19 Tooth Front Sprocket

[SparkyMan]

No worries Red, and I appreciate your being humble rather than defensive and angry. The last thing I want is to offend (unless we’re talking about the mother-in-law).
-Sparky

 

[Maersk]

Thank you Sparky.

It's been I while since I tried to use gearing to explain an answer. I made mistakes along the way. Crunching numbers the way I was; it's easy to confuse yourself.

Thank you, Sparky and Red Dog. Valuable information!

 

[SparkyMan]

Please let us know what, if anything, you do or discover.
I’m also pretty sure you’d be able to recalibrate your Speedo via tuneECU or the like.
Lastly, I noticed that auto-correct has been capitalizing the word Speedo throughout this thread; must think we’re discussing swimwear!
-Sparky

 

[LoneRanger]

Did you correct the speedometer.

Changing the sprocket(s) will only affect the speedo if it runs off the engine or final drive. If the speedo is driven off a wheel, the wheel is still travelling across the road at the same speed, but the engine revs for that speed will be higher (smaller front sprocket) or lower (larger front sprocket). A larger front sprocket will also reduce the rate of acceleration.

 

[SparkyMan]

Changing the sprocket(s) will only affect the speedo if it runs off the engine or final drive. If the speedo is driven off a wheel, the wheel is still travelling across the road at the same speed, but the engine revs for that speed will be higher (smaller front sprocket) or lower (larger front sprocket). A larger front sprocket will also reduce the rate of acceleration.

LoneRanger,
Hate to say, but I think you’ve got it reversed.
Presuming that changing to a larger sprocket will increase the rate of wheel rotations per engine rotation, a wheel-driven speedometer will show increased speed at a given rpm (e.g.: 18-tooth @ 3K rpm = 60 mph; 19-tooth @3K rpm = 63 mph).
A transmission or crankshaft driven speedometer will indicate 60 mph @ 3K rpm regardless of sprocket size because it is unaware of the increased rotation of the wheel. All it know about is what gear it’s in and how fast the crank or transmission is turning.
Make sense?
-Sparky

 

[Red Dog]

Sparky,

You do make sense. I have a cable speedo off the front wheel. That clouded my thinking, forgetting that the OP has a transmission/crankshaft speedo. And why my gearing charts are based on front wheel circumference.

 

[PieMan]

No need to presume, a larger front sprocket will definitely give more speed for a given rev reading, it's a mechanical thing and speedo drive methods won't make a difference.

In theory you should be able to achieve a higher top speed with a larger front sprocket, but if the engine doesn't have enough power to build revs to achieve the higher top speed over wind resistance, your top speed and engine revs will be lower and of course your acceleration will be slower.

You would think it would be reversed for a smaller front sprocket, but the truth is that although with a smaller front sprocket the top speed will be lower at the same revs as with a larger sprocket, the engine finds it easier to over come wind resistance and a higher top speed may be achieved and of course acceleration will be quicker.

In the early days on my Bonnie, I could achieve a higher top speed with a 17 tooth front sprocket, than a 19 tooth front sprocket purely because it would rev out to the red line in top gear, where as it couldn't with a 19 tooth sprocket. And of course the acceleration was magic with the smaller sprocket, but very revvy for motorway riding.