High RPM on highway

[Ultanium]

Been working on my 69 C20 Panel truck all week, and have made lots of progress, to the point where I can drive it pretty much anywhere. The PO had put the two piece driveshaft in wrong, leaving the carrier to handle the flex of the rear axle. The truck has a 350 crate engine, and 700R4. Once we got the engine tuned up, and took it out for a test drive, the 350 was spinning at 3400 RPM at 60 MPH. I thought for sure that the 700R4 wasn't going into 4th, but it was. The truck has a Dana rear, and we are guessing 4.56 gears. I hate to lose the Dana, but I think I need to swap something a little more highway friendly, if I'm going to use this truck for cruising. I am thinking a 3.73 gear will bring it down, and maybe a 10 bolt or 12 bolt, with posi wIll be just fine. Will only be towing a light trailer with this panel, nothing heavy. What rears will direct swap without too much hassle? Is the Dana worth anything?

Tom

[Steeveedee]

Other Dana axles are out there. I have a Dana 60 in my truck, with a 3.55 ratio. It's a leaf spring rear, as I have a Custom Camper, but you ought to be to find what you need. The 700 has a real digger first gear to get you off the line, but you probably don't want to go higher than 3.55 or 3.73, if you plan to tow (you'll possibly get detonation in OD if you go to a tighter ratio). I also wouldn't recommend using the lockup feature when towing, for that matter.

[TwoFiftyShifter]

3400 at 60 in a 1:1 would be 4.57 gears. When OD engages it would lower your rpms by 30% I.E to 2400.

Something is up alright.

[70STOVEBOLT]

Converter probably not locking up.

[davepl]

Originally Posted by 70STOVEBOLT

Converter probably not locking up.

BY 3000rpm the centrifugal force locking up the converter is pretty tight, unless it's some wild aftermarket converter.

I'm thinking you're incorrect in your assumption that you're seeing fourth gear. I run 4.10s and with the 4L85E I put along around 2K on the highway (actually just a bit above that) at 60+ I think.

Really sounds like you're in third. I've been wrong before though!

[trac209]

Id concur with above,doesn't sound like it's going into fourth or you have some real steep gears. What's your tire size ?

[Ultanium]

It definitely is in 4th. In first gear, I can get up to maybe 8mph, and the 350 is screaming, I swear I think I can lift the wheels from a standstill. The truck has a megashifter, so I walked it through all 4. There is no lockup on the trans, I will look for the plug when I put it back up on the rack. I just think with the low first gear of the 700R4 and the tall gears, everything is mismatched for highway running. I'll try to get some pics and video this weekend.

Tom

[Ultanium]

https://youtu.be/RUJgozmedIM

[RustyBucket]

I watched your video... don't see how the numbers are possible. 700r4 has a .70 O.D. ratio. On a 30" tall tire 3500 rpm in O.D. at 60mph works out to a 7.44 rear gear ratio. Is there a setting on your digital tach. that could account for this? Maybe a bad converter. I wouldn't go changing rear gear ratios til I figured it out. Curious to see what it is.... Like to see pictures of the truck too.

PS.. the 30" tire is just an average... don't see your tire size in your post

[outfield]

Everyone is spot on, your RPM's are too high to be in overdrive. Unless you have 5:38 gears...
Here is the RPM chart for 4.56 gears and 29 inch tires:

4.56 gears
Tire Dia. 29 inch
45 mph 2377
50 mph 2642
55 mph 2906
60 mph 3170
65 mph 3434

[davepl]

I watched the video, and it does seem like you go through four discrete gears. But in the video it doesn't sound quite like the engine matches the tach, almost. You don't have a Pertronix Ignitor module that could be throwing off the tach, do you?

I'd almost want to verify that the tach is right. If your multimeter has a Hz function or if you have a spare tach, I'd verify it neutral at 4000 and see.

Or maybe you have 5.13s in there!

[Ultanium]

Tires are 265/70R16, plain old HEI with tach lead going to the Intellitronix cluster. I will make sure the cluster is set correctly this weekend. https://youtu.be/cpGzweo539k

[trac209]

Without verification of gear ratio we really can't be of much help. Jack up the diff and check how many times drive shafts rotates to one rotation on the tires. If you have an open diff leave one tire on the ground to check.

[Stocker]

Yep, gotta verify a couple things.

Determine the gear ratio, even if it means opening the diff and counting teeth.

Compare your tach reading to a known accurate tach. It could be reading too high.

[RichardJ]

I think the tach is off, also.

My Dana had 4.56 gears. I found a Dodge pickup in the wrecking yard with 3.54 / Posi and swapped carrier and pinion. You have to check if the axle splines are the same and change the yoke for the bigger u-joint that GM uses.

[BILT4ME]

Is the tach set to a 6 cylinder instead of an 8 cylinder? The idle does NOT sound like 800 RPM. The 60 MPH at 4000 RPM shown does not SOUND like 4000 RPM. If it is set to 6 cyl it would be reading 25% higher. In other words, your idle is 600 RPM and the top end is 3000 RPM.

JMHO.

[davepl]

That tach defaults to 8 cylinder mode. You cut the blue loop for 4-cylinder, the orange loop for 6-cylinder. Be worth checking to make sure those loops of wire are still intact. If they've been cut, removed, or damaged your tach goes into 4/6 cyl mode.

[Ultanium]

Got it, thanks man. I will pull it out and verify everything this weekend.

Tom

[Jrainman]

I have to agree with others about that tach not registering the correct RPM , I drive my truck every single day about 200 miles round trip to work mostly highway, and I know what 2000, 2500, 3000 rpm sounds like , from going by what I here in your video that Tach is way off . I would install another tach to check things out . I run a 350/th400/ 3.08

[special-K]

Sounds right, reads wrong. C/20s have short or deep gears, not tall. Tall as in long legs. Like Bonneville racers run super tall gears

[schwoch1]

Have you verified the speedo reading with a GPS of some sort? 4000 RPM at 63 you would have to have a 20" tall tire or at least a 5.xx:1 gear in it!!!

Mike

[Ultanium]

Hey gang, ya'll were totally right. I pulled the cluster and mine has dip switches, not wire loops. The dip switches in the manual are supposed to be for ohm adjustments, for different fuel sending units. All is well now, 2550 RPM @ 55 MPH. This weekend, I'm going to check and see if this has a lockup converter too. Thanks everyone!

Tj

[LongBox]

Good to hear that the problem is not there anymore!

[67ChevyRedneck]

That's great you got the tach figured out!

You still have a lot of gear out back if you're turning 2550 @ 55mph.

MY 350/700R/3.73, 28" tall tire spins 2400 RPM at 75 MPH.

[RustyBucket]

http://www.wallaceracing.com/calc-rgr.php

Have you checked your speedometer reading with a GPS? With a .70 overdrive ratio in the 700R4, using a 31.5 tall 265/70R16 tire, 2550 rpm, & 55mph the numbers work out to a 6.21 rear gear ratio. Sounds like something's still wrong in the figures 'cause 6.21 sounds way too steep to me.