New wheels and speedometer
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Joined: Mar 2004
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From: Kansas City, MO, USA
1989 560SEL, 1999 S600, 2005 S600, 2008 S550 4Matic
New wheels and speedometer
I'm considering purchasing a set of new 16 inch chrome wheels mounted on P225/R60 tires for my 84 300SD. I was just wondering, since the car's original equipment was P195/70R14 tires, will the change in aspect ratio affect the accuracy of my speedometer? Any assistance would be greatly appreciated.
Thanks all.
Thanks all.
Last edited by braverichard; Jul 22, 2004 at 11:35 AM.
Thread Starter
Member
Joined: Mar 2004
Posts: 235
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From: Kansas City, MO, USA
1989 560SEL, 1999 S600, 2005 S600, 2008 S550 4Matic
Somewhat significant. With the P225/60R16, the speedometer will be showing that I'm doing 55mph when I'm actually doing 60.2mph!! Sure way to get the cops' attention!
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From: Nashville
300SEL
Originally Posted by braverichard
Somewhat significant. With the P225/60R16, the speedometer will be showing that I'm doing 55mph when I'm actually doing 60.2mph!! Sure way to get the cops' attention!
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MBWorld Fanatic!
Joined: Jun 2003
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From: Nashville
300SEL
Originally Posted by howdosaymercede
Are you sure about the speedometer being off by that much?
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Joined: Mar 2004
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From: Kansas City, MO, USA
1989 560SEL, 1999 S600, 2005 S600, 2008 S550 4Matic
Originally Posted by howdosaymercede
Are you sure about the speedometer being off by that much?
On a car whose speedometer is factory calibrated to work with 195/70-14 tires, the proper size for a 16" wheel mounted tire would be 205/55-16. Reason for this is because as the width of the tire increases, the aspect ration should decrease so as to try to retain a diameter as close to the original tire as possible. It is recommended that you do not exceed 3% of your original tire's diameter as you 1) mess with your speedometer 2) add excess, un-sprung weight to your suspension 3) thanks to the increase in weight, you can experience decreased braking ability and/or face brake failure 4) mess with your transmission shift points - a direct consequence of messing with the speedometer.
A 225/60-16 tire has an overall diameter of 26.62 inches, while the stock 195/70-14 tires has an overall diameter of 24.74 inches. That's a 7.07% increase in diameter from the stock ones to the above mentioned 16 inchers. End result? Well, when your car with the factory wheels is doing 60mph, the car with the 225/60-16 will be doing the same speed but the speedometer will be showing it going at 55.7mph. Why? Because of the new, larger diameter of the tires, with which the speed sensors tell the speedometer what to show as your current speed. Of course, you could always re-calibrate the speedometer, but for someone like me who doesn't want to get that deep just for the want of bigger tires, I'd rather stick to what I know will work: 205/55-16. With these tires the diameter would only change from 24.74 inches for the stock tires to 24.84 inches. Speedometer speed would only vary by 0.53%. So when, with the stock tires, I'm doing 60mph, my speedometer would be showing that I'm doing 59.6mph with the new tires. Not bad!
