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Can someone tell me if it is possible

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nocturnalnasty

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Can someone tell me if it is possible that I have done my math wrong or maybe I'm doing something else wrong?

I have a 1990 suburban with a 700R4/208 and 38" TSL/SX's with a D60 front and 14BFF rear that I just put under the truck.

The PO of the axles told me that the axles had 4.10 gears. I have not had the inspection plate off the 14 bolt rear end, but my buddy who is a really good mechanic has and he confirmed that the ring is stamped with the correct numbers for a 4.10 gear set.

Now from what I can tell my 700R4 should have
1st gear 3.06-1 ratio
2nd gear 1.63-1 ratio
3rd gear 1.00-1 ratio
4th OD .70-1 ratio

So here is my math. Based on a 1.00-1 final transmission (3rd Gear) at 65 MPH.

RPM = MPH x Gear Ratio x 336 / Tire Diameter in inches

RPM = 65 x 4.10 x 336 = 89,544 / 38" TSL

RPM = 2,356

But my RPM is 2,800 at a GPS'ed 65 miles per hour on flat highway. Which would mean my gear ratio would be closer to 4.88 to 1.

So does anyone have any idea where I am going wrong?

Thanks
John
 
Torque converter slip and your tires don't measure out to a true 38"
 
I don't know of a way in driving conditions with a slushbox.
If it has a locking converter, then you can get close. Look at your tires and you see the flat spot on the bottom that makes them effectively smaller than rated.

Some of that will disappear with speed.

The easiest way is to jack up a tire and count the shaft revs as you turn it by hand.
 
Can you run a tape around your tire like a dirt tracker does and figure true Diameter? You can only check it in D with the converter locked up. I don't know for sure it that locks up in drive or not. The hillbilly way to check it is locking up is to keep your foot steady on the gas and tap the brake to see if the rpms jump up about 500 rpms then come back down. Tapping the brake unlocks the converter.
 
Good stuff thank you all for the info. I think I'm just going to jack it up and spin the tire to see what the gear ratio is. I am also going to call the shop that built the tranny and find out the specs on the torque converter. That way I won't be guessing, I'll know for sure.
Thanks all
John
 
Why don't you just take the cover off and read the numbers stamped on the gear?

I'm sure the fluids could use a change and also you can inspect the condition inside the diff.
 
As others have said, its due to your tire being efectivly less than 38" and the slip in the converter/trans.
 
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