in diameter. The ones still on the tree in the below pictures are actually about 4-5 inches across and still very green. There is also not a protruding tip at the end like I've seen on others - the ends/bottoms are round.

Help! Thank you!