What is the difference between the John Deere 6M 180 and 6R 180?

Here is how the John Deere 6M 180 and John Deere 6R 180 compare on John Deere's published specifications:

• Horsepower: John Deere 6M 180 178 hp vs John Deere 6R 180 178 hp
• Pto horsepower: John Deere 6M 180 140 hp vs John Deere 6R 180 177 hp
• Weight: John Deere 6M 180 19180 lb. vs John Deere 6R 180 19621 lb.
• Fuel capacity: John Deere 6M 180 85.9 gal. vs John Deere 6R 180 95.1 gal.
• Wheelbase: John Deere 6M 180 110.2 in. vs John Deere 6R 180 110.2 in.