nick harmon

im begining to think my tach is fubar. with 308 gears, and under WOT, what mph are you guys shifting from 4th to 5th? i shift around 90-100 mph. something tells me this just isnt right, but the car seems to run faster that way. to sum it up, should i be running 4th gear to 100mph? the tach is reading like 6000 plus. i know, im the king of over reving.

ShiftyCapone

iTrader: (7)

It has been awhile since I drove mine but mathmatically you should be at 109mph at the top of fourth at 5000rpm. Now this is with a 245/50/16 tire. I do ermeber not quite being able to hit the limiter at the top of fourth.

r90camarors

Nick,

Factory tachs just plain suck. I am on my third cluster. 1st had a bad tach. Replaced it and watched the speedo die a year later. So I went with a 145 mph cluster to find out that tach was bad!

Anyways, there are two easy ways of checking for accuracy.
1) The equations on this page https://www.thirdgen.org/newdesign/t...ulations.shtml . You know that fourth gear is a 1:1 ratio, and you are running 3.08 gear, at say 5,000rpm. The only thing in question is your tire radius. Just assuming 12", it comes out to be 115.9mph at 5,000 rpm.

2) Winaldl. It displays the rpms you are actually running at what speed, given that your computer is reading the correct rpms. From my experiance, a bad tach (i.e. on that reads more than actual) does not effect the signal going to the computer. Though I guess it is possible for the dissy signal to be messed up, but I don't think that is a common problem

Dewey316

i've never seen a dizzy sending problem, that would cause a host of other issues.

like other said, chances are your tach is wacked, i have a picture of my car on the dyno, the pickup on the #1 plug for the dyno was reading 4800rpm, my tach is burried in the dash . and if it helps, with 3.08's T5, 235/60/15's on my car, when i ran my car up close to 5k, i was doing about 120mph according the dyno. i can try to copy/paste the dyno print out of showing MPH/RPM from the run, when i get home from work, i will try to get it and post.

Dewey316

here you go.

nick harmon

wow! my tach is junk. time for a nice one thanx fellas

DM91RS

I'm with Dewey.......no dyno but it's closer to 130 when mine needs shifting at over 5k. Too busy watching the road to get exact readings. This is with 255 50 16's and Autometer tach.

Jokerman

Just to chime in, while i do have an auto, my tach will bury itself past 7k on a a WOT romp from a stop through first into second. That is with the tranny doing the shifting. I don't think i have seen or know of anyone with a 305 that can rev like that. (could be wrong, there are some wicked combos lurking on this board) Gm put some crap tachs into these cars. If your speedo is accurate just go by that untill you find a nice aftermarket.

jlbolin

yeah nick mine would be right at about 110-115 at 5000,give or take a couple of rpm.

nick harmon

thanx on the info guys. i took her out on the highway last nite and woo hoo! 4th gear up to 110. i was very impressed. i was shifting around 90-100, totaly shorting myself. that felt incredible, felt as though i had made some serious mod. and all it took was to run the damn engine to its actuall shift point. destroyed an SHO last nite, though i coulda really embarresed his *** if i had no speed limiter. i think mine was just a touch before his which means that had we continued, i wouda been way farther up front. thanx so much for your replies guys. while i used to think my whip was patheticly slow, i now have rejuvinated hope...shhh...i know, its still slow. but it burns the ricers stop lite to stop lite

Dewey316

been there doen that, i was shifting mine at the 4500-5k mark for the longest time, and i thought it kept getting slower. hooked up a winaldl, and found out how off i really was.

at the stip i let the tach hit the dash, then count to 2 mississippi