Gear Ratio by Tachometer

[80goldenburb]

I have a tach gauge hooked up in my '80 suburban. i am trying to determine whether i have a 3.21 or 3.42 ratio in the 14 FF. Heres the stats

31.6 overall tire size
1:1 trans ratio
35 mph


Im confused about what reading it is while Im driving. It dead on at 1200 rpm when COASTING (implying i have a 3.21) yet at light throttle (to maintain 35) it reads 1400 rpm (implying I have a steeper gear closer to 3.42)
I know its at 2k exact at 58 mph light throttle. coasting it goes down between 1900 and 2k mark (again implying 3.21)

I ask you guys this since theres no rpo sheet in my glove box and i just wanted to verify what specific reading (light throttle vs coasting) determines it. Thanks again!

 

[foamypirate]

I have a tach gauge hooked up in my '80 suburban. i am trying to determine whether i have a 3.21 or 3.42 ratio in the 14 FF. Heres the stats

31.6 overall tire size
1:1 trans ratio
35 mph


Im confused about what reading it is while Im driving. It dead on at 1200 rpm when COASTING (implying i have a 3.21) yet at light throttle (to maintain 35) it reads 1400 rpm (implying I have a steeper gear closer to 3.42)
I know its at 2k exact at 58 mph light throttle. coasting it goes down between 1900 and 2k mark (again implying 3.21)

I ask you guys this since theres no rpo sheet in my glove box and i just wanted to verify what specific reading (light throttle vs coasting) determines it. Thanks again!

You most likely have 3.21s. The RPMs will go up under load due to converter slip since the TH400 doesn't have a lock-up torque converter.

 

[yevgenievich]

Easier would be to jack up the rear and spin the tire while counting driveshaft revolutions

 

[crazy4offroad]

I know its at 2k exact at 58 mph light throttle.

With an auto trans we would have to go with a higher speed in 3rd gear to get an accurate guess. Assuming you have a stock stall (normally 1400-1600 rpms), at that speed this gear ratio calculator shows 2,007 rpms at 58 mph with 31.6" tires and 3.08 gears in 3rd gear (1:1).
http://www.grimmjeeper.com/gears.html

 

[74propu]

to find out what gear is in the rear end on the passanger side axel tube on top about 6 in from the 3rd member is a code stamped in the tube clean it off so you can read it then Retroc10sport has a post with the codes.
GM Square Body - 1973 - 1987 GM Truck Forum > '73 - '87 GM Truck Modifications > The SquareBody Reference Library
Reload this Page Squarebody rear axle codes

 

[80goldenburb]

this gear ratio calculator shows 2,007 rpms at 58 mph with 31.6" tires and 3.08 gears in 3rd gear (1:1).
http://www.grimmjeeper.com/gears.html

This is the one I use. At 59 mph its 2014 rpm with 1:1, 3.21, 31.6 tire.
http://www.crawlpedia.com/rpm_gear_calculator.htm

Lol previous owner had custom gears made?? As 3.08's were never a factory option in the FF. Only semi floater.

 

[redneck_reed78]

My th350 and 4:10's runs right at 2500 rpm at 55 mph. It's scary on the hwy at 75 mph!

 

[crazy4offroad]

This is the one I use. At 59 mph its 2014 rpm with 1:1, 3.21, 31.6 tire.
http://www.crawlpedia.com/rpm_gear_calculator.htm

Lol previous owner had custom gears made?? As 3.08's were never a factory option in the FF. Only semi floater.

Yeah I must have missed the 14b ff, which also never came with 3.21 gears. Highest (lowest numerically) is 3.42

 

[J Knight]

You speedo is probably off a few mph skewing your input to the gear ratio formula.

 

[yevgenievich]

From factory spedo reading generally would be off by 2-4mph. Even on my 91 spedo is set to show 70mph, while the ecm logs the speed as 66mph.
Spining the rear tires and counting driveshaft rotation can be most acurate without taking the cover off

 

[80goldenburb]

Yeah I must have missed the 14b ff, which also never came with 3.21 gears. Highest (lowest numerically) is 3.42

3.21 was standard ratio, everything else was an option. Its in the sales brochure I have. And of course its a rare gem.

From factory spedo reading generally would be off by 2-4mph. Even on my 91 spedo is set to show 70mph, while the ecm logs the speed as 66mph.
Spining the rear tires and counting driveshaft rotation can be most acurate without taking the cover off

EDIT: after close examination while driving with a GPS speedo app on my GS4, the truck speedometer is NOT off at all. Same speed on both. So at 35 mph its revving 1200 rpm, indicating 3.21 ratio.Yea I am gonna change the differential fluid eventually and then I will take a look at it.

to find out what gear is in the rear end on the passanger side axel tube on top about 6 in from the 3rd member is a code stamped in the tube clean it off so you can read it then Retroc10sport has a post with the codes.
GM Square Body - 1973 - 1987 GM Truck Forum > '73 - '87 GM Truck Modifications > The SquareBody Reference Library
Reload this Page Squarebody rear axle codes

would you happen to have any pics of that? would help to know where to look

My th350 and 4:10's runs right at 2500 rpm at 55 mph. It's scary on the hwy at 75 mph!

how are you even able to get to 75 with 1:1 trans ratio?? My 7.4 hates anything above 2k, 55 60 feels topped out. Ive done 70 before at 26XX, but that felt I was pushing it. Not a smooth ride.

 

[redneck_reed78]

3.21 was standard ratio, everything else was an option. Its in the sales brochure I have. And of course its a rare gem.




EDIT: after close examination while driving with a GPS speedo app on my GS4, the truck speedometer is NOT off at all. Same speed on both. So at 35 mph its revving 1200 rpm, indicating 3.21 ratio.Yea I am gonna change the differential fluid eventually and then I will take a look at it.




would you happen to have any pics of that? would help to know where to look



how are you even able to get to 75 with 1:1 trans ratio?? My 7.4 hates anything above 2k, 55 60 feels topped out. Ive done 70 before at 26XX, but that felt I was pushing it. Not a smooth ride.

Usually keep it on the back roads staying below 60 usually. I hate to push her too.

Sent from my SAMSUNG-SM-G890A using Tapatalk

 

[HotRodPC]

You most likely have 3.21s. The RPMs will go up under load due to converter slip since the TH400 doesn't have a lock-up torque converter.

Exactly Right, Converter slip is why the difference of coasting and on the throttle and this is true with or without a converter lock up in most cases but not all.

Do keep in mind, you are not getting a full 1:1 ratio ever due to converter slip. You're probably losing 300-400rpm.

And yes, you for sure have 3.21's. 58mph w/ 31.6 tires and 3.21 ratio computes to 1979.64rpm.

Another tell take it's NOT 3.42's is 3.42's were not offered in the Full Float rear axle and 3.73 is way out of the ball park to your calculations. IIRC, you can now get 3.42's for a 14b Full Float but they're aftermarket.

 

[HotRodPC]

Yeah I must have missed the 14b ff, which also never came with 3.21 gears. Highest (lowest numerically) is 3.42

The FF DID come with 3.21, and NOT 3.42's. 3.42's are in the Semi Float. My 86 C20 came factory with 3.21's in the FF with Big Block and C6P suspension.

 

[highdesertrange]

jeez just pull the cover, count your teeth and do the math. when was the last time you changed the fluid anyway. highdesertranger