What rear axle gear do I have?

[Renxus]

Good day,

On my 73 I have C6 gearbox and it revs a bit too much for me, about 3000 RPMs on 60 mph. Way too loud. So I was thinking of the rear axle and this info I got from there:


What could be the gear ratio in my axle?

 

[Stanglover]

First, I'm not the expert here, but does your axle have a tag attached to it behind one of the bolts. The tag has the info of what is supposed to be in the axle. I say supposed because the gear could have been changed.
If your motor is a 351C 2V the tag could be WES-T or T1 which is a 2.75:1 ratio. WES-AD is 3.25:1. If the car has the original driver door sticker, axle codes would be 2 or 9 respectively which are open diff. If it has a WFB-D2 door code K it is a 2.75:1 Trac lock 2L75:1. A WFB-C, door code R is the 3.25:1 trac-lock diff.
The only list the book shows for the 351C4V is WES-AK2 door code A, 3.50:1 open or WFD-F3 door code S is the 3.50 traction lock diff. No other ratios were available for automatics.
It's possible someone has swapped the gears for a higher ratio, but if you jack the back up with the trans in neutral, you can turn the back wheels and count the rotations of the drive shaft. With a 3.25:1 one turn of the wheel will turn the driveshaft 3 and 1/4 turns, (3.25) get the idea?
If both wheels turn in the same direction, that is both forward or backward, it is a traction-lock diff. If just the right wheel turns, it is an open diff. The left wheel may turn backward when the right wheel turns forward.

 

[c9zx]

This may help. Chuck https://www.moderndriveline.com/how-to-determine-your-rear-axle-ratio/

 

[midlife]

An easy way to tell your rear axle ratio: put the car up on a lift, and in neutral. Rotate one tire 2 complete rotations and count the revolutions of the drive shaft. If the rear is open, the ratio is the number of drive shaft rotations. If the rear is a posi, then divide by 2 to get the rear axle ratio.

 

[Renxus]

Thanks, gentlemen! There are no labels, so turning test it will be.

 

[Renxus]

Wheel turned and rotations measured, it is 3,25. I plan to swap to AOD but doubt that will it make it much better on the highways. Want to have it just for cruising, old enough already (that is me then). Should I look for a different gear?

 

[giantpune]

I wonder if something is amiss. Your current C6 transmission has a 1.0 ratio in its highest gear. With a 3.25 rear end, you would need to have itty bitty wheels (22" tire diameter) on the back to be pulling 60MPH @ 3000 rpm. Either your vehicle speed or your RPMs or your 3.25:1 rear end is not jiving in the equation, or you have comically small tires.

With 26" tall tires, it would be doing 60mph @ 2500rpm.

And then if you had 26" tall tires and swapped to an AOD with a 0.67:1 overdrive, 60MPH would be 1690rpm.

 

[basstrix]

I've got 3.25's, a C6, and running 255/60/15 tires. At 3000 RPM and no converter slippage, mine would be just shy of 70mph. With 6% slip, the calculator says just shy of 66mph. A stock converter would probably be less than 6%, on average.

You may have a phone app that has a GPS speedometer built in....I think Google Maps has one....use that and your tach to see where you're at. Another good GPS app is GPS Status....no ads and very few permissions needed.

Edit: With an AOD, you'd be at 65 @2000 RPM and 75 @2300RPM assuming 6% converter slippage and same size tires.

 

[Stanglover]

Wheel turned and rotations measured, it is 3,25. I plan to swap to AOD but doubt that will it make it much better on the highways. Want to have it just for cruising, old enough already (that is me then). Should I look for a different gear?

I'm curious, did your wheels turn 1 revolution to 3 1/4 turns of the drive shaft? I'm a bit confused by Midlife's reply where he say 2 revs of tire. Also is it an open diff or limited slip (Trac-Lock)

My car has a 3.25:1 trac-Lock diff with a 4 speed an 1:1 final gear. At 60 mph it is turning 2700 give or take a few rmps and that is with 245-60-R14 tires. The speed is correct to GPS speed.

 

[Hemikiller]

My 71 XR7 has 3.50 gears, 245/60R15 tires (26.5" diameter) and an AOD. She pulls down 2500rpm at 75mph.

