Ford Focus Forum, Ford Focus ST Forum, Ford Focus RS Forum - View Single Post - crappy fuel mileage
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Old 01-10-2013, 03:18 PM   #25
Magus2727
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What I Drive: 2001 MB ZX3, 2004 SY SVT ZX5

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Tire size can do cause different readings in MPG but will not cause a drastic difference "over night".

A larger diameter tire: 1 rotation = further distance traveled. So you will have actually traveled further than the OD reads. 0.5" difference not sure how much... their was a chart available that shows the MPH differences vs tire size. Larger tire may provide better freeway MPG, could have a slightly lower RPM to maintain a specified speed. City driving will cause higher MPG due to starting and stopping a tire with a larger rotational mass.

A smaller tire: 1 rotation = Less Distance. So you will have actually traveled less than the OD reads. Your actual MPG will be less, also freeway you will need to maintain a higher RPM to maintain the same speed.

Have you checked to see if you might have a dragging caliper or partially stuck drum? if you hit a pot hole and somehow had a huge change in your alignment that could be another possibility.... try testing the DPFE its easy to test.
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