gear ration-rpm-mph question

[OneTonGoldfish]

my bronco has a fresh c4, 5:30 gears (so the previous owner says), and 38.5 tires.

In 3rd gear at around 4k rpm it only does 39-42mph. The motor runs good, tranny shifts fine and I am using gps to get the speed so I know its not a faulty tach, or not shifting to 3rd, or an off speedo like you would assume.

I checked randys rpm gear ratio calculator and with a c4 (1.00 3rd gear ratio) 5.30 r&p, 40 mph should be at 1943rpm

Do you think the tranny doesnt have stock gearing or the r&p are not 5.30?

I am stumped

 

[7bronco6]

Are you sure your tach is set up for a 8 cylinder? It might be switched to a 6 or 4??

 

[Viperwolf1]

Are you in low range? You should be able to tell what gear the trans is in by just shifting manually.

 

[OneTonGoldfish]

the tach is right and the trans is in 3rd....

 

[OneTonGoldfish]

also in 2h

 

[Socal Tom]

do the roll test to determine gear ratio. Put a peice of tape on the drive shaft and roll the rig forward for 1 tire revolution. The number of time the driveshaft turns is the gear ratio.
Tom

My vote is the trans is in 2nd

 

[RB Precision]

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

seems like the rpm is a bit high

 

[Viperwolf1]

3rd gear in a C4 is 1:1. It can't be changed. No one makes 10:1 gears for a 9". Either your tach is wrong, your tcase is in low range, or your torque converter is broken.

 

[garberz]

...Do you think the tranny doesnt have stock gearing or the r&p are not 5.30?
I am stumped

3rd gear is always 1:1, there's no way to change that. I'll agree with Viperwolf1, with that speed and RPM, it sounds like your in low range.

 

[OneTonGoldfish]

the tcase shifter is all the way to the right then up for 2h on a stock 74 d20?

 

[Viperwolf1]

J-shifter, that explains it. You're in low range. Pull it all the way back, then right, then up slightly.

 

[OneTonGoldfish]

J-shifter, that explains it. You're in low range. Pull it all the way back, then right, then up slightly.

from N I had it down all the way then right all the way then up all the way. so thats wrong?

 

[garberz]

Now it will be like having overdrive.;D

 

[carter2772]

Here is a pic of the shifters.... Top right knob is a J patter n..

 

[OneTonGoldfish]

Here is a pic of the shifters.... Top right knob is a J patter n..

i know, thats the same diagram i have on my shifter knob. see my post above yours...isnt it correct?

 

[Moon's74]

from N I had it down all the way then right all the way then up all the way. so thats wrong?

Define "up all the way".

Like Viperwolf said, up slightly. If it went up on the right as far as it does on left for 4L, then your not in 2H, you're probably still on the left and in low range.

Not trying to insult your intelligence, but I've been there.;D

 

[OneTonGoldfish]

Define "up all the way".

Like Viperwolf said, up slightly. If it went up on the right as far as it does on left for 4L, then your not in 2H, you're probably still on the left and in low range.

Not trying to insult your intelligence, but I've been there.;D

haha insult it all you want, I would LOVE for this to be the problem!!;D

but I am pretty sure its in 2h as it does not go up near as high as when i put it in 4L

 

[canrel]

Just for giggles, pull it all the way back and try it. this should be 4 high which will tell you what ratio you have for high. Then go to left and forward to see if there is a difference in low. If there is a difference you will now know what it will feel like when you do get it in 2 high.

 

[OneTonGoldfish]

does 4H and 2H have the same gearing?

 

[carter2772]

does 4H and 2H have the same gearing?

Yes.... And you could always try and turn your front drive line by hand, with hubs unlocked, to see if it is in 2 or 4 wheel.. I was also going to suggest what was said above, and just see if there is a difference between what you think 2 and 4 wheel drive is.