Can you adjust a speedometer?

John Lieberman /

Now that I've firmly established that my CEV speedometer reads about 7 MPH fast, I'm trying to figure out if there's anything I can do about it.

The speedometer unit itself appears to be sealed. The part that goes onto the axle, on the other hand, has a suspicious looking bolt on it. I don't know what it's for, but it seems doubtful that it would have no purpose whatsoever. I just changed my speedometer cable yesterday and it didn't require me to fiddle with it to accomplish that. (Or, was I supposed to do something with it?)

Does anyone know (1) what that bolt is for and (2) is there a way to adjust the speedometer? Does the second question answer the first???


Re: Can you adjust a speedometer?

I think that bolt is to help keep the gears aligned. Not the best idea to just yank it out.

It seems like you would need a new sending unit, or whatever they call the gear on the wheel. I think yours is for a moped with a smaller front tire.

Measure, or calculate the total outside radius of your wheel, and that could be a starting point to find a new gear.

Yeah, but...

John Lieberman /

I think you've got it backwards. The sending unit is for a moped with a LARGER front tire. It assumes that I'm covering more distance than I really am with each revolution of the wheel.

Nevertheless, you've given me an idea. I note from the Moped Warehouse site that moped tires come in two sizes: 16" and 17". Mine are 2.25 x 16. The distance from the rim to the tread is another 2", so my total diameter is 20". The circumference, then, is 62.8".

Let's assume a 17" tire is also 2" from the rim to the tread, giving it a total diameter of 21" and a circumference of 65.9", which is 5% larger than the 16". Therefore, if the sending unit were intended for a 17" wheel, I'd read about 5% too high.

Looked at another way, a 17" wheel is spinning at 561 RPM at 35 MPH (35x5280x12/60/65.9). At 561 RPM, a 16" wheel is only going 33.4 MPH, or 95 percent as fast as the speedometer reads.

Unfortunately, I'm off by more than this amount. If the sending unit were designed for an 18" tire, I'd only be going 31.9 MPH when the speedo read 35 MPH, but who the hell has 18" tires for a moped?

I'm perplexed. So many of the parts on my bike are universal Italian components. I've got universal tires, wheels and speedometer, but they don't work together properly. Maybe they're front different universes?

I bought an extra, beat-up City Bike to use for parts. Maybe I'll try using the sending unit from that one to see if it makes any difference. Or, at least, I'll take apart the extra sending unit to find out what that silly little screw really does!

Re: Yeah, but...

Well, it also depends on how many RPM's the speedo uses per mile. One of them may be from a diffrent brand. I don't think there is a set standard. I guess that they should sell speedometers and the gear in a set, according to tire size.

Re: Yeah, but...

John Lieberman /

I suppose I assumed that they were provided in a set, since they're both original equipment with the bike. Or, at least, one would hope that Cimatti engineers would have been careful enough to specify matching operating characteristics for the two components.

Here's a theory: The original speedometers must have been metric, seeing as they're from Europe. Somebody designed a new MPH face plate for the speedometer for the U.S. market and screwed up the placement of the numbers.

Yeah...that's it!!

Re: Yeah, but...

Ze Frenchmen /

You're probably right on the number mess during KMH to MPH conversion. Some mopeds do have such problems (od reads miles for Km or reverse, gears are for MPH and display is KMH, and such...) I'm guessing a non-mechanical problem.

BTW, I do have a moped with 18'' tires. It's a 79 Torrot, (the one in the photo section).

-Ride in Peace-

Ze Frenchmen

Re: Yeah, but...

Makes more sense thatn just changing the K in KPH for an M, which also happened on a lot of them.

Annother option is to place the marks yourself, on the glass over the guage. Fred did this on his Honda to tell how fast he was going after he got off of the marks.

Re: Yeah, but...

John Lieberman /

That's a different-looking machine.

It figures that a French moped would have differently sized tires from most every other bike on the market. My experience with French motor vehicles is that they seem to be designed by engineers who are either (1) ignorant of designs being used elsewhere or (2) indifferent to the way anyone else is solving the same problems.

I suspect it's the latter. My Peugeot car, as comfortable and stylish as it was, seemed like it was engineered on another planet.

The Italians, on the other hand, are all willing to share the same designs and components. Unfortunately, when you get one bad component, it shows up all over the place!

Re: Yeah, but...

You might get your best answer by asking Ron Brown .. he lives out in Howell .. and seems to know more about electrical components and things like speedos than anybody here .. I'm sure you can look up some of his old posts and then email him.

He usually gave very thorough detailed answers.

(though you're probably screwed no matter what)

Some motorcycle rider have made stick-on decals and paste them over the speedo face with the true speeds shown.

(this would ge after they made gearing changes that threw off the original numbers on bikes that ran the speedo off the gearbox)

That would almost surely be the most painless way to go.

Re: Yeah, but...

david f martin /

I recently read an article about this guy that restores speedometers for classic cars (mainly VW). One of the tools he uses to calibrate the speedo accurately is a magnetizing tool. Apparently, the speedo uses a magnetic rotor and pickup to swing the needle, and a clockspring to return it to zero.

The odometer is directly geared from the cable. If your odometer is off, you're probably using the wrong size tire. If it quits working completely, you've got a gear that has come loose inside the odometer. I've fixed a few of these (VW), and I assume a moped speedo and odo work similarly.


Re: Yeah, but...

Chris MWH /


Thats a grease fitting so you can keep the internal worm gear lubricated. Speedo drive units are calculated to the wheel size and the speedometer. and they do come in different ratios. What are the markings on the driveunit itself and what bike is it on?

Chris MWH

Re: Yeah, but...

John Lieberman /


The only marking that I can see on the drive unit is the CEV logo. If there's anything written on the side facing the hub, I'd have to remove it to find out.

The bike is a Cimatti City Bike with the stock wheels and tires described earlier in this thread. Everything is original (except the speedometer cable, which made no difference.) If it's got the wrong drive unit, then it was assembled that way in 1977!

« Go to Topics — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account