When i got my new bike (Trek FX 7.4) i also purchased the Bontrager 5W...are they pretty accurate when set to the exact wheel size? (700x32)

I can't speak to that computer but my most accurate readings come when I measure the wheel by rolling it out.

How accurate to you need it to be?

If you set the computer to the nominal tire size, it will be about as accurate as a car odometer. That's accurate enough for me. If you roll out your wheel, you can gain some increment of accuracy. Some riders roll out and average multiple wheel revolutions for even more precise accuracy. Unless you are a person who pursues accuracy for it's own sake, I don't see that it makes a lot of difference.

I wouldn't use one to do a property line survey, how obsessed with down to the last CM .
Of accuracy on the road are you?

a tire size menu is probably close enough .. Were the input MM of wheel radius, or roll-out as said above,.
measured while you are sitting on the bike with the standard tire inflation, would gain more accuracy..

Car odometers are supposed to be, by law within 5% which means a car with 100,000 miles on it, could read as low as 95,000 miles or as high as 105,000 miles. I measured out both the road bike and the mountain bike, the road bike measured out 2100mm where as the chart that came with the computer read 2124 a difference of a little under 1.2%. Which is close enough.

My wife bought a 5W for her recumbent trike and had nothing but problems with it. It was terribly inaccurate, even after setting the tire size several times (I use the rollout method), it seldom started when the bike started to roll and sometimes took a mile or two before it would start tracking. She took it back to the dealer and got another one, but it wasn't any better than the first one. We still have it but it's no longer used. She replaced it with an inexpensive Sigma after she bought her Garmin Forerunner.

I finally got around to checking my Bontrager 5W against my vehicle odometer...on a 1.7 mile course, it was within 10-20 ft. of right-on with my minivan odometer!...close enough indeed!...it was my old bicycle computer which was quite a bit off...when i got my new 5W i noticed the difference and that was the reason for the original question...thanx for all of the interesting responses!

[QUOTE=Gnosis;15849805]When my cousin purchased his 29er hardtail he also had the LBS install a new Bontrager speedometer (around $49 as I recollect). While it worked well enough, its button usage wasn’t intuitive and it was far too easy to accidentally reset the wheel size. For that reason alone, I’d likely not purchase a Bontrager speedometer, but it also perpetually indicated the actual time (hence never shut completely off when not in use), which meant its battery would be depleted sooner.

My numerous cheapy 12-function Schwinn speedometers have worked fine for the last 2+ years now and they were only $13.00 each at Walmart. I only require the following functions:

Trip Distance
Max Speed
Average Speed
Trip Time
Actual Time
Odometer

My cheapy Schwinn speedometer allows me to re-enter the odometer reading if the battery fails or I have to perform a reset to re-enter a different tire diameter.

In the picture I provided, I temporarily attached my Schwinn 12-function speedometer to monitor the rear wheel of my Trek 2.1 road bike, as I wanted to see how fast I could crank the rear wheel by hand in its highest 50/11 gearing with the bike flipped upside down on its handlebars and seat. With the correct tire diameter entered, you can see that I achieved 74.6 MPH. I had also just balanced the rear wheel with its 700 x 28 tires installed, so I wanted to see how smooth it was when cranked up as fast as possible. It was very smooth, but I wouldn’t want to get my fingers, hands, or arms caught in its spokes at that speed, as it would have no problem severing them. The wind generated by the rear wheel was blowing papers off my table located several feet away!

I recently bought an Iron Horse computer,same company as Schwinn.Easy to program,easy to read ,large numbers.No cadence but features such as temperature and a light for nite rides.20.00 bucks.I had nothing but problems with the cateye strada afterabout 5months.