Gear Ratios

Aircooled · August 30, 2002

What are the differences in RPM of these two final drive ratios?

Bigbluemax · August 30, 2002

That is hard to say. You must take into account the tire size, transmission ratio, 3, 4 or 5 speed. Personal observation on my vehicles is 200 to 300 rpm increase from 3.42 ratio on Trailblazer, to 3:73 ratio on 2500HD., At 70mph on TB with 3:42 it tachs about 1,800 rpm. At 70mph on HD with 3:73 it tachs about 2,100 rpm. Hope this helps. So I would assume a 4:10 would tach about 2,400 at 70 mph. Just my .02 cents worth.

Beaux Duke · October 16, 2002

As a rough estimate consider this.

The difference between a 3.73 and a 4.10 is roughly 10% so your engine rpms will increase approximately 10% at any given speed.

Let's say your're doing 1800 rpm at 70 with the 3.73's. With the 4.10's your rpms will be approximately 1980 or 2000.

But you need to remember that if you don't change your speedometer to reflect the new gears then it will not reflect your true speed so you will see a different rpm reading at 70 with an uncorrected speedometer.

And this is why we all needed to pay attention in math class during school. :thumb:

Hope this helps and remember, it's an approximation.

Aircooled · October 17, 2002

Another factor is the variable valve timing which will automatically adjust the RPM. At 120KM which is about 75MPH my RPM is 2400.

tblazer503 · October 20, 2002

hmm... from what I know about variable valve timing(VTEC, VVT-i, etc) it does not adjust your RPM's. What it actually does is at X-RPM's, which is noted either by oil pressure, or a switch; a solenoid will activate that will change the "cam phase" in the case of the Triplets, there is an electronic setup that allows the engine to retard the exhaust cam up to 24deg. This allows in overlap of the exhaust/intake cam thereby eliminating the PCV valve(for emissions) and also allows the computer to advance the exhaust cam for higher RPM's. Just a general basis. In the case of VTEC, there are actually two cam lobes for each valve, at oil pressure X, a valve opens up, and oil pressure builds in a channel in the block, at the top of the block is an actuator that shifts the cam over to the higher duration lobe.