Quick check on gear ratio

[ubtripn]

Hi All,

I have a 350 with a 700R4 tranny. At 2200 rpm in 4th I am at 60 mph and after hitting the OD it is 2000 rpm equal 60. These are roughly accurate, any stab at what gear ratios my diff is. All of my power is at the top end. I want to bring the power down to where it is usable and increase torque, thanks for the help!

[kwmech]

Tire size plays and important role here.

[ubtripn]

235/75 R15 - Thanks, great point!

[Keith Seymore]

Originally Posted by ubtripn

At 2200 rpm in 4th (sic - 3rd) I am at 60 mph and after hitting the OD it is 2000 rpm equal 60.

I'm showing a 235/75R15 is approx. 29" in diameter.

Using 1:1 for third gear and .70 for overdrive I get:

3.07:1 = 2134 rpm at 60 mph in third; 1490 rpm at 60 mph in OD

3.23:1 = 2245 rpm at 60 in third; 1570 rpm at 60 mph in OD

3.43:1 = 2380 rpm at 60 in third; 1670 rpm at 60 mph in OD

3.73:1 = 2590 rpm at 60 in third; 1815 rpm at 60 mph in OD

4.11:1 = 2860 rpm at 60 in third; 2000 rpm at 60 mph in OD

So something is not adding up; I'm getting a way bigger drop in engine rpm from 3rd to OD than what you are reading.

K

[Keith Seymore]

Originally Posted by ubtripn

At 2200 rpm in 4th (sic - 3rd) I am at 60 mph and after hitting the OD it is 2000 rpm equal 60.

Trying a different online calculator:

3.07:1 = 2200 rpm at 62 mph in third; 2000 rpm at 80 mph in OD

3.23:1 = 2200 rpm at 59 in third; 2000 rpm at 76 mph in OD

3.43:1 = 2200 rpm at 55 in third; 2000 rpm at 72 mph in OD

3.73:1 = 2200 rpm at 51 in third; 2000 rpm at 66 mph in OD

4.11:1 = 2200 rpm at 46 in third; 2000 rpm at 60 mph in OD

So - I'm thinking somewhere between 3.23 and 4.11, based on your data.

K

[ubtripn]

Thank you, yeah something is off, I will re-measure.

A 700R4 has 4 gear plus OD right?

[kwmech]

4th gear is overdrive. Then the convertor locks up, which may be what you are thinking. And yes more than likely your speedo is not accurate. Have GPS on your phone?

[sanddunecoon]

http://www.grimmjeeper.com/gears.html

Ive used the calculator for years. works great - it will tell you what you need to know and more.

[ubtripn]

Thx for the calculator, its a GPS speedo so not sure what is up.

[kwmech]

Trying to figure out your actual MPH. Sound like you have that.

With what is given: 60 mph, 29'' tall tire at 2000 rpm--if my math is correct you should have a 3.07 rear. To bring you into the power band I would shoot for a 3.42 or 3.73

[gmc684x4]

3.42 you can use same carrier 3.73 you will need to change the carrier for the ring and pinion have you ever changed the gear oil in the diff ?
Easy way to to change the gear oil and look at the numbers stamped on the ring be something like 43:14

Other way is to look on the axle tube for codes stamped there one issue with axle stampings is did anyone change gear ratio in 48 + years your truck has been on the road

[ubtripn]

Thank you! I will.

[cj847]

The 2200 to 2000 rpm does not make sense. There should be a 30% drop between 3rd and 4th.

But, to find what you are looking for, I would ignore 4th gear. Hold the truck in third as that is 1:1 ratio and makes everything easier to calculate.
Second issue, what are you using to measure RPM? If it is a factory tach it is not accurate enough for this job.

The easiest way to find your ratio is to raise 1 rear tire off the ground and turn the tire 2 revolutions while counting the rotations of the driveshaft. You can find a clearer explanation on google. Take precautions blocking the tires as the truck has to be in neutral.

https://shiftsst.com/blog/post/how-t...25%3A1%20ratio.

[kwmech]

He thought the convertor locking up was 4th, hence the 200rpm drop

[Keith Seymore]

Originally Posted by kwmech

He thought the convertor locking up was 4th, hence the 200rpm drop

Ah - I see what you are saying.

The OP took data in OD (converter unlocked) and OD converter locked.

Originally Posted by ubtripn

At 2200 rpm in 4th (OD converter unlocked) I am at 60 mph and after hitting the OD it is 2000 rpm equal 60.

Having said that, this data becomes:

4.11:1 = 2000 rpm at 60 mph in OD

So - 4.11


K

[Keith Seymore]

Originally Posted by ubtripn

Thank you, yeah something is off, I will re-measure.

A 700R4 has 4 gear plus OD right?

3 forward gears plus OD.

Regular "D", or "3" = 1:1

OD = 0.7:1

K

[Tom70C20]

Originally Posted by ubtripn

All of my power is at the top end. I want to bring the power down to where it is usable and increase torque, thanks for the help!

Running 2000 rpm at 60 mph (2,333 at 70) for normal street use doesn't sound bad to me. Do you really want more torque/power at lower rpms, or just want your engine to generally operate at lower rpms and get better mileage? If all the data is correct, your 4.11 differential is already pretty high numerically (low gear ratio) and to get more torque with just the diff would require an even higher number/lower ratio, but your truck will run at higher rpms and get worse mileage. Does your engine have any performance mods (e.g. cam/heads)? I agree you should check the factory tach for accuracy if that's what you are using.

[ubtripn]

Thank you all for the knowledge! I thought about it and I built this truck to be a farm truck so I thought, "Why do I want more power on the low end?" I live in the country and cruise well at 2000rpm so I am not going to mess with the gears. That being said, I sure wish it was limited slip or posi. I have already demonstrated my lack of understanding so I will just ask if there is a way to change the guts of the dif to get that? A way besides welding it that is.