Citrus isn't my forte, but maybe a pomelo? What color is the flesh? There are other members that are more citrus knowledgeable, and they will probably pop in before long to help out with the id.

Looks like a pomelo to me too.... Fore barer of the grapefruit.

Agree, pomelo. Depending on the variety it can be great for raw eating. Otherwise, you can always candy it or use it for zest or use it in cocktails!

Pummelos generally have very smooth skins. It might be a rootstock that has taken over the graft. I can't see the base of the tree but the multiple trunks would support that. What does the trunk of the tree look like? Is there a dead stub above where all of the current trunks grow? Here is a page from UC Riverside which has a citrus breeding/preservation program, it lists common types of citrus rootstock:

http://www.citrusvariety.ucr.edu/citrus/rootstocks.html

It might also be a ponderosa lemon, they grow large, have thick skins and have a bumpy exterior

http://www.citrusvariety.ucr.edu/citrus/ponderosa.html

The citrus forum is very active and has several people who could probably do a more definitive ID.

http://forums.gardenweb.com/forums/citrus

Looks to me like a lemon root stock. Big Thorns?

It is most likely root stock that is a random blend.

Well, pomelo would taste like a sour grapefruit, and a Ponderosa Lemon would still taste like a lemon.

As the discussion here should be showing by now orchard fruits are identified using several samples including ones cut in half to show the interior, may still need to be examined in person in order to find out things like aroma, flavor and texture.

Cuban Shaddock is commonly used as a rootstock for grapefruit. That may be what it is.

It's not really a variety intended for eating, but it is a form of pomelo.

Cuban Shaddock commonly has wrinkly skin and is often mistaken for "giant lemons".

See another similar thread here:

[https://www.houzz.com/discussions/ponderosa-lemon-cuban-shaddock-dsvw-vd~4033229[(https://www.houzz.com/discussions/ponderosa-lemon-cuban-shaddock-dsvw-vd~4033229)

I would identify it as "Rough Lemon" rootstock, which has overgrown and pushed out the scion wood entirely. Rough Lemon is NOT really an edible or desirable lemon, nor is the trifoliate orange rootstock which is about as commonly used as citrus rootstock, nastily bitter smallish fruits that resemble miniature oranges in appearance. I think trifoliate is not even in the citrus genus, but rather in the genus poncirus. But a good rootstock, and that's all.

by looking at the shape and size, I vote ponderosa lemon.