Sprockets and Speedometer

[John1454]

I am sorry if this has been discussed before... I did a brief search, but didn't look thoroughly.

I have a 620 how much will my speedo be off by if I lose a tooth in the front and go with 13 & 46?

[Speeddog]

Sprocket changes have no effect on the speedo accuracy on a 620.
The sensor is over on the right side, in the rear brake caliper bracket.

If you've got a 14 on the front already, and you want it to be geared lower, you'll need to go larger on the rear sprocket.
I'm not aware of a 13 tooth front sprocket.

[The Bearded Duc]

IIRC it's not the speedo that will be off, it's the tach.

What year is the bike? What kind of riding do you do, mostly?
IMHO, If you do a lot of highway riding I would leave stock gearing. Your stock gearing could either be 15/46 ('02-'05) or 15/48 ('05 +). Either way, I'm not sure there is a 13T front sprocket available and even if there is I would not go that low in the front. There will be too much tension on the chain and probably quite a bit of rubbing on the drive side of the swing arm.
Most duc riders go 14T in the front, although there are mixed feelings about this putting tension on the chain as well. The better recommendation may be to go up a couple teeth on the rear sprocket. I went to a 14T on the front and like it quite a bit, you  loose a few hundred RPMs but nothing too major.

[John1454]

speeddog, That is good news! Thanks.
The shop that put my tires on was mentioning I needed a new chain and we started talking about sprockets he thought it would effect the speedo, but he didn't know about Ducati's. I guess other bikes are different.?

duc750, I need to investigate the sizes some more. If it is not going to make my speedo be way off like the guy at the tire shop led me to believe, then I am going to do it. I thought I had a 14 on the front now, but maybe I am wrong... that or the guy that owned my bike before me already changed it... I do all kinds of riding: day trips in the twistys and long touring type trips too.

[The Bearded Duc]

I won't put the speedo off.
To find out what size you have now just take the sprocket cover off and look at the front of the sprocket. There should be a bunch of numbers followed by a " - something".
That "something" is your sprocket size. Or you can try counting the teeth. 

[Speeddog]

Some bikes it affects, but not your 620.

Tachometer won't care either.

What year 620 do you have?

[Slide Panda]

As noted Ducs don't use engine speed for the speed reading - it's taken from the wheel. Pre electronic dash its is from a cable drive on the front wheel. Your 620 has the electronic dash and takes the speed from a magnetic sensor mounted on the rear break caliper support. It;s that black wire running along the swing arm, to the caliper mount.

The tach get's it info from the ignition system.

a 13 tooth, besides possibly not existing has problems like probably grinding the hell out of your swing arm, due to the reduction in clearance. Some folks get a bit of rub with a 14 tooth - 13 would be worse.

[John1454]

I guess the previous owner had already upgraded; as I have a 14 tooth front. I was thinking you go down in the front or up in the rear to get more ummmph, but everyone is right, they don't make a 13, so my bike has already been modded.

I have a 2003 BTW.

[The Bearded Duc]

I guess the previous owner had already upgraded; as I have a 14 tooth front. I was thinking you go down in the front or up in the rear to get more ummmph, but everyone is right, they don't make a 13, so my bike has already been modded.

I have a 2003 BTW.

I'd leave it the way it is. When you replace the chain you should get new front and rear sprockets as well. And like Speeddog said, if you still want it geared lower than it is you can go up a couple teeth in the rear.