The owners manual states that tire should be replaced when the wear bars become visible and only 1/32" (0.8 mm) tread depth remains. The following cut and pasted has some very interesting points.

What is your opinion of the following?


http://reviews.ebay.com/Measuring-Ti...:-1:LISTINGS:2



The Law

According to most states' laws, tires are legally worn out when they have worn down to 2/32" of remaining tread depth. To help warn drivers that their tires have reached that point, tires sold in North America are required to have indicators molded into their tread design called "wear bars" which run across their tread pattern from their outside shoulder to inside shoulder. Wear bars are designed to visually connect the elements of the tire's tread pattern and warn drivers when their tires no longer meet minimum tread depth requirements.



Common Sense

However, as a tire wears it's important to realize that the tire's ability to perform in rain and snow will be reduced. With 2/32" of remaining tread depth, resistance to hydroplaning in the rain at highway speeds has been significantly reduced, and traction in snow has been virtually eliminated.

If rain and wet roads are a concern, you should consider replacing your tires when they reach approximately 4/32" of remaining tread depth. Since water can't be compressed, you need enough tread depth to allow rain to escape through the tire's grooves. If the water can't escape fast enough, your vehicle's tires will be forced to hydroplane (float) on top of the water, losing traction.

If snow covered roads are a concern, you should consider replacing your tires when they reach approximately 6/32" of remaining tread depth to maintain good mobility. You need more tread depth in snow because your tires need to compress the snow in their grooves and release it as the they roll. If there isn't sufficient tread depth, the "bites" of snow your tires can take on each revolution will be reduced to "nibbles," and your vehicle's traction and mobility will be sacrificed. Because tread depth is such an important element for snow traction, winter tires usually start with noticeably deeper tread depths than typical All-Season or summer tires. Some winter tires even have a second series of "wear bars" molded in their tread pattern indicating approximately 6/32" remaining tread depth to warn you when your tires no longer meet the desired tread depth.

 

My personal opinion would at first be to not buy used tires. JMHO. (your ebay reference)
Use the gage as a guideline. I always replace tires BEFORE there is any issue.
On a bike you have only 2 things between you and the road. Dont try to save money here. Safety first and use common sense.
MikeM

 

My State uses the wear bars as the legal standard for safe tires. Good enough for me. No need to change a tire before it's time imo.



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2008 Heritage Classic: His
2008 Deluxe: Hers

 

If I replaced at 6/32, I'd never get out the door. My D402's only come with 5/32 brand new.

 

I got a HD tire gauge as a present and it has a dial indicator that you use to measure the tread. When it goes to the red zone, i get new tires

Bubba