I just got my tranny switched from a 4L60 to a 5-sp from a '89 RS. I was playing with the calculators on here for top speed at RPM, etc. and according to the info charts on here I should have a 3.23 rear gear ratio, and up to a .76 5th gear ratio. That would mean a 5500 RPM redline in 4th gear (1:1) would get me to 130mph.

So here's the facts:

235/60/15 tires, 26.1" diameter, 13.05" radius
'89 T-5

60 MPH at 2750 RPM in 5th!!

I played around in the calculators with my known speed/rpm/tire radius but couldn't get a gear combo that made sense. The only combo that came close was if I was running a 1:1 ratio in 5th, or a 4.5 rear gear ratio. Is either of these scenarios even possible?

 

In 5th at 60mph, I got you at 1903.4

And you top out at 132 @ 5509rpms in 4th.

 

That's what the calculator says, but my actual speed in 5th is 60mph at 2750, which would mean 120mph at 5500 rpm. That's why I'm confused.

 

What rear gear ratio do you have? Did you change the gears in your new trans to match your rear?

 

Nope, rear gearing is bone stock, which should mean 3.23.

 

...and what gear ratio was in the car the trans came from, and did you change the speedo gears in the trans to match your 3.23 rear?

 

The rear ratio in the donor car should have been 3.42. I didn't change speedo gears. The difference in rear end gearing from 3.42 to 3.23 shouldn't put it off by that much though right?

 

Time to pull the cover and see what gears you have, and get the correct ones for the manual trans. What RPMS did it turn before the swap at 60?

 

About 2500 in 4th with TC locked

 

Should be 1750 with 3.23 gears. You had a problem before the swap.

 

Good idea to back check what used to be going on. I just did a calc with the actual measured tire radius (12.5, not 13) and 3.23 gears. Based on a 3rd gear ratio of 1:1 (4L60), it would be running at 115mph at 5000 rpm, which is about right. I topped out at 115 before the swap with the tranny in 3rd (governor won't allow an upshift to 4th at full throttle) The rpm in 4th at 60 was a best recollection.

So the back check confirms I should have 3.23 gearing in the diff. Now I have more factual data:

Tire radius = 12.5
Rear end ratio = 3.23
rpm @ 60 in 5th =2750

So the only 2 possibilities are either
(1) I have a 5th gear ratio of 1:1 or
(2) My speedometer is off by about 20mph

 

is it possible that your tach is off? i know that more often than not, the factory tachs in these cars suck and will be off by quite a bit.

 

It could be, but it shows idle at 800 and stops making power at about 5200, so it seems right at both ends.

 

Way too many assumptions going on here. Verify the tach is correct, verify exactly what rear gear is in the car, and then report your findings.

 

I have to agree with lo-tec here. The errors you are seeing are stacked. The tachs and speedo's in these cars are off. The faster you go the more they are off. Also, if you don't change your speedo gears than your innaccurate speedo will be even farther off.

The math on the tech page will tell you what numbers you should read. A scan tool will tell you your exact RPM. Pull the cover and find out the ratio. It is probably a good time to change the fluid anyways.