Final drive gear reduction question

I have just purchased a new-to-me 2003 Tacoma with a 6" Fabtech lift. It has 35" BFG / MT tires on it. It has a 5-speed manual transmission.

The question I have is regarding the RPM and the road speed. At a measured 75 MPH the tach shows 3100 RPM. At about 50 MPH I am looking pretty hard for 5th gear and it would be great if it had a 6-speed.

I lifted the rear-end of the truck and counted driveline revolutions and the tire revolutions and it seems as if it is about 3.5 to 1. That doesn't jive at all with the 3100 RPM at 75 MPH on the highway. Is it possible the previous owner put some kind of a doubler in the transfer case?

I had a Ford Ranger which was the most gutless wonder you have ever seen. This truck has plenty of pep. I was wondering about 37 x 12.50 x 16 tires for it to bring the highway RPM down a little.

Thoughts? Quicky

 

Engine RPMs (what the tachometer shows) don't have a relationship to the number of revolutions per minute your wheels are turning at.

If you want to reduce engine RPMs at highway speeds you need to change the gear ratio in your differentials, bigger tires will make it worse.

 

The speedometer indicates 81 MPH when I am actually going 75. Had my wife drive beside me on the freeway to measure speed. Going to a 37" tire should reduce my speedometer indication about 3.5 MPH if I am figuring right, and should also reduce the RPM to go a given speed in relation to what I am doing now. Unless I am bassackwards, which I have been before.

 

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