Alright, in my 2001 Sierra HD, the speedometer only goes up to 100, so I was pegged out before I could see an actual speed. It has the LQ4, 4L80, 3.73 gears and 35" tires (33" if you measure them". I was pegged at 100 and was climbing and let out when it was at 4,750 rpms. I've cruised at just under 100 at 3,000 rpm so I'm wondering if that was in overdrive? Someone help!

 

And my speedometer has been recalibrated for larger tires.

 

Nobody..?

 

Google?? But likely even with the info you know, it might be impossible to figure out.

I dunno, grab a gps or use your phones gps/nav directions thingy, most have vehicle speed on there.

And yes at 3,000 rpms going above 90, you'd be in 4th gear. 3rd gear would be way way higher.

 

Well it's like, sometimes it pulls not so hard and I hit 100 at 3,000 rpm, but then if I start in third with my tow haul on it hits 100 at 4,000, like it's in a different gear. It's quite confusing, haha. My tow/haul's set to shift at 6,000 rpm, it shifts right at 90 into what I guess is fourth, and that lands me at 3,700 or so. So I'm thinking it's in overdrive when I hit 100 at 3,000 rpm, it's quite a turd. Either way, just setting up a simple ration it was 118 miles per hour, just wondered if someone else had an actual formula to calculate with.

 

I like this calculator http://www.f-body.org/gears/

3rd at 4750 is 125
4th at 3000 is 105
3rd at 4000 is 105

You need to have exact tire measurements to get it 100% right but this will get you close.

 

Are all GM transmissions the same gearing internally? Can't remember the F-Body auto trans off the top of my head. Is it a 4L60 or something else?

 

You input your ratios on the site.

80e is: 2.48, 1.48, 1, .75
60e is: 3.06, 1.63, 1, .7

 

Put those numbers in and it put me at 125 at 4,750 in 3rd gear. Tonight I made some pulls messing around and manually shifting and actually learned all my gears, haha, runs pretty good for an old jacked up work truck. Thanks for all the help guys!!