I was wondering what the rpm's were for a stock 305 TBI with a T5 and 3.08 Gears in the real world. I've done math and saw what it should be and cant figure out why my car is revving so insanely high. 2600 RPM's at 65. I've checked my speedo with a new vehicle and yes it's accurate. The only other possibility is someone swapped the rear gears in my car before me. If the revving sounds familiar, post your gearing combo so i could compare.

 

If you ride in fourth gear at 60 mph your rpms will designate your rear end ratio. You should be running 3080 rpms at 60mph in 4th. Assuming your 4th gear is 1:1. The best way to find out is to actually look at your gears though.

 

Thanks I never thought of running in 4th to see. The tranny's been rebuilt too and i contacted the company and they gave me the stock ratios. Borg Warner is the same as T5 right? I've never understood if there was a difference

 

Yes you have a Borg Warner T5.

 

Most likely your tach is inaccurate like just about everybody else's that posts this question.

The tach (I use the word loosely) in these cars ALMOST ALWAYS reads high. That's how they break. They get worse and worse with age. I have one that reads twice the actual RPM; and there are people on this board that have reported theirs reading 5 times actual.

Check it against a piece of real test equipment, such as an engine analyzer. I'll bet you'll find that it's reading 2500 RPM when the engine is really doing 1800, or some such.

 

For exactly this purpose, I made this spreadsheet:

https://webdisk.ucalgary.ca/~jmknopp...eed%20calc.xls




It was written to check speed vs RPM with different rear gear sets, tire sizes, tranny combos, etc etc. Top speed, RPM at speed, speed at RPM, whatever. So handy, so very handy.

Enjoy.

 

Will the tach become more inaccurate as the RPM's increase?

 

Not usually; it's generally linear, but off by a uniform percentage. Like, it reads 25% high or something like that. Such as, it reads 1000 when the engine is doing 800, 2000 when the engine is doing 1600, 4000 when the engine is doing 3200, etc. Always off by the same proportion.

It's not "500 RPM high" or something like that; that would be a "zero" error, where there's some fixed offset from reality, as opposed to a "reads high" error.

 

Yeah, my dash tach reads about 1250 on a 650 idle and is pegged by 3000.

 

off topic, but the tach seems to be an easy fix. theres a big red capacitor on the circuit board that if you replace, will register RPM correctly again. Or, at least as correct as factory stock. Its not an autometer or anything, but still shouldn't be too far off...

I am waiting for mine in the mail. Theres another thread stating 4 or 5 members doing this and fixing their tachs. Can't hurt for 59 cents a pop.

This is of course for82-89 models.

Back to the transmission talk!

Brandon

 

care to snap a pic of that big red cap? Or tell me where to find it? Right behind the tach? Maybe the value of the cap (if it's universal), and the size/watt rating of it? I'd rather do that then strap in a 5" tach on my dash pad, or have to replace all of my gauges for upwards of $400...

 

Sonix,

https://www.thirdgen.org/forums/elec...89+tach+repair

everything is in this thread, including which capacitor to order, and where its at on the circuit board to replace it.

some else even mentions another capacitor that dictates the v6/v8 calibration.

i am waiting for my capacitors in the mail, i plan to fix both my buddies and i's tachometers.

enjoy!
brandon

 

very cool, thanks.

how on earth do you get to that PCB? my speedo is in my cluster, and I can't get it out for the life of me.,..