40ft end dump trailer should i go tandem or spread axle ?

Will you ever go to Canada?

 

Then will having a spread not be a problem.

 

Oh yes it will! You'd better consult the federal bridge formula:

https://ops.fhwa.dot.gov/freight/publications/brdg_frm_wghts/index.htm

There is an exception which states that "two consecutive sets of tandem axles may carry 34,000 pounds each if the overall distance between the first and last axles of these tandems is 36 feet or more." The problem you'll run into with a spread is that you DON'T have two consecutive sets of tandem axles...you have ONE tandem axle and a pair of single axles...therefore you do not qualify for the exception. Unless you have 39' between the hub on the front drive axle and the hub on the rear trailer axle, you cannot put 68K on your back 4 axles. You actually LOSE weight carrying capacity when you put a spread axle on a short trailer!

 

What's your axle spacing? Consult the bridge law formula.