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Old 03-20-2007
Thread Starter
Join Date: Dec 2006
Location: San Francisco, CA
Posts: 26
Speedo Adjustment

Now that I have 30x950x15 BFG ATs on my truck
where I USED to have 225/70/15

with the old tires, it could always run down the hiway at 72MPH
now the best it would do is about 68MPH

It's only a 4 wz a 5 spd, so it seems with the bigger tires I lost some speed, power and MPG

How can I tell what the real speed is on the sppedo since the change??
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Old 03-20-2007
Marcaronio's Avatar
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Join Date: Oct 2006
Location: Spring Hill, FL
Posts: 560
Convert Metric Tire Sizes to Inches:

225/27R15 = 27.4 inches diameter

Checking your Speedometer After Changing Tire Sizes:

Original Tire Diameter: 27.4 inches
New Tire Diameter: 30 inches

At 100mph actual speed is 109.5mph

109.5 / 100 = 1.095

Multiply whatever speed you are traveling by 1.095:

60mph * 1.095 = 65.7mph

Additionally for calculating gas mileage:

1. Fill tank completely
2. Reset Tripmeter
3. Drive until tank is close to empty
4. Fill tank at same pump
5. Take mileage on tripometer and multiply by 1.095
(ex: 200 miles * 1.095 = 219 miles actual distance traveled)
6. Take amount of gasoling required to fill your truck and divide actual distance traveled by amount of gasoline
(ex: 219 miles / 12 gallons = 18.25 mpg)

Taa Daa!
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Old 03-21-2007
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Join Date: Dec 2006
Location: San Francisco, CA
Posts: 26
just the info I needed
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Old 03-21-2007
lifted97ranger's Avatar
Join Date: Nov 2005
Location: Charlestown, IN
Posts: 1
if you have an '01+ get a SCT XCalibrator 2.....it will correct your speedo and give you performance tunes...
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Old 03-22-2007
Thread Starter
Join Date: Dec 2006
Location: San Francisco, CA
Posts: 26
get a what??
more details please
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Old 03-22-2007
01xltranger4x4's Avatar
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Join Date: May 2005
Location: MI
Posts: 3,200
Superchips programmer. It reprograms your computer to advance timing to accomidate for a higher octane fuel, which generates more power. It also can read diagnostic codes, firm shifting in auto transmissions, reprogram your speedometer for different tire sizes and or gear ratios, etc. Search google for "Bamachips" for more information.
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