Mark Hodowanec

What is the overdrive ratio for 6th gear, rear axle ratio, & tire size for a SRW 4X4 six speed SLT? I want to figure out engine revs at highway speeds.

What is the 'sweet spot' for the 600?

I know that as vehicle speed goes up, milage goes down, but is this because of aero drag, or because the engine is past it's 'sweet spot'.

Would an auxilary overdrive unit (like US Gear or gear vendors) increase the highway milage at 75 MPH assuming stock tire sizes are used?

Captain

I thought I heard something about gearvenders breaking on thies trucks... I *think* 6th gear is like .74-.76 something like that. I think its the .74 though and a 5 speed 5th is .75 dont remember exactly but thats real close.

Captain

err theres more. I found the "sweet spot" to be whereever max tourq is made so like 1800rpm or so on our 98 12v. With 33" pizza cutter tires we get 23mpg w/ a 3.5x rear. My truck 4.10 and 30" tires gets like 15mpg. Were going up to 315s and hopeing to increase our highway millage by droping the rpm to what we estimate to be peak tourq for the speeds we run at. So basicly I need 37" tires on my 4.10 to get the final ration the 3.5x has with about 34" tires. I'm considering a gear change. I'll try to dig up the formula for caculating speed/tire size/gear/rpm for you

bulabula

OD is .73 on the 6 shooter. There's a number of ways to do the math; tire rev's/mile is one way, and using tire diameter is another (the way I do it).

Mark Hodowanec

The Formula is easy enough:
Engine RPM = MPH*FG*88 /(3.1416*D)
where:
MPH is road speed
FG is Final Gear ratio = tranny OD ratio * rear axle ratio
D is tire diam in ft.

So given the following assumptions:
Tire = 285/70-17, then D = 32.71 in. = 2.726 ft.
FG = .75*3.73 = 2.7975


Engine RPM (@ 70 MPH) = 70*2.7975*88/(3.1416*2.726) = 2012 RPM

2012 RPM @ 70 MPH
This seem about right?

bulabula

Yeah, that seems about right for a 5 speed. The 6 speed OD translates to about 1950 rpm.