Mine could probably hit 600 with cruise set at 55 and no A/C during moderate spring/fall temperatures. I've got to about 550 before but I've never kept it an honest 55 for the whole tank.

Right now with 50% A/C use my reserve light comes on at about 460-480 miles. In the winter it comes on closer to 400-420 miles. This is with mixed driving.

There is no way to get 600 miles to a tank under normal conditions. You would have to try really hard under the right conditions.

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1984 300 SD 384,000 Miles