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Old 10-16-2002, 12:03 AM
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Post Bigger tires Speed-o-meter is off

I put bigger tires on and dont know how bad my MPH is off.

Old tires are P205/70R/15
New tires are P225/75R/15

can someone tell me how much that throws off the ol Speed-o-meter?

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Old 10-16-2002, 12:05 AM
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The quick and easy way to findout is to drive beside a car with a digital readout dash like a cadillac or buick. take a glance over see what their speed is and what yours says and you'll know how far off you are.

HK
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Old 10-16-2002, 12:23 AM
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I have tried but on coming traffic is sort of a pain

The only two lane road around here is the express way. My new toy cant go 75 MPH and everyone speeds on that road.

Drove side by side my ex wife, but there is no way that thing is right. Acording to to her car, my samurai is about 12 MPH off.

The most common road i am on is a 25 MHP then up to a 30 MPH. that is it, one in a great while i zip down a 40 mph road but not often.

The samurai is for mudding and off roading, sucks on the hwy.
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Old 10-16-2002, 05:55 AM
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Hallo; That is unique advice! I have a minds-eye view of you driving next to a big Caddy, about 1' away, straining to cop a look into the space of a pink haired old lady. Jim Carey stuff!
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Old 10-16-2002, 06:04 AM
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In California our interstates have little mile markers on the side of the road that say "mile 1", "mile 2", and so on so you can check your speedo. Also, if you are on an interstate that is sort of out in the country with overpasses at regular intervals, they are usually for roads that are on section lines (except for Louisiana which is a weird state all around!) and sections are a mile wide. Either of these markers can be used. Just drive 60mph, check the time to pass between two of the markers, and divide the time into 60. The number will be the percentage your speedometer is off, fast - over 1.0, slow under 1.0.

Also, check previous posts for links to websites with mph calcualtors. If you know the diameter of your tires you can plug the numbers into the calcualtors and know how farr you are off. If you are a math whiz just calculate the number. Each rotation of your tire sill move your car

3.141 x tire diameter

down the road. Thus,

diameter of the new tire / diameter of the old tire

will also give you the percentage of error in the speedo reading.

[ October 16, 2002: Message edited by: willys36@aol.com ]</p>
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Old 10-16-2002, 06:14 PM
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excellant. Thanks
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Old 10-17-2002, 01:43 PM
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CompFX, try www.hotrodworks.net/hotrodmath/tire.html
You will need circumference of both sizes and rear end ratio.
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Tom
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Old 10-18-2002, 10:31 AM
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Hey,
The best thing to do is: go to www.richmond.com . Do you know the pinon gearing? This web site asks you the gearing the tire size and the rpm. this calculates the mph. this ia a hell of alot easier than trying the greek formula above. have fun
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Old 10-18-2002, 12:40 PM
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Hey, that ain't no greek it's Amrcun!
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Old 10-18-2002, 12:56 PM
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Willys theres been many-a-time I goosenecked into an old ladies cloud to see how fast I was creepin by. I don't think I've ever had a car or truck that the speedo worked, always did it by feel after about the second week in the car. I think I about gave one poor lady a heart attack, she was driving one of those new caddies with the speedo on the windsheild and i was on my motorcycle. Chopped stretched and had a flaming skull on the tank, I got so close to her car without knowing it I could have reached the center consol, I followed her to a gas station and appologized. Pretty nice lady, but her on the other hand...

HK
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