Topic: Just got back from my first "long ride"

[Pinhead]

I was driving around Kansas City yesterday when I realized that it all looked the same. Driving around town is like exploring your own house. It'll only be interesting the first few times. On a whim I decided to head west. I took I-435 to I-10 towards Lawrence, and then got off of I-10 between Lawrence and Topeka, and continued on Old Highway 40 into Topeka. Went through a few small towns that looked interesting. It was nice to get back out of town for a change. Once I got to Topeka I couldn't think of anywhere else to go so I hopped onto I-70 and went west. By the time I had started paying attention to where I was going I was west of Manhattan. I thought to myself "Since I"m this far, I might as well go all the way." After a good 5 hour ride I was at my parents' farm in central Kansas. I got there at 10pm, stayed over night and left this morning at about 9:30am. On the way back I took I-70 the whole way back to Kansas City. On the way I had my speedo set right on 70mph and my tach showed right at 5k. The only discrepancy was the fact that I got home 3.5 hours later instead of the more normal 4 hours that it takes me in my car (I usually set my speedo right below 85 mph in my car).

This brings up my question. Is my speedo way off? According to my odometer I got 47mpg both ways, which is pretty much what I expected for a pretty steady 70mph. My rear sprocket is stock size, and my front sprocket is up-sized by one tooth to 17. 1979 CB650. Could my speedo be off but my odometer still be accurate? If my speedo is showing slower than I'm actually going than my mileage would likely be higher than figured, too.

What should be my speed at 5000rpm in 5th gear on a '79CB650 with a 17-tooth front sprocket and a stock rear?

[Gordon]

It's entirely possible for your speedo and odometer to read differently because there is an actual physical connection between your front hub and the odometer, but only a magnetic connection to the speedometer.  You can check your odometer next time you're on the highway by watching the mile markers and comparing them to what the odometer shows. 

And next time you find yourself heading west, just keep on going and give me a ring when you get to Denver!

[Uncle Ernie]

I set my clock ahead 10 minutes so i can fool myself and always be on time.
Maybe I should have my speedo set ahead 10moh so I'll always be as fast as my ego says I am.

[Pinhead]

If my theory is correct, my speedo is showing less than I'm actually going. In other words, when it was showing 100 mph for a short stretch of highway, I was actually going faster...

[Chris Schneiter]

Perhaps all our speedos are wrong. Mine reads exactly 70 @ 5000rpm...perhaps the time difference is in the logistics of riding a bike vs driving...it's easier to pass on a bike vs car..not as much slowing down. I'm always faster in city trafic on a bike vs car.

[Pinhead]

I'm pretty sure the 750's and 650's have different gearing. Mine's not stock, either, having one extra tooth on the front sprocket.

I didn't pass anybody on the trip home, as I was trying to maximize my fuel economy while still making good time. I ran 70 mph indicated pretty much the whole way, and when I accelerated from a pit stop I did so very moderately. Also, when I take the trip in my car I make no stops (drive through the whole way). It takes me 4 hours in my car with my speedo above 75mph the entire trip, and it only took me 3.5 hours on my bike with my speedo on 70 mph. On this trip I stopped a few times and drove 60mph between Topeka and Kansas City, which should have added time to my trip.

All of this would indicate that I'm going faster than my speedo is showing, and if so, I may well be getting better mileage than I thought (47 mpg at 85mph would be better than 47 mpg at 70 mph, for instance).

Does anyone know where I can get a cheap digital speedometer that I can easily install on my bike, just for a calibration?

[Pinhead]

I think I'll get a Sigma Sport 800 digital bicycle speedometer and install it on my bike.

I got the idea here.

I can get it here for less than $15 and it's stated accurate up to 183 mph (+/- .03mph) but tested accurate up to 350mph. No others tested would work above 75mph.

[Uncle Ernie]

Cheap?  No.  Check a bicycle shop.
Any worm holes in Kansas?