2000 has 4 bearings and the 2500 has 5. plus the retrive ratio and line capacity are different. I believe the 2000 Rd is an older model
Here is the Shimano catalogue link.
http://fish.shimano.com/publish/cont...eels06.pdf.pdf
2000 RD
Style #: SR2000R
Rear Drag
The Spirex 2000RD has many exciting features that include four anti-rust ball bearings. One Super Stopper II roller bearing anti-reverse for instant hooksetting power. Solid, unflexing aluminum frame. Lightweight graphite rotor and sideplate. Cold forged aluminum spool for increased strength and durability. Cold forged aluminum spare spool. Fluidrive II polished, oversized drive gear for increased efficiency. Floating shaft reduces friction between the spool shaft and pinion gear for a smooth, effortless retrieve. PowerRoller oversized line roller reduces line twist. Dyna-balance eliminates wobbling during the retrieve by counter-balancing the rotor. Line Capacity(yds./lbs.): 120/8. Gear ratio: 5.2:1. Weight: 10.2 oz.
SR2500 RE
Rock-solid reliability. Quick Fire IIŪ automatically centers the bail trigger for fast casting. Double paddle handle. Rear Drag has lever-adjustable Fightin' DragŪ for quick drag adjustment during the fight. at least 10 Each includes a FREE spare spool, front drag spare spool is graphite, rear drag spare spool is aluminum.
6.0:1, 5 bearings, 10.8 OZ 120 yds 10 lb test.