Tank Size??? (SOLVED)

I've had this tank for about a year and it has been pointed out to me that it may not be a 55gal. I'm not real sure how to calculate it .
The tank dem. are 24" deep X 16" wide X 40" long.
I know that there is 7.48 gallons in a cubic foot, but I'm not real sure how to figure it.
Any help would be great!
 
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