When I search tires with vehicle model,
tires with wrong weight index appear.

I emailed about this to tire rack, but I did not get answer yet.

1992 300D: weight index 91.
But tire rack shows tires with 88 and 89.

when Mercedes selected the load index for you car they did so, with the idea to allow lower tire pressures for softer ride quality and still be able to run at triple digit speeds in Europe. While those types of speeds are rarely run here in the states most people also run higher pressures for better steering response and less of a "floaty" feel.

Mercedes actually recommends 26 psi in the tires on the your 300D.

At that pressure the tires load capacity is only 1116 lbs.

So, when you look at a p-metric tire's load capacity it more than meets the need. You will find a majority of Euro-metric tires do have a 91 load index while most P-Metric tires will have an 89 load index.

an 89 index is more than capable of supporting the weight of your W124

hope this answers your question

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