Speedo Inaccuracy - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 5 (permalink) Old 02-21-2010, 08:52 PM Thread Starter
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Speedo Inaccuracy

Its like stuffing a tube sock in your speedo. I rode today for the first time with my GPS attached. My speedo is off @ 5%, as previously reported here. While I thought I was going 50 mph (acccording to the speedo), I was only going 56 or 57. I ran 80 mph for 1 minute, and sure enough, the GPS said 76; at 90 the GPS registered 85. I used to be satisfied at running up to 100 every now and then; I guess I'll have to run up to 105 now. I guess the inaccuracy can be a function of the front tire size, but I only have time to ride, not worry about that crap.

Now I have to explain to the wife, the tube sock in the speedo reasoning.

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post #2 of 5 (permalink) Old 02-21-2010, 09:05 PM
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Don't understand the sock thing, but yes the Vulcan 750 is highly optimistic. I don't have an issue with it now that I know about it. The fact is, I have never seen ANY vehicle that didn't have an optimistic speedometer. I have had cars and trucks that I put oversized rear tires on (speedometer was driven off the transmission) which corrected the speedometer error. I just make it a point to ride 5 mph faster than the speed limit, or in some cases, on interstate highways, as much as 15 mph above the posted speed limit. That's what it takes to keep up with left lane traffic around here. I have had my Vulcan up to 110 mph by GPS, it won't seem to go any faster, though I've heard of people getting 120+ out of them. Jerry.

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post #3 of 5 (permalink) Old 02-22-2010, 07:07 AM
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I have had the same issue in the past. The matter partially corrected itself when I put new tires on the bike. Still didn't make the speedo dead-on-balls accurate, but it dropped the difference by several miles per hour. You wouldn't think that the tire wear alone could cause such a difference in the 'size' of the tire, but the new tires cut the difference by a bit more than half.

Jeff

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post #4 of 5 (permalink) Old 02-22-2010, 10:34 AM
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After I installed Pirelli tires with the upsized 110 up front I noticed my RPMs went up when the speedo indicated 60 mph, meaning my speedo was probably indicating higher than actual speeds before the tire change, and was more accurate now. According to my knowledge of where my baby's RPMs are at particular speeds, the difference was just under 5 MPH. Don't have a GPS to verify, but I always pay close attention to my speed & RPMs, along with the general feel of the bike at all speeds, just to know if anything needs adjustment.

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post #5 of 5 (permalink) Old 02-22-2010, 05:55 PM
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Quote:
Originally Posted by VN750Rider/Jerry View Post
Don't understand the sock thing, but yes the Vulcan 750 is highly optimistic. I don't have an issue with it now that I know about it. The fact is, I have never seen ANY vehicle that didn't have an optimistic speedometer. I have had cars and trucks that I put oversized rear tires on (speedometer was driven off the transmission) which corrected the speedometer error. I just make it a point to ride 5 mph faster than the speed limit, or in some cases, on interstate highways, as much as 15 mph above the posted speed limit. That's what it takes to keep up with left lane traffic around here. I have had my Vulcan up to 110 mph by GPS, it won't seem to go any faster, though I've heard of people getting 120+ out of them. Jerry.


I am a big guy at ~#300 and have a full size windshield on my 750, and have had it up to 150 km/hr on the speedo, which I now know was probably more like 140km/hr, (87 mph). I had more throttle, but ran out of nerve.

I just did some calculations last week, which I posted on a thread here, which indicate that the gearing on the Canandian and American bikes will yield ~14.5 mph in 5th gear, per 1,000 rpm.

So at 7K rpm, your speed is ~101.5 mph.

7,600 rpm yields ~ 110 mph. I think speeds higher than this with a large windshield or a rider much bigger than a jockey, are not going to be easy to acheive. You are past the torque and horse power curve peaks, and that last 900 rpm is going to be hard won.

Our redline is 8,500 rpm, which would theoretically yield 123.25 mph.

Gordon

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Purchased May 16, 2008
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