What rear axle gear do I have?

[Renxus]

Good point.

Easy today to check up with the phone. I have a metric face on the speedometer and it shows me about 10 km/h more than actual. With my 351 she could be nice with 1500 rpms I guess. But AOD first and let's see from there.

 

[basstrix]

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

Using same tire and gear with an AOD (0.67:1 vs 1:1 on the non OD trans).

 

[Renxus]

Using same tire and gear with an AOD (0.67:1 vs 1:1 on the non OD trans).

View attachment 84144

Thanks, Basstrix!
That's a nice tool you have there Self-made?
Seems that AOD will shave off about 1000 RPMs at 60 mph. That would be a much more enjoyable ride.

One more question: is there also math available on what speedometer driven gear I should find for this setup? Like, how many teeth will correspond?

 

[basstrix]

Thanks, Basstrix!
That's a nice tool you have there Self-made?
Seems that AOD will shave off about 1000 RPMs at 60 mph. That would be a much more enjoyable ride.

One more question: is there also math available on what speedometer driven gear I should find for this setup? Like, how many teeth will correspond?

Thanks, yes, just an excel spreadsheet I put together...comes in handy every so often.

For speedo gear, I used Don C's spreadsheet. Here's a link to it:

https://7173mustangs.com/threads/speedometer-gears.15595/

 

[CandyRcode]

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.

Forgor the most important thing. The other axle must be held in place.

 

[CandyRcode]

Measure the radius from the ground to the center of the rim. Double that number. Divide 20,178 by that number. That is your tire RPM @ 60 MPH. Mulitiply by your gear ratio and that is your RPM @ 60mph. (no slip) example. Tire =14" x 2 =28. 20178/28 = 720. 720 x 3.50 gear =2520 @60 mph

 

[Renxus]

Thanks guys! Hopefully, I will get the AOD dropped in within the next two weeks now and see then what the GPS shows me as a difference.

 

[CandyRcode]

To make spedo correct you need to turn the cable 1000 times per mile.

 

[t.kohlmeyer]

Okay, my Q code '72 convertible with a C6 has a 3L25 rear setup. CJ cars have a smaller, higher stall converter on it.

My '72 H code Mach 1 had an FMX and 2.75 open gears from the factory. It's now running a '70 closed chamber head 4V engine, Holley 700 DP carb, '71 CJ cam, and Hooker SC headers, C6 with the CJ converter and 3.91 gears with a Detroit Locker and 31 spline axles and a nodular iron case. I managed to get two gear reduction speedo cables gearboxes from Ford before they became extinct. It pulls like crazy, hooks up well, but goes 70MPH at 4500 RPMs. It's a blast to drive.

 

[cjstaci]

https://www.strangeengineering.net/gear-ratio-calculator/