Please excuse me if this was asked already but couldnt find the answer,but here goes:
Im changing to a 34T front trans pulley & running a 70T on the rear pulley. Was wondering if any one knows what the deviation is on the speedo? will it read 2MPH slower? or 5 MPH slower? not too concerned if its 2mph but if 5 mph will re calibrate. speedo .Does anyone know? This is on 2002 Deuce, Thank You

 

I don't know but a GPS would show you while you ride.

 

I believe your stock set-up is 32/70...So changing just the trans sprocket from 32 to 34 will put the speedo off (slow) by 6%, or 6 mph at 100, 3 mph at 50 etc.

 

Drove a Deuce same year with the same rear pulley and w/the 34T pulley it was a lot smoother than mine and was told best bang for the buck mod if looking for a faux 6 speed. 80 mph on the converted deuce was about 3k on the tach and smooth , mine at 3k is about 70-72 aprox mph and not buzzing horribly bad but noticeably more than the 34 T set up I drove. I guess its a Dakota Digital speedo calibration module..Was hoping there was a less $ version available but I didnt see any ,the only other re-cal.module I saw was S&S 's and it was 2x the price . Chime in if any know of other modules for less ? Thank You again Gents

 

Yeah...
I did the 32 to 34 pulley swap on 2 consecutive bikes.
I can't tell you the exact equation, but it was easy enough to know that it was off. Keep it a few MPH below what the Speedo reads. Sometimes science isn't required. Use "the force" Luke..... You'll be OK.

 

Don't know about "after" but before, it's about 6-8%.

 

The 6% quoted by RockerB is correct. GPS will verify that.
Of course that's assuming your speedo was accurate to start with.

 

 

 

Here is some math for you to use. This should tell ya.

http://www.customvalley.com/technical/speed/index.php