braverichard

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.

schwarzwagen

Go with 225/55-16 for proper aspect ratio

howdosaymercede

Not by very much

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!

schwarzwagen

that's why I use the proper tires...

braverichard

Yeah, well P205/55R16 will not mess with the speedometer.

howdosaymercede

Are you sure about the speedometer being off by that much?

schwarzwagen

It should, the proper 205 sizing for a 16 inch wheel is 205/60.

schwarzwagen

I think its really only 1-2mph, nothing really that big. I have seen many many w126s with 225/50s or 225/60s. In fact, I am the only one I have seen with a w126 w/the proper sizing, at least in parking lots and such.

braverichard

OK...

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!