DETROIT -- The Buffalo Bills' depth at running back figures to be on display in the team's final exhibition game tonight against the Detroit Lions.

It also figures to create some tough choices for Bills coach Chan Gailey this weekend when it comes time to cut the Buffalo roster to 53 men.

Bills backup runners Joique Bell and Chad Simpson are making a hard run at a roster spot. Bell ranks fifth in the NFL in preseason rushing yards with 142 on 22 carries. His 6.5 yard-per-carry average is best in the league among the top 40 ground-gainers. Simpson has 93 rushing yards and a 5.2-yard average.

Running back is a position of strength for the Bills, since they added rookie C.J. Spiller to the mix with veterans Fred Jackson and Marshawn Lynch. Fullback Corey McIntyre makes it four sure bets at the position to make the roster.

Gailey is a coach who likes to run the ball, and running looks to be the strength of the Bills' offense. Many teams keep five backs. Jackson probably will be questionable for the season opener due to a broken bone in his left hand.

"They've done a good job," Gailey said of Bell and Simpson. "They're both very unique runners. They have different styles, but they've both done a great job. That's one of those where we've got to take a hard look at when we get to Saturday [and make the roster decisions]."

The competition for the final five to eight spots on the roster probably will be the most significant outcome of the preseason finale.

The Bills and the Lions are expected to give their starters limited action -- a quarter at most -- so they stay healthy for the season-opening games Sept. 12.

Other players on the Bills' roster bubble include: defensive tackle John McCargo, defensive backs Ellis Lankster and Cary Harris, linebackers Antonio Coleman and Keith Ellison, and offensive lineman Nick Hennessey.

Bell, 5-foot-11 and 220 pounds, is an undrafted rookie who rushed for 6,728 yards and 88 touchdowns at Wayne State in Michigan. He has hit holes hard and broken some tackles. He showed good balance after getting knocked back from a pile on a 1-yard TD run vs. Indianapolis. Simpson, 5-9 and 216, is smaller and quicker than Bell. He spent the last two seasons as a backup with the Indianapolis Colts. He had a 43-yard run against Washington in the preseason opener.

Here's a position-by-position breakdown of the roster battles heading into the summer's final tuneup (the number of players likely to be kept at each spot is in parentheses):

Quarterback (3): Brian Brohm is expected to get a lot of snaps vs. the Lions. Ryan Fitzpatrick has played well behind starter Trent Edwards. Presumably, the Bills will keep Brohm and Fitzpatrick and put rookie Levi Brown on the practice squad. As a seventh-round pick, Brown is likely to make it through waivers. If Buffalo opts not to expose Brown to waivers, then Brohm might be in trouble.

Running back (5): Bell would seem to have an edge on Simpson but the Bills could keep just four backs in order to go heavier at another position.

Receiver (6): The pecking order seems to be Lee Evans, Stevie Johnson, Roscoe Parrish, David Nelson, Chad Jackson and James Hardy. Rookies Donald Jones and Naaman Roosevelt could go on the practice squad. Some teams keep a seventh receiver.

Tight end (3): This spot is tricky because Shawn Nelson is suspended the first four games and won't count against the 53-man roster. Jon Stupar starts, and J.P. Foschi has solid credentials to make the team. Derek Schouman has missed virtually all summer and currently has a knee injury. He could be back for the opener or out the first month. Rookie Andrew George is on the roster. Will the Bills keep just two tight ends until Nelson comes back?

Offensive line (9): The top four backups are Jamon Meredith, rookie Cordaro Howard, Kirk Chambers and rookie Ed Wang. Wang, a fifth-round pick, is a lock to make the team. Hennessey is probably 10th on the list. Teams keep anywhere from eight to 10 linemen.

Defensive line (6 or 7): Spencer Johnson is the third defensive end. The backups who are locks to make it are draft picks Torell Troup and Alex Carrington. Veteran John McCargo, the former first-round pick, has played well in the second half of the preseason games. He has adjusted well to the end spot in the 3-4 scheme.

Linebackers (8 or 9): The top three backups are Aaron Maybin and Chris Ellis outside and Kawika Mitchell inside. Sixth-round pick Arthur Moats looks like a keeper. That's eight players. Do the Bills want to keep undrafted rookie Coleman as another pass-rush prospect? Very possibly. Ellison is making a tough conversion to the 3-4.

Defensive backs (9 or 10): George Wilson and Bryan Scott are the backup safeties. The top backups at cornerback are Leodis McKelvin, Reggie Corner and Ashton Youboty. That makes nine. Lankster had a good preseason last year but has given up some plays the last two preseason games. He did have a pass breakup in the end zone vs. the Bengals. It will be a tough call on him.

Special teams (3): Brian Moorman, Rian Lindell and Garrison Sanborn are the specialists.

mgaughan@buffnews.comnull