Speed estimation by Tach reading?

[biggersm]

I can't remember if it was in this forum or the other one but I remember a post about estimating the speed (in 5th gear) by looking at the tach. It was something like doubling the tach reading but then it involved subtracting, or adding, 5 or 10 mph (and then dropping a couple zeros, otherwise we would all be supersonic). What was the exact formula?

We just had a speedometer cable break. I guess we should have saw it comming since the turn signal auto cancel failed, followed by the cruise control getting harder to set. As long as the gas guage still works I guess we are OK, but now we are more cognizant of all the Law Enforcment around here now.

[Charlie from Illinois]

The speedometer cable is not hard to replace. Contact Carl Leo for the parts and you can be ready to go. A new cable is a whole lot cheaper than a ticket. It's hard to remember what the tach is for 30 mph and 55 and 45 etc. The speed cop really won't care.

[flip18436572]

The speedometer cable is an easy replacement part for the Voyager. I actually try to take it apart and clean it twice during the riding season and once right before the season starts for me in Iowa.

Contact Carl Leo and get a new cable shipped to you. He probably has a few of these already in stock. I haven't purchased one from him, but I probably should, even though my cable seems to be in decent enough shape.

[Mike W]

I can't remember if it was in this forum or the other one but I remember a post about estimating the speed (in 5th gear) by looking at the tach. It was something like doubling the tach reading but then it involved subtracting, or adding, 5 or 10 mph (and then dropping a couple zeros, otherwise we would all be supersonic). What was the exact formula?

We just had a speedometer cable break. I guess we should have saw it comming since the turn signal auto cancel failed, followed by the cruise control getting harder to set. As long as the gas guage still works I guess we are OK, but now we are more cognizant of all the Law Enforcment around here now.

That be mine....Here is what I originally wrote:
"I don't promote this for long term use for anyone else, but in a pinch, it works great. The Tack in 5th gear is actually more accurate than the speedo when trying to figure out your speed . Compared to my Garmin GPS, this formula is dead on: Take the RPM number times 20 plus 5. Example: If you are running along at 3000 RPM - take 3 * 20 = 60 + 5 = 65 mph. This seems to be accurate from 1,500 rpm (35mph) up to 4,000 rpm (85mph) as checked against my GPS. "

or if you don't like doing math on a ride:
1500 rpm = 35 mph (I think this one is off just a bit)
2000 rpm = 45 mph
2500 rpm = 55 mph
3000 rpm = 65 mph
3500 rpm = 75 mph

Of course, the accuracy of this answer can still vary depending on the amount of tread left on the tire.

As mentioned earlier, it's better to just get a new speedometer cable or coupler asap and be done with it (unless you don't mind not having cruise, self canceling signals and the auto adjustment on the radio volume).

[David (N. Alabama)]

I believe the cable cost less than a meal out. Takes 15 minutes to replace. Like others have said, Carl Leo is your man.

[ghostler]

Motion Pro replacement speedometer cable for the XII is $15.29 at:

https://smile.amazon.com/gp/product/B000OO0W4G/

I did recall a relationship between speed and tach setting when mine broke. I found it easier to use the GPS to establish speed then notice what RPM was on the tach, since it is easier to watch the tach than the GPS speed numbers. Using tach is a bit more tricky as slight movement can be a couple miles per hour. I'd notice where setting was at, notate say the edge of the nearest RPM numeral to keep the needle pinned at for a constant speed. Besides use as an emergency speedometer, I found the GPS to help me identify my actual versus indicated speed, then use Kentucky windage when I don't have the GPS with me so I travel the right speed.

I also learned that the cables need periodic attention because they tend to gather condensation inside the casing, causing the cable to rust, then freeze before snapping. Should be removed and re-lubricated at least yearly.

[voyager55]

This thread is 9 years old!