I'm considering the purchase of a 1980s 300SD. The 1991 560SEL is on one side of the garage, the other side has a gas heater and gas hot water heater taking up approximately three feet which means the 1991 560SEL will not fit on that side. Question; is a 1980s 300SD a shorter car than a 1991 560SEL?

They're both the same chassis. The 300SD is the "short" wheelbase, the 560SEL is the "long" wheelbase. The 300SD will be a bit shorter. My SDL fits in my garage with the license plate touching the front wall and has ~3" clearance to the garage door in the rear. House was built in '59, so I can only assume they designed the garage for a Nash Metroopolitan...

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

This is true, unless the OP actually ends up with a 1980 model...

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Christopher
'06 Mercedes E350 station wagon (silver/black)
'85 Mercedes 300D (black pearl/palomino)
'85 Mercedes 300SD (smoke silver/burgundy)
'79 Cadillac Sedan DeVille

'05 Toyota Camry (because always running is nice)

'85 Mercedes 300D sold back to orig. owner 8-1-06
'84 Volvo 264GL Diesel, owned 2000-2013

Wheelbase difference - 140mm/5.5in.