Some of your noise is probably the factory flex fan. A simple swap to a thermostatic clutch unit and matching fan will free up some HP, lower engine noise and save you gas to boot.

 

[Renxus]

Interesting discussion. My wheels are 215/60 R14. I have an open differential (turning opposite). Flex fan is an interesting thought. However, will shift to AOD in spring (when the snow melts) and we'll see from there.

 

[Stanglover]

Interesting discussion. My wheels are 215/60 R14. I have an open differential (turning opposite). Flex fan is an interesting thought. However, will shift to AOD in spring (when the snow melts) and we'll see from there.

The tires show a height of 24.2" / 61.47cm and 835 revs per mile / 1.61 km. Could that be what's causing high rpm @ 60 mph.
If I'm right, a tire closer to 26" height would give you a bit better speed to rpm ratio.
Am I correct Hemikiller?
Google "Tire size calculator" and you can determine a better size, not that I'm trying to spend your money!!
If your car has a factory flex fan, get rid of as soon as possible, they are dangerous.
On my 71 with a 351C, I installed a Hayden 2710 fan clutch with a Derale 17118 18" fan
Works perfectly.

 

[Don C]

Good day,

On my 73 I have C6 gearbox and it revs a bit too much for me, about 3000 RPMs on 60 mph. Way too loud. So I was thinking of the rear axle and this info I got from there:
View attachment 84059View attachment 84060

What could be the gear ratio in my axle?

The spreadsheet in this post will let you play around with tire sizes and differential ratios so you can see what your speed will be with different combinations.
https://7173mustangs.com/threads/tire-sizes.11266/The link to the speedometer gears, in the first post, will not work, but a corrected link is further done in post #5

 

[midlife]

There's another factor that has been overlooked: if the speed is from the speedometer, the transmission speedo gear may not be appropriate for the tires and rear axle ratio, giving an erroneous speed.

 

[Renxus]

There's another factor that has been overlooked: if the speed is from the speedometer, the transmission speedo gear may not be appropriate for the tires and rear axle ratio, giving an erroneous speed.

Speedometer is off about 10 km/h according to the GPS indeed but I calculated this error.

 

[delawarebill]

mine is factory. 351c 2v with 275 rear. if my memory is close I'm running under 2k rpm at 60 mph. it still has spunk in takeoff and cruising is great. i'd say the cheapest and easiest is rear Grear change. if u choose that option don't forget to recalculate the driven speedo gear and change it. you will need to pull the speedo cable out of tranny to count the teeth on the end of that cable.

 

[Renxus]

mine is factory. 351c 2v with 275 rear. if my memory is close I'm running under 2k rpm at 60 mph. it still has spunk in takeoff and cruising is great. i'd say the cheapest and easiest is rear Grear change. if u choose that option don't forget to recalculate the driven speedo gear and change it. you will need to pull the speedo cable out of tranny to count the teeth on the end of that cable.

Nice RPMs. Do you drive 3-speed automatic, not AOD? Will count the teeth for sure.

 

[basstrix]

Interesting discussion. My wheels are 215/60 R14. I have an open differential (turning opposite). Flex fan is an interesting thought. However, will shift to AOD in spring (when the snow melts) and we'll see from there.

The radius for your tires calculates to 12.1", unloaded. To get an accurate calculation, use the loaded radius. I estimate your would be .6-.7" less than the unloaded (11.5"). Here's table of RPM vs speed for no slip and 6% slip, assuming a 11.5" loaded radius.

 

[Stanglover]

There's another factor that has been overlooked: if the speed is from the speedometer, the transmission speedo gear may not be appropriate for the tires and rear axle ratio, giving an erroneous speed.

Good point.

 

[Renxus]

The radius for your tires calculates to 12.1", unloaded. To get an accurate calculation, use the loaded radius. I estimate your would be .6-.7" less than the unloaded (11.5"). Here's table of RPM vs speed for no slip and 6% slip, assuming a 11.5" loaded radius.

View attachment 84112

So 3000 rpms is about correct with 60ish. Can you tell what AOD should give me?