Gear Ratio/ Enging RPM Question

[muckyp]

At 55mph my truck is turning 3000 RPM. That seemed a little high to me. The truck has a 454 and a th400 transmission. I attempted to figure out the rear gear ratio by jacking up the rear and counting the number of revolutions the driveshaft makes with a single rotation of the wheel. I came up with less than two...OBVIOUSLY I am doing something wrong. Anyone have any idea what I am doing wrong. I don't really want to pull the diff cover if I don't have to. Does that RPM sound high? By the way, I also made sure that the transmission was shifting into all three gears.

[engineer_gregh]

What size are your rear tires?

[engineer_gregh]

Sounds like your tach is off. Even with 27 inch tall tires and a 4.10 rear end you wouldn't be at 3000. 55x4.10x336/27=2800 rpm's.

[69SCSS]

Assuming you have a 29" tire in the back, that would mean you would probably have 4.10/4.11's. If you want to know for sure then you would need to remove the cover and look for the stamp or count the teeth. Easier to just spin the rear end until you see the stamp.

My truck with 3:73's and 29" tire will spin 2800 or so at 55, that's why I put a 700R4 in there!

[69SCSS]

Originally Posted by engineer_gregh

Sounds like your tach is off. Even with 27 inch tall tires and a 4.10 rear end you wouldn't be at 3000. 55x4.10x336/27=2800 rpm's.

I also found that multiplying the result by 1.1 for converter slippage gets you more in the ballpark, although not all converters will be the same.

This is how I checked mine: I took one of the straps for the drivesahft and pointed it straight up. Then I rotated one tire one full rotation. The strap was pointing almost directly at me, so 3/4 of a rotation after 3 full rotations, 3:73. Sounds like you already did something similar though.

[muckyp]

Tires are 28 1/2" tall. How can I verfiy the tach accuracy?

[71K20chevy]

I believe if it's an open rear end that if you do the wheel/driveshaft ratio thing then multiply it by 2, that should be your actual ratio.

[muckyp]

Oh yeah, any idea why the spinning of the wheel/drive shaft thing is not even coming close to a reasonable number?

[engineer_gregh]

I've used my timing light that has a digital RPM indicator to compare. Mine is original and way off.

[69SCSS]

Originally Posted by engineer_gregh

I've used my timing light that has a digital RPM indicator to compare. Mine is original and way off.

This ^^^ Mine reads 200-300 high. On the freeway, it says 2500-2600 when I'm going 65, which should be 2200-2300 or so when the converter isn't locked in the 700R4.

[71K20chevy]

If the tach is indeed off, reach up behind tach with a small flat blade screwdriver, there is tape covering a hole that has a small adjustment screw.

[mechanicalman]

Originally Posted by muckyp

At 55mph my truck is turning 3000 RPM. That seemed a little high to me. The truck has a 454 and a th400 transmission. I attempted to figure out the rear gear ratio by jacking up the rear and counting the number of revolutions the driveshaft makes with a single rotation of the wheel. I came up with less than two...OBVIOUSLY I am doing something wrong. Anyone have any idea what I am doing wrong. I don't really want to pull the diff cover if I don't have to. Does that RPM sound high? By the way, I also made sure that the transmission was shifting into all three gears.

If you only spun one wheel then you need to multiply "less than 2" x2. You probably have 3.73 by the description given.

[mechanicalman]

Originally Posted by muckyp

At 55mph my truck is turning 3000 RPM. That seemed a little high to me. The truck has a 454 and a th400 transmission. I attempted to figure out the rear gear ratio by jacking up the rear and counting the number of revolutions the driveshaft makes with a single rotation of the wheel. I came up with less than two...OBVIOUSLY I am doing something wrong. Anyone have any idea what I am doing wrong. I don't really want to pull the diff cover if I don't have to. Does that RPM sound high? By the way, I also made sure that the transmission was shifting into all three gears.

If you only spun one wheel then you need to multiply "less than 2" x2. Sure it wasn't "more than two"?

Try turning the driveshaft 2 turns for a more accurate wheel count and no need for multiplication.

[68wannabe]

On an open differential, or non position, you turn the wheel/tired twice and count the drive shaft revolutions to get the actual gear ratio. Or multiple one turn of the wheel by 2 like stated above.

[68wannabe]

Posi*

[muckyp]

Thanks guys, I had never heard of multiplying by 2. I guess I will get a timing light with a rpm display.

[muckyp]

Originally Posted by engineer_gregh

I've used my timing light that has a digital RPM indicator to compare. Mine is original and way off.

Any suggestions on what light to get?

[imjeff]

Mine has 3:73s and a T400 and is over 3K at 60. It's pretty normal.

Jeff

[LockDoc]

Originally Posted by muckyp

At 55mph my truck is turning 3000 RPM. That seemed a little high to me. The truck has a 454 and a th400 transmission. I attempted to figure out the rear gear ratio by jacking up the rear and counting the number of revolutions the driveshaft makes with a single rotation of the wheel. I came up with less than two...OBVIOUSLY I am doing something wrong. Anyone have any idea what I am doing wrong. I don't really want to pull the diff cover if I don't have to. Does that RPM sound high? By the way, I also made sure that the transmission was shifting into all three gears.

As far as finding the ratio by jacking the vehicle up there is actually more to it than that. You have to jack up one tire if you have an open differential, or both tires if you have a positraction or locking differential.

Rotate the tire one full revolution for positraction and lockers, and 2 full revolutions for open differentials. Count the number of full revolutions the driveshaft makes. This is your gear ratio. If the drive shaft turns just over 3 turns it should be a 3.08 if it turns 3 ¾ turns, it will be a 3.73 gear ratio.

LockDoc

[jocko]

here's the math on what you should see with 4.10s, TH400, and 28.5" dia tire.

[mechanicalman]

Originally Posted by muckyp

Thanks guys, I had never heard of multiplying by 2. I guess I will get a timing light with a rpm display.

First off, I made a mistake on a previous post.
"Try turning the driveshaft 2 turns for a more accurate wheel count and no need for multiplication".

Should have said "Try turning the wheel 2 turns for a more accurate driveshaft count", (sorry, got it backwards), and that is with one wheel only the other on the ground. Reason is, due to how the spider gears have to be designed, if one wheel is not turning the other is turning twice as fast so we have to double the wheel turn count when only turning one wheel.

There are aftermarket gears of like 3.90 and such, unlikely you would have them but really need to be sure exactly how many turns so use some kind of marking system on the tire and the driveshaft so you don't make a mistake.

If you know someone who has a high end digital multi meter, like a Fluke or Snap-on with a tach, that will be as accurate as the digital timing light.

OR

After you confirm your rear gear ratio for sure, use this to calculate what RPM you need at what speed then adjust your tach to match.

http://www.ringpinion.com/Calculators/Calc_RPM.aspx