thanks for the reply gmbusa another thing guys everyone said my rpm's would be higher well they are in the same spot at consant 80 on the highway down the rpm's were at 4100,on the way back the same is it because my mph gauge is way off now,and if so why did everyone say it would be higher because mine is the same as it was before.
The tach and speedo relationship remain the same due to the speedo picking up its speedo pulses from the countersprocket. If speed was detected from a wheel, then the correlation between tach and speedo would have changed but never when picked up from the countersprocket. The countersprocket method of detecting speed requires a correction device such as a Speedo Healer or Yellowbox to make corrections to the speedo, which would then show a difference between indicated RPM and speed on your gauges when making gearing changes.
This issue is often questioned and can be a bit difficult to envision as to why the tach and speedo remain the same after changing gearing. Allow me to provide a scenario which should help in understanding why this is so.
Let's use the following numbers to make things easier to understand:
Let's assume the transmission is appropriately geared so a countersprocket of 20 teeth and a rear sprocket of 20 teeth (a simple 1:1 ratio) produces a true 60 MPH at 4,000 engine RPM while in 6th gear. Let us also assume that the speedo sensor detects 1,000 pulses per minute at 60 MPH. Notice that the ratio between engine RPM and detected speedo pulses is a 4:1 ratio. 4,000/1,000 = 4:1
With a 1:1 primary/secondary gear ratio (20/20 gearing), each time the rear wheel completes one full rotation, the counter sprocket has also completed one full rotation.
Now let's change the rear sprocket to 40 teeth yielding a 2:1 ratio (leaving the countersprocket 20 teeth). Realizes this 2:1 ratio requires doubling the engine RPM to accomplish the same 60 MPH speed as each time the countersprocket completes one full rotation, it only spins the rear tire half a rotation (therefore only half the distance, which equates to half the speed), therefore, the countersprocket would have to have its speed doubled to achieve the same 60 MPH with this 2:1 gearing ratio. The only way to double the countersprocket's speed in 6th gear is to double the engine RPM. So now we are at 8,000 engine RPM when traveling at 60 MPH.
Since the countersprocket's speed has been doubled to produce the same 60 MPH speed, we now detect twice as many speedo pulses at the same speed which is now 2,000 pulses detected per minute. Notice the ratio between engine RPM and detected speedo pulses remains exactly the same (a 4:1 ratio). This is why changing gearing does not alter the relationship between speedo and tach when the speed is detected from the countersprocket.
No matter what actual gearing you elect to employ, the ratio between engine RPM and detected countersprocket pulses remains the same.
Looking at it from the rear wheel's rotational speed (using the 20/40 gearing) to actually produce 60 MPH, each time the rear wheel rotates one time, the countersprocket must rotate twice, producing twice as many pulses at the same given rear wheel speed required to produce 60 MPH. Since the countersprocket clearly spins twice as fast, so too must the engine (now 8,000-RPM to achieve 60 MPH). The ratio of 8,000 engine RPM to 2,000 speedo pulses remains a 4:1 ratio therefore, from the speedo and tach's perspective, all appears unchanged even though a drastic reduction in gearing has been made.
Hopefully, this has made this issue easier to comprehend.
