And im thinking its the Tranny. Cause its used.

When I top out 3rd, stock tach reading 5900-6000ish, my speedo reads 85MPH.

The car absolutly has 3.42's.

The transmission is used, not sure what year, or what car it came from. It keeps sticking in my head that it IS a Vette M6.

The speedometer, at 75-80ish, was always within 1 MPH of the radar things on the sides of the road, in 6th gear.

So what the hell kinda gear set do I have? 1st, 2nd, and 3rd, are pretty tight, but after reading that a M6, even revving to the stock redline of 5700, should be going damn near 100mph, and knowing my speedo is that close to the radars, something is not right.

Anyone know if this site still has the thing that calculates the MPH's for the top of each gear, for a given tire size, rear end gear, and RPM ?

 

Vettes used the ZF behind LT1s, so if yours is giving inaccurate speedo readings (and its factory), its possible a 93 F-body model trans. Those came with 3.23s, not 3.42s, and is the only year "LT1" T56 that has different gearing. I'd look for numbers and run them by T56rebuilds.com or D&D Performance.

 

Year Model/Optn 1st 2nd 3rd 4th 5th 6th
---- ------------- ---- ---- ---- ---- ---- ---- -----
1993 Manual/M28 3.36 2.07 1.35 1.00 0.80 0.62
1993 Manual/M29 2.97 2.07 1.43 1.00 0.80 0.62
1994 Manual/MN6 2.66 1.78 1.30 1.00 0.74 0.50

The '95s and later have the same ratios as the '94s. Aside from the CAGS provision and different gearing/input shaft (on the post 93's), all of the 6-speed transmissions are basically the same.

So now, that said, since I am having a brain dead moment, which of these would put me at the 85MPH mark on my speedo, at the top of 3rd, with 3.42's? Gotta run to work now, but im really curious. Also, my speedo seems to be dead on at highway speeds in 6th gear for what its worth.

 

Jack it up and run it at 60MPH in 1-1 gear on the spedo. Time it to one minute and see if it has gone a mile on the OD.

 

None of those trannies would put you close to 6,000rpm at 85MPH in 3rd gear.... at least not if you have 3.42 gears and stock tire diameter.

3rd Ratio MPH@6000rpm

1.30 -------- 103 mph
1.35 -------- 99 mph
1.43 -------- 93 mph

The ZF 6-speed used in the Corvettes had a 1.31 3rd gear ratio. That doesn't explain it.

 

http://www.f-body.org/gears/

 

Injuneer, I definatly have 3.42's cause last year when I first realized that mine tops 3rd early MPH wise, I thought I had 3.73's or something. So I marked the drive shaft with chalk, and had someone spin the tire to check the gear ratio. Just under 3 & 1/2, so its definatly ot 3.42's.

Tires are 245/50R16's

 

So after looking at them, I think its possible I may have the M29 gear set ??? That would put me the closest. And I know my odometer works correctly. Just tested. Likewise, my speedo is always within 1 +- to the highway radar things.