Debt Ratio calculations ? car or house option , i want both ?

Why is debt ratio calculated on gross amount and not the net amount ? When bank factors in car payment, they are not factoring how much the insurance would be for car going toward det ratio ? should they ? this is a hidden expense with car payment that varies ?
credit score 655, B rating.
My pay is 2990 gross amount, i live by net amount.
-Student loan 333 a mth. because of 30,000 dollars debt.
-Rent $350 mth.
-Utilities,heat $150 winters mth..
-Cable $30 mth
-Groceries 200 mth.
my honda accord lx ,car valued at 1700 blue book. worth nothing to banker she said.

Ratios will vary depneding on credit and income. They figure everything on gross. Loans can be made on up to 50% total debt of your gross. If you make 2990, a loan officer COULD give you a loan on total debt of 1500 or so. 330 student loan + 300 car deducted from 1500 leaves 870 for a house payment. The remaining bills; cable utilities, insurance , groceries etc… are assumes to be paid from the other 50%. If you take home 2300 of your 2990, and spend 1500 on bortgage, car payment etc.. that leaves 800 to pay your other bills. If this leaves you money to live on, then go for it. If not consider keeping your old car and just getting the house.

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