Just put new belts on my '85. Why do I always seem to have to get a slightly larger belt than the ones put on by a shop? I know how to adjust the tension. There must be a gap in my knowledge.
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Chuck B '89 245 DL, '85 244 DL

These are the right belts for your car: (thanks FCP Groton)

10X850 P/S Belt 85-92 240 Gas Models w/ AC $4.00 Continental
13X975 A/C Belt 85-92 240 all Gas Models $5.95 Continental
973537 Alternator Belt Set (contains 2) 85-93 all 240 Gas $8.50 Continental

That last item is simply 2X 10x920 belts. Anything else, while probably workable, is not *correct*. I've never come across a problem using these belts on all of mine and all that I've worked on.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE: 625K total

One of two reasons:

Either the one you bought is longer, or the one the shop put on was shorter. Parts books usually list one part, but maybe the shop had a 1000mm instead of a 1013mm or something like that. Usually you can adjust some of the 200 tensioners that much. Even more than that if necessary. There is little downside to doing that also; if the part is there, they'll sometimes just put on whatever is close but works well.

Also, if you don't order belts by exact size in a parts store, they will sometimes give you the closest match as well. They're usually "right", but not necessarily exact.
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1992 940 wagon, very low miles
as well as others.