Rear Axle: rear end ratio

[dallastx75208]

I have a 62 hawk. How do I know what my rear end ratio is?

[Skip Lackie]

There should be a tag attached to one of the bolts on the back cover of the center pumpkin. It will contain two numbers that give the number of teeth on the ring and pinion. Divide the larger by the smaller.

[rockne10]

In the absence of the tag, the easiest way is to jack up the rear, put the trans in neutral, crayon mark the bottom of the tire. Then, spin the tire and count the number of revolutions the driveshaft makes until your crayon mark returns to the bottom center.

[RadioRoy]

There are also "standard" ratios that are usually applied. The automatic equipped cars generally come with 3.31 ratio except the super hawks are often equipped with a 3.54.

An overdrive hawk will usually have a 3.73.

I do not know what usually comes with a 4 speed.

All these are subject to differences caused by special orders and/or previous owners modifications.

[dallastx75208]

The higher the ratio the better the MPG? Car had a 3 speed. Now has a 5 speed

[Warren Webb]

Your 5 speed is most likely an overdrive in 5th gear. Lets guess your car has a 3:73 rear end & the overdrive ratio is .7, then divide them & the result is your final drive ratio.

[Jerry Forrester]

In the absence of the tag, the easiest way is to jack up the rear, put the trans in neutral, crayon mark the bottom of the tire. Then, spin the tire and count the number of revolutions the driveshaft makes until your crayon mark returns to the bottom center.

That's almost correct, but you need to add a little more info.
With the transmission in neutral,
If it's an open differential, jack up one rear tire. Put a mark on the tire. Notice where the pinion yoke is. Turn the tire exactly two revolutions while counting how many times the pinion yoke goes around.
If it turns 3 1/3 times it's a 3.31, if it turns 3 1/2 turns it's a 3.54 and so on.
If you have a TT , jack up both wheels and if they turn in the same direction when you are turning only one tire by hand, turn the wheels only one revolution.

[nibbs53]

There are also "standard" ratios that are usually applied. The automatic equipped cars generally come with 3.31 ratio except the super hawks are often equipped with a 3.54.

An overdrive hawk will usually have a 3.73.

I do not know what usually comes with a 4 speed.

All these are subject to differences caused by special orders and/or previous owners modifications.

Good morning Roy.
My 62 GT Hawk 4 speed as a 331.

nibbs53 Patrick.

[Mike Van Veghten]

nibbs -

I'd bet that's a 3.31 to 1.

A 331 isn't possibe in that housing.

Mike

[dallastx75208]

Transmission is a T5 from a mustange

[Dan Timberlake]

Do you drive it? Does the tach read reasonably close? Does the speedometer come close to agreeing with a GPS about instantaneous MPH? How does it go down the hiway in 5th? If rpms are around 2000 rpm at 55 mph the final drive ratio is pretty decent for gas mileage.

[dallastx75208]

Yes I do drive it on highway and around town when it is not 101 outside, no air yet. I have a tach but it is not hooked up, I do not have a tach sending unit, need to get one. I had to switch the grear on the sped cable, got that from a mustange shop. It is real close to GPS. I have checked gas on a longer trip and it seem to get around 25mpg. I have not been on another long trip. Need to check it again. Around town I do not know what it gets, have not checked that. I like the 5th grear a lot. I am making this into a daily driver so I wanted a different transmission in it.

Do you drive it? Does the tach read reasonably close? Does the speedometer come close to agreeing with a GPS about instantaneous MPH? How does it go down the hiway in 5th? If rpms are around 2000 rpm at 55 mph the final drive ratio is pretty decent for gas mileage.

[wittsend]

How was your speedometer gear determined if you do not know the rear ratio?

As a side note to this I recently put a 700R4 in my car with 3.07 final gears. I then compared it to the effective rearend ratios I would have needed to use if I had installed the GM non-overdrive 350 transmission and had the equivalent numbers. The ratios were:

700R4 1st gear w/3.07 = 350 w/3.73
700R4 2nd gear w/3.07 = 350 w/3.31
700R4 3rd gear w/3.07 = 350 w/3.07
700R4 4th gear w/3.07 = 350 w/2.15 (4th gear is .70 overdrive)

I found it ironic that the non-overdive ratios were all equivalent to common ratios.

Tom

[dallastx75208]

I got lucky. I did read some place on here to try two different gears for the sped cable. I got lucky with the correct one.

How was your speedometer gear determined if you do not know the rear ratio?

As a side note to this I recently put a 700R4 in my car with 3.07 final gears. I then compared it to the effective rearend ratios I would have needed to use if I had installed the GM non-overdrive 350 transmission and had the equivalent numbers. The ratios were:

700R4 1st gear w/3.07 = 350 w/3.73
700R4 2nd gear w/3.07 = 350 w/3.31
700R4 3rd gear w/3.07 = 350 w/3.07
700R4 4th gear w/3.07 = 350 w/2.15 (4th gear is .70 overdrive)

I found it ironic that the non-overdive ratios were all equivalent to common ratios.

Tom

[wittsend]

That's great that it worked out. Perhaps you can reverse engineer this based on speedometer gears and figure out the ratio of the rear gears. An FYI, the size of the tires also effects speedometer accuracy. But if they are close to original size... .

I seems we both have .70 overdrives. My calculations had me at about a "diesel like" 1,600 RPM at 65 MPH with the 3.07 gears.

Tom