T140V Bonneville sprocket tooth count and miles-per-hour comparison . . .

[Lordmac]

You guys already know if you put a smaller final drive sprocket on the rear your engine turns fewer RPMs at designated speeds. T140Vs came stock with twenty-tooth front and forty-seven tooth rear sprockets which had the bike traveling at CERTAIN RPMs at (let's say) seventy miles-per-hour. I bet there are conversion factor specs for different sprockets going from the forty-seven to the forty-five and the forty-four and the forty-three. Does anybody happen to have that information handy who might post a reply? That would also be using a stock tire size of 4.10x18.
Thanks . . .

 

[Rudie]

You guys already know if you put a smaller final drive sprocket on the rear your engine turns fewer RPMs at designated speeds. T140Vs came stock with twenty-tooth front and forty-seven tooth rear sprockets which had the bike traveling at CERTAIN RPMs at (let's say) seventy miles-per-hour. I bet there are conversion factor specs for different sprockets going from the forty-seven to the forty-five and the forty-four and the forty-three. Does anybody happen to have that information handy who might post a reply? That would also be using a stock tire size of 4.10x18.
Thanks . . .

You are confused - speedometer indicates road speed, tachometer indicates engine speed, neither is connected to the other.

If you read https://triumphtalk.com/threads/speedo-error.62076/ :-

From about 66 onwards, 650 Triumph speedo driven by the rear wheel; sprockets don't make any difference

Triumph fitted the same speedo drive to UK & General Export 650's - that had 3.50x18 rear tyres before '71 - and all triples - that mostly had 4.10x19 rear tyres (some Hurricanes had 4.00x18, others had 4.25/85x18).

Applies to 750 twins also, Triumph never changed the Smiths speedo gearbox even for 4.10x18 tyre. When they swapped to Veglia speedo drive, it has exactly the same ratio as the superseded Smiths.

Rear wheel drive nullifies the tranny sprocket... it's not a tranny-driven speedo!

The tacho is driven by the exhaust camshaft, geared to the crankshaft.

Standard final drive sprockets, an observant rider will notice a correspondence between the speedo and tacho indications depending on the gear in use. If either final drive sprocket is changed, the correspondence between the speedo and tacho indications will change, but the speedo still indicates the road speed, the tacho still indicates the engine speed.

 

[Lordmac]

You are confused - speedometer indicates road speed, tachometer indicates engine speed, neither is connected to the other.

Naw, not confused.

I happen to know this much: in the past, when I have had the 20/47 sprocket combination installed on my 1978 T140V with a 4.10x18 rear tire, actively traveling on the bike, showing seventy miles per hour on the SPEEDOMETER whilst simultaneously viewing the TACHOMETER, it, the TACHOMETER, would indicate approximately, in the neighborhood of 4300 RPMS, as best I can recall.
What I was wanting to know is: what would the TACHOMETER read if I was traveling the same speed (70 MPH by the SPEEDOMETER) but had a 20/45 or a 20/44 or a 20/43 sprocket combination installed on said bike. I know I have in the past, somewhere, seen such 'conversion factors' that someone posted, so I know the information has already been produced. And thus, the engine RPMs and the road speed are directly connected and do affect each other, unless the bike is on a stand with the rear wheel spinning freely . . .