my 2008 mazda tribute will not go over 50mph. why?

It is possible the catalytic converter may be starting to get plugged. Would the rpm's of the engine only go so high winding out a gear? Have that checked at an exhaust shop.

i had that checked out and was told the converter was not plugged.

It is possible the fuel regulator would be bad, not able to create enough fuel pressure. Its not because of the transmission is it? RPM's of the engine stay the same when at 50 mph?

Test/replace the DPFE ... differential pressure feedback EGR sensor...known issue or remove and clean maf and trottle body butterfly valve. .known issue, u may even have to replace entire valve body..cheap part about $70 us?

Definatly EGR valve, just buy a new one not too pricey.