The speed limitr is two bytes in the PROM - one for the fuel cut off ($6C is 108 mph, $7D is 125) and another for the resume (generally 2 mph lower). There is a similar pair that govers the redline (believe 6000 rpm for $5B - 3800 - engines). Those cars that came with "H" rated tires most likely have a 125 mph cutoff. (e.g. 1990 SE).

stupid speedo only goes to 85mph, any way i can tell how fast im goin? the only thing i can think of right now is using my dads gps with the ground speed on it and judge by thsat but im not sure how accurate it is

The speedo in my 79 Firebird used to end at 85 mph or 140 kph, so I installed a geared 1.6 ratio converter so now my gauge reads to 140 mph.
Even with that, I used to bury the speedo so I made up a chart that corresponded to engine rpm as read off the tach. 5200 rpm took me to 160 mph with a 2.41 gear ratio. :P

If you have a tachometer then you should use that to give you a pretty close estimate of knowing how fast you are going after 85 mph.

MPH = (.076 x RPM)/Overall gear ratio

Math= Headache

ill just go as fast as i can and see what the cops have to say

mine would have the 2.84 gear ratio right?

Well if you assume that your sweet spot is around 4200 rpm, then with your gearing you could be doing 112 mph.
4500 rpm = 120 mph

So go for the rpm and you will get speed. :P

depends...the SE in your Gen could have had the 2.97 ratio....only way to know??? go check the RPO codes

F17 - ratio, transaxle, final drive 2.84

F79 - ratio, transaxle, final drive 2.97

look for one of these

i just checked and its got the 2.97 no wonder my numbers seemed off

So with 2.84 in 3rd gear.....
At 4200 rpm, then you could be doing 112 mph.
4500 rpm = 120 mph

But you have 2.97 in 3rd gear so....
4000 rpm = 102 mph
4200 rpm = 107
4500 rpm = 115
4750 rpm = 121
5000 rpm = 128

Of course you could shift into OD 4th gear but you will go slower in 4th than in 3rd with that final ratio, unless you force feed that baby. :P