found something cool under my truck today

[71chevylowrider]

First off, the truck was built by the PO over a 20 year period, so I'm slowy finding out what all has actually been done to it.

I pulled the diff cover & wanted to see what gears I had in my 12 bolt today, unfortunately it's not a posi as I was hoping (some burnouts left 2 marks) anyway, I found it had markings that calculate to 3.73 gears, the rear ratio calculation chart that i've been using shows it shoud be 4.56 gears according to transmission,tires size, rpm's & speed (TH-350, 27" 3,500 rpm's, & 60MPH) but I was pleasantly surprised when i got a look at my nice custom made aluminum drivshaft, I love it when I keep finding cool stuff on my truck. Now I gotta figure out why my RPM's are so high at 60MPH.

[special-K]

Inaccurate speedo or tach?Undersized tires?Wrong speedo gear in trans?How does trans-type effect the gear ratio formula?The rear gear ratio is based on 1:1 final drive

[71chevylowrider]

1:1 is correct, I just wanted to throw in that its a TH350 so that number would known, like if it was a 700R4 or any other overdrive, it would be different then 1:1, I'm not sure why it's revving so high, I know the tach is right, I'm pretty certain on the speed & the tire height, it's perplexing, with 3.73's I should be closer to 3,000 rpm's. I guess I'm gonna have to get taller tires.

[JAKES 68GMC]

Torque converter or tranny slipping? That would be my first guess.

[71chevylowrider]

The Tranny feels nice & tight, it has a stage 2 B&M kit in it, shifts firm, smokes the tire in second, I haven't noticed/heard/felt any slipping at all, the only thing wierd about the truck was it would go through the gears too soon & lug, if you were just putting , it would go first second & then third just going across an intersection, I had a guy put softer springs in the governor & it stretch's out the shifts much better now. If the torque converter is slipping, would it be fairly obvious?

[JAKES 68GMC]

I really don't know, I was just throwing out what popped into my head. Those were the two prime suspects, and the only things I can think of that would make a engine crank a lot higher than it should be at a certain RPM.

[Longhorn Man]

go easy on the burnouts... well, actually,more like spinning and getting sudden traction... we replace a LOT of those shafts in the winter from ppl spinning in show and then hitting dry pavement.

[68C15]

if your tach wire is too close to a plug wire it can pick up some EMI and give an increased reading.
try this, spirally wrap a 18 or 22 gauge wire around the tach sense wire and connect one end to ground. about 3 or 4 wraps per inch is about right.

[piecesparts]

The first thing I would do is to go out on the highway and use a stop watch to figure what speed you are actually going at a certain RPM. This is easy, 60 MPH at 60 seconds is equal to 3600. So with that you can calculate the speed at different times through a set of mile markers. Use more than one set of markers to get a good rounded number on your time.

Example: 55 seconds in a mile will equal 65.45 MPH. (3600 divided by 55 seconds and so on)

Once you know this, then you can use the calculations to figure your speedo gear or go to GM and open their old books and get the right number gear for a certain tire size and gear ratio. I have done both and now have my truck within .05 MPH of what it should be at 70 MPH. Speedo readings are not linear, so if the gearing is off a little it will change across the band of the speedometer, so pick the speed that you drive the most and work for that calculation.

[71chevylowrider]

Hey thanks guys, there are some good ideas to try, all I know is on the freeway the truck definetely sounds like it's wound up to 3,500 at 60MPH & I let mini vans pass me cause I don't want to drive for long periods with it too wound up, on another note, the truck is faster then ****zu around town, but once I get it up over 80 it runs out of legs, & the tach is then almost 5,000rpms. It's been making me want a 700R4 but this TH-350 is bullit proof & if I'm cruising at 25 MPH & stomp on it, it literally fries the tire in first & second gears, I'd have to spend big $$$ on a 700R4 that would have this kind of shift firmness, I'm gonna try these ideas & see what data comes out of it. Thanks you everyone,HEy Andy, didn't you think my aluminum driveshat was cooler then 2 girls kissing?

[Longhorn Man]

ummm, no.

[Sonny]

Your rpms at 60mph are correct. That is what my truck turned before I swapped it out with a 700r4.

[71chevylowrider]

wow Sonny, this was normnal? I guess back in 71 they didn't take the old farm truck out on the highway much then huh? It still doesn't calculate out correctly though, with a 27" tire, 3,500 rpm's at 60 MPH the gear ratio chart says I would have 4.56 gears.

[WorkinLonghorn]

Count the teeth on the pinion.Or turn the DS with one wheel down and count the revs and divide by 2.

[72MARIO]

If the motors are happy turning 4000 let it spin.

My 355 is happier with it in 3rd which is 1:1 than in OD i HAVE 3:73'S unless I am going 80mph THEN IT GOES INTO od.