My Trans Am's gear ratio is insane

[hgffrank]

I know this is gonna sound nuts and I dont understand it either, but I recently swapped my wheels/tires from my 90 Bird to my 83 T/A. On the 90 the car had a 3.08 gear and 5 speed trans, it went 80mph at 2000rpm exactly. now with the same EXACT wheels/tires on the 83 T/A w/5 speed trans, it went 80mph at 3200rpm. both cars were in 5th gear.

I did the math and used the ratio calculation (3.08X3200=9856 divided by 2000= 4.928). Yes I came up with a 4.928 gear ratio. HOW THE HELL IS THAT POSSIBLE. I didnt even know a 10 bolt was capable of a 4.93 gear. WTF?????? Someone please help me here, I am so confused. I know the tachs were both accurate. Do they even make a gear that high?

In case your wondering the T/A came with 205/70/14's which are 819 reves per mile and 25.4 diameter. The wheels I swapped on are 245/50/16's which are 808 revs per mile, and 25.7 diameter, so the difference is only like 1.2%.

[transampimp]

I think alot has to do with the possibility of having a different final drive ratio in your gear box. I highly dought you have a 4.928 gear ratio.

[hgffrank]

As far as I knew ALL 3rd gens with 5 speeds had the SAME final drive ratio of .73

[00firebird]

at 80mph tires can throw your speedo off 5+ mph easily even if they are only slightly different

[b.a.camaro]

wow that is messed up!!!!!!!!!!!8-O

[81DaytonaPaceCar]

I know this is gonna sound nuts and I dont understand it either, but I recently swapped my wheels/tires from my 90 Bird to my 83 T/A. On the 90 the car had a 3.08 gear and 5 speed trans, it went 80mph at 2000rpm exactly. now with the same EXACT wheels/tires on the 83 T/A w/5 speed trans, it went 80mph at 3200rpm. both cars were in 5th gear.

I did the math and used the ratio calculation (3.08X3200=9856 divided by 2000= 4.928). Yes I came up with a 4.928 gear ratio. HOW THE HELL IS THAT POSSIBLE. I didnt even know a 10 bolt was capable of a 4.93 gear. WTF?????? Someone please help me here, I am so confused. I know the tachs were both accurate. Do they even make a gear that high?

In case your wondering the T/A came with 205/70/14's which are 819 reves per mile and 25.4 diameter. The wheels I swapped on are 245/50/16's which are 808 revs per mile, and 25.7 diameter, so the difference is only like 1.2%.

Hey, Frank, it's not possible. Not quite sure what math you're using there, but there is no way you have a 4.92 gear. Even if you had 5.11 zoomies or something similar, you'd be turning 4200+ at 80 mph.

I'm just guessing, but you likely have between a 3.73 or 4.11 gear in the new car.

And I wouldn't be surprised if one or both of the speedos are off.

[meissen]

I'd have the speedo recalibrated, or drive past one of those cop signs that gives you the speed as you drive by. When I went from 15" rims to 17" rims my speedo was 5 mph off.

[phoenix_rising]

Something smells fishy. I think you need to have the tach checked or have someone check your math. 4.9 gears would never been put in a T/A from GM. I am guessing you have either 3.08s or maybe 3.23s.

[81DaytonaPaceCar]

Something smells fishy. I think you need to have the tach checked or have someone check your math. 4.9 gears would never been put in a T/A from GM. I am guessing you have either 3.08s or maybe 3.23s.

I'm betting he's got aftermarket 4.11s in there...

[Yoeasy69]

doubtful...

i would say a nice set of 3.73's... my on my dodge i put some bigger tires on it.. not too much bigger, just one size.. (if i could only remember.. its been so long)
and at 50 on the speedo, im doing 58.. at 70. im doing 81...

thanks to the nice michigan state trooper association of ass raping ticket givers.. i now know that.. (j/k j/k they were actually really nice to me)

[meissen]

yeah it doesn't take much to knock the speedo out of alignment.

[kammi10]

nope,

and speaking of that i need to get my speedo gear in the tranny swapped, it says im doing 48 when im doing 35

[hgffrank]

The speedometer is mechanical cable driven so I'm not sure about the recalibration being needed. I wish we had some of the mile marker signs around here in Jackson, thats an easy way to figure it out.

The math I used is posted already and its accurate. Its a simple ratio equation:

3.08 ....__?___
----- : .------
2000.... 3200

Basically, 3200 X 3.08 divided by 2000 = 4.928

[81DaytonaPaceCar]

The speedometer is mechanical cable driven so I'm not sure about the recalibration being needed. I wish we had some of the mile marker signs around here in Jackson, thats an easy way to figure it out.

The math I used is posted already and its accurate. Its a simple ratio equation:

3.08 ....__?___
----- : .------
2000.... 3200

Basically, 3200 X 3.08 divided by 2000 = 4.928

Well, I guess either one of two things is happening.

1. You're running 5.00 rear gear...or...
2. You're speedo and/or tach is jacked up.

Both are possible, but I'm betting on the speedo/tach.

[AJ 87SC]

Your speedo is probably off by a little and you have a nice set of 4.88 gears which is possible in a 10bolt.Its the biggest you can fit, I am 90% sure of it
WHY you would have that is beyond me, but it is possible..

Could the 10 bolt have been taken out of an old drag car, or a truck maybe??