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Old 12-11-2008, 10:16 PM
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http://www.miata.net/garage/tirecalc.html

Go here and you will see the size difference. Take that difference in % and multiply that by how many miles you drive before filling up. This will be around 4% by what I can tell. Add what you get to your total mileage and that is the actual miles you have drove.

Example: You drive 250 miles before empty and then fill up at 19 gallons.

250 miles X 4% = 10 miles...... So that gives you 260 miles.

260 miles / 19 gallons = 13.68

Instead of: 250 miles / 19 gallons = 13.15

Little better. You also have to take into consideration the rolling resistance of the new tires + the weight difference + the fact that you have now numerically lowered your gear ratio. Around town bad ====== Interstate Good.

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