The chart below shows the various gear ratios for the 1970 Harley Davidson
Sportsters. The engine, clutch and rear wheel sprockets were not changed.
The transmission sprocket is the only change. The engine RPM at 60 MPH is
assuming high gear and a standard rear tire (4.25x18) measuring 26.5
inches in diameter. If you run a larger or smaller rear tire, your engine
rpm's will be different. I have included the gear ratios from a 2000
Sportster just for comparison purposes.
19, 20, and 21 tooth sprockets are available from Harley Davidson. 22, 23 and 24 are available from other vendors.
|Sprocket & Gear Ratios||XLCH||XLH||Optional||Optional||Optional||Optional||2000 883|
|Fifth (Over Drive)||N/A||N/A||N/A||N/A||N/A||N/A||3.615|
|Engine RPM @ 60 MPH||3545||3368||3207||3062||2929||2806||?|
|MPH @ 6,800 RPM||115||121||127||133||139||145||?|
1970 Sportsters are geared very high from the factory. With the standard gearing, you have lots of power but a terrible highway cruising machine. Any Sportster can go up to a 22 tooth transmission sprocket without modifications, but going to a 23 or 24 will require some cutting of the sprocket cover. A 22 is a good compromise and does not require any modifications to the sprocket cover.
Be careful, the size of your rear wheel will effect your engine speed at any given m.p.h. This next table shows how the diameter of your rear wheel effects your engine rpm at 60 m.p.h. The 19/51 means 19 tooth front sprocket, 51 tooth rear sprocket. The number in the table is engine rpm at 60 m.p.h.
One other thing to note. Changing the overall gear ratio will affect the accuracy of your speedometer. 1970 Sportster speedometers are driven off the transmission. If that is a concern, see my BC800 page.
P.S. I am still running a 19 tooth sprocket but am going to try a 22 tooth very shortly.
Have fun... Ride safe...