just wondering if anyone could tell me a good way to test its accuracy? thanks.

Roll out test... https://www.sheldonbrown.com/cyclecom...libration.html

What is your tire size?

Most highways have mile markers on them, reset to zero and check distance between them. Or, what DSott says.

Reset your car's tripmeter and drive to any marking or distance of your choice, then do the same with your bike and compare...viola!

Cars have some seriously inaccurate speedometres and odometres.

Most of them have a 10% tolerance.

Uhhhhhhh................which one's right? Car odometers are notoriously bad.

Tire sizes and inflation may be the influence in the inaccuracies. Maybe also vehicle brands? My good ole' Chevy has been checked with a mile marker with good condition stock sized tires with proper inflation, and it was on the button within a couple hundredth/mile, so I suppose I was only talking about my situation. YMMV?

edit: I forgot to mention my Garmin GPS speed is also the same as my speedo and Heads-Up Display.

What information from the computer would you like to test? Speed? Distance? Cadence?

Sections in my part of the world are one mile on a side. Ride a section and see if you get 4 miles.

Sheldon's rollout is the best method. Most computers use your wheel circumference in millimeters which means you can give it a value with four significant digits. You can measure a single wheel revolution rollout accurately to within two or three millimeters.

You can improve your measurement accuracy to get that last millimeter by doing the following:

1. Get a long tape measure, 30 feet or 8 meters, long enough for a four revolution rollout.

2. Pick smooth pavement with a straight line to ride along. Pump tires up to your usual pressure. Load bike with yourself and whatever you car including water.

3. Put tape down on the pavement for the start point, tape on the tire sidewall to line up with the pavement tape, and have a friend put tape down on the pavement at the end of the 4 revolution rollout.

4. Measure to nearest millimeter, divide by four, put in bike computer. This number will be quite a bit different from the one on the instruction sheet.

5. Repeat after one year as your tires wear down and get larger.

You will have the most accurate mile measurement of all your buddies, including the GPS equipped ones. +-0.05% You can bring your tape measure and tape with you to your next ride and offer to help calibrate your friends' computers and they will all decline because they rather keep their wildly inaccurate bike computer settings.

Car odometers are inaccurate. Sometimes purposefully so.
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