Tach Install Where and Whats The Trucks Redline?

[man7sell]

Driving the latest acquisition, I find the truck revs too high for the speed it attains. It's a manual 4 speed. First gear the revs are high and the wheel turn about half way around so easy to take off in second then quickly to third and now I'm doing 20 mph ready for forth. At around 40 MPH it seams to be pretty high revs.
So number one, I'm fitting a tach. Got it all hooked up except the green signal wire. This wire will go to the distributor, I think there is a tach connection there right? It got too dark to continue last night.
What should I expect for revs at a cruise speed?
Took a pic of the rear axle diff. Note on the right some suspicious numbers on the right axle. I think this axle has been replaced with one from a wrecking yard. Probably with the wrong ratio.
Not a manual transmission should be on to one in forth gear or am I totally off base?

[K5owner]

SM465 probably for a trans right? Possibly 4.10's in the rear of that 12 bolt? That could be it. I would open that cover and take a look as well as service that differential.

[TBONE1964]

http://www.advanced-ev.com/Calculators/TireSize/

Use this calculator for correct RPM based on speed, tire diameter and rear gear ratio.

The calculator will tell you what your RPM should be at a given speed but you need to know the exact gear ratio and tire diameter.

If you do not know your rear gear ratio, it is easy to find out.

Jack up only one rear wheel. (I always do right side but it does not matter)

Make sure you have a good safe jack stand and block the wheels so the truck will not roll. Place the trans in neutral.

Mark the pinion at the u joint and mark the tire. Rotate the tire two turns and count how many times the pinion turns. If the pinion turns just a little over 4 turns, you have a 4:10. If it turns 3 3/4 turns, you have a 3:73. This only works for an open non posi traction differential.

On a posi traction, you need both wheels off the ground and only turn one of the tires one turn.

Of course the only way to know exact way is to pull the cover and read the ring gear.

Hope this helps,
Tom

[man7sell]

Originally Posted by TBONE1964

http://www.advanced-ev.com/Calculators/TireSize/

Use this calculator for correct RPM based on speed, tire diameter and rear gear ratio.

The calculator will tell you what your RPM should be at a given speed but you need to know the exact gear ratio and tire diameter.

If you do not know your rear gear ratio, it is easy to find out.

Jack up only one rear wheel. (I always do right side but it does not matter)

Make sure you have a good safe jack stand and block the wheels so the truck will not roll. Place the trans in neutral.

Mark the pinion at the u joint and mark the tire. Rotate the tire two turns and count how many times the pinion turns. If the pinion turns just a little over 4 turns, you have a 4:10. If it turns 3 3/4 turns, you have a 3:73. This only works for an open non posi traction differential.

On a posi traction, you need both wheels off the ground and only turn one of the tires one turn.

Of course the only way to know exact way is to pull the cover and read the ring gear.

Hope this helps,
Tom

Thanks Tom, I'll try that

[TBONE1964]

By the way,

Yes the green tach wire will go to the "tach" terminal on an HEI ignition or the negative side of the ignition coil on a points style distributor.

Good luck and keep us posted. Everyone learns from this site.

Take care,
Tom

[mattybilt79]

The 2/3 gear ratio is very close in a 465 so it usually is a quick shift into 3rd. Technically its a 4 speed but the first gear is considered to be low gear most shift knobs show the pattern as L123. Its been awhile since I've had one but the shift points are close to a 400 auto and my 400 shifts into 3rd at roughly 38 mph with 4.10 rear. 3rd gear in 465 and 400 are both 1/1 ratio so its only going to go as fast as the engine can rev.
If your truck is only 2wd sounds like clutch might be slipping if its revving high in gear. Been awhile since I've run a tach on a 3 speed but 2600-2800 rpm seems to come to mind running 65-70 mph.

[man7sell]

Originally Posted by mattybilt79

The 2/3 gear ratio is very close in a 465 so it usually is a quick shift into 3rd. Technically its a 4 speed but the first gear is considered to be low gear most shift knobs show the pattern as L123. Its been awhile since I've had one but the shift points are close to a 400 auto and my 400 shifts into 3rd at roughly 38 mph with 4.10 rear. 3rd gear in 465 and 400 are both 1/1 ratio so its only going to go as fast as the engine can rev.
If your truck is only 2wd sounds like clutch might be slipping if its revving high in gear. Been awhile since I've run a tach on a 3 speed but 2600-2800 rpm seems to come to mind running 65-70 mph.

That interesting, would make sense that 1st gear is a Low gear. I have started pulling away in second and using it as a 3 speed. I tested the clutch and there is no slip, although the release point is right at the end of travel.
I will check the speed vs revs as soon as I hook up the tach.

[csdineley]

Mine runs about 2400 rpm's at 55 mph. With the sm465 and 4.10 gears.

[man7sell]

Originally Posted by csdineley

Mine runs about 2400 rpm's at 55 mph. With the sm465 and 4.10 gears.

Thanks that's a good reference point

[angertheloner]

I'm interested, but those with a factory tach and sbc 350, what is the factory redline?

[TBONE1964]

Originally Posted by angertheloner

I'm interested, but those with a factory tach and sbc 350, what is the factory redline?

Not really a red line but most factory Chevy truck tachometers register to 5000 RPM's

[angertheloner]

Originally Posted by TBONE1964

Not really a red line but most factory Chevy truck tachometers register to 5000 RPM's

Thanks.

[man7sell]

Hooked up the tach (Later permanently but for the test a zip tie). Test run 2500 revs at 55 MPH and a nice 60 PSI on the oil and 185 F on the new temperature gauge, nice to have it all working.

[86c20]

let me see here. my 4.10 trucks low is 5-8mph 1 is 15 2 is about 25-35 and that is up shift points. unless you are really winding it out. my 3.42 truck is low 15 1 30 2 50-55 then 3 to a tuch over 115. and thats about all she has

[man7sell]

sounds like I have the 4.10

[MalibuSSwagon]

A good gear ratio calculator is good to have --> http://www.grimmjeeper.com/gears.html

Me being me, I regularly run the original 305 up to 5k RPM.