Guest cudgel Posted April 9, 2007 Share Posted April 9, 2007 I've read about four pages of comments on the Blackbird's speedo being inaccurate and of ways of correcting it on this forum. Yesterday I was clocked by a county deputy going in the opposite direction at 75 at the same time I was looking at my speedo reading 75. Of course he had to spin his green and white Dodge pickup around so we could discuss it. I told him about the commonly held belief of the bike's gauge being optimistic and he said mine is right on. I thanked him for the help. I also thanked him for not citing me for speeding. Guess age has it's advantages. If he'd seen me about 2 minutes sooner he could have confirmed my speedo at 110! Quote Link to comment Share on other sites More sharing options...
Keith Posted April 9, 2007 Share Posted April 9, 2007 Are you sure you have stock gearing? I know from checking my speedo against my GPS that with stock gearing, my speedo was off 7% optimistic, i.e. the speedo was reading high. Now, with the 18T front sprocket (1 tooth larger), the speedo is dead nuts on, again according to my GPS. YMMV, of course. Quote Link to comment Share on other sites More sharing options...
rodantking Posted April 9, 2007 Share Posted April 9, 2007 I was gps confermed 83, it read 85. Close enough for me. Stock gearing Quote Link to comment Share on other sites More sharing options...
feelergaugephil Posted April 9, 2007 Share Posted April 9, 2007 At 150mph, mine is showing on radar 146, Quote Link to comment Share on other sites More sharing options...
Furious Posted April 9, 2007 Share Posted April 9, 2007 According to Dr. Garmin, mine is off about 7% as well. Quote Link to comment Share on other sites More sharing options...
shovelstrokeed Posted April 9, 2007 Share Posted April 9, 2007 According to 3 different GPS systems, the speedometer on my bike, with 17/45 gearing and a 180-55/17 Pilot Road rear tire is off by 7.9%. 90 indicated is 83.4 GPS. Pretty much agrees with my local LEO who graciously sprays me with microwave and lately laser radiation every Sunday morning on my way to breakfast. I have U-turned a couple of times and asked for a reading. 40 mph zone, indicating 45 and he/she always says 41 or 42. Oddly enough, that is exactly the same differential I got on my BMW 1100SBX, which runs the same tires. When I did my gearing spreadsheet, I corrected for this by changing the tire section modulus. Figured it had to be that as the calculations for speed in gears in terms of rotational numbers are too simplistic for the engineers to have gotten wrong. Had to be the rolling diameter of the tire. Turns out a modulus of 53 is about spot on. Quote Link to comment Share on other sites More sharing options...
CALCXX Posted April 9, 2007 Share Posted April 9, 2007 Are you sure you have stock gearing? I know from checking my speedo against my GPS that with stock gearing, my speedo was off 7% optimistic, i.e. the speedo was reading high. Now, with the 18T front sprocket (1 tooth larger), the speedo is dead nuts on, again according to my GPS. YMMV, of course. Same results here also. 18T CS made my speedo dead on, according to my GPS. Question for Keith, have you compared your od reading against your GPS? My odometer now records less than the GPS shows. Quote Link to comment Share on other sites More sharing options...
Guest cudgel Posted April 9, 2007 Share Posted April 9, 2007 Yes, I do have stock gearing, 17-45, just put new sprockets and chain myself. Tire is a 180/55 Diablo about 90% worn. Maybe my cop's wrong, we're a small, backward county. Quote Link to comment Share on other sites More sharing options...
EVLXX Posted April 9, 2007 Share Posted April 9, 2007 So... you believe the Radar gun over everything else..... I wouldn't. Depending on what type of radar gun... and how it's used.. there's error in it's numbers as well. Only 2 ways to tell for sure... Time distance measurement (my priffered method), and GPS Quote Link to comment Share on other sites More sharing options...
Keith Posted April 9, 2007 Share Posted April 9, 2007 Question for Keith, have you compared your od reading against your GPS? My odometer now records less than the GPS shows. Yes, exact same results here. I guess if you fix one, you are going to through the other off. As I understood it, with stock gearing, the odo was exact while the speedo was off, so it follows that fixing the speedo has to mess with the the odo. I compensate for the diff in mileage when I compute MPG by increasing mileage by 7%. Quote Link to comment Share on other sites More sharing options...
Furious Posted April 9, 2007 Share Posted April 9, 2007 So... you believe the Radar gun over everything else..... I wouldn't. Depending on what type of radar gun... and how it's used.. there's error in it's numbers as well. Only 2 ways to tell for sure... Time distance measurement (my priffered method), and GPS I can tell you for certain that the radar is not always right. I was pulled over in Illinois with the cruise control set on a nice flat stretch of interstate with the GPS reading 72 flat. The cop told me I was running 71 and that I had a headlight out. Not that it's THAT big a difference, but it's a difference none the less. Quote Link to comment Share on other sites More sharing options...
Crazy Dave Posted April 9, 2007 Share Posted April 9, 2007 Normally the radar shows a lower speed than actual because it is usaully not a true head-on reading. They farther off to the side, the lower the reading. I found that the best way to check it is find one of those stationary 'The Speed Limit Is __- Your Speed Is __ ' radar units. I find them around constuction zones and speeding hot spots. My speedo is off about 5 or 6%. It is also fun to nail the bike in 1st gear and see if the radar can keep up. Quote Link to comment Share on other sites More sharing options...
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