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/

 

[Renxus]

Eventually got my AOD installed. The speedometer (in kph) is over by 16 kph. While driving 100 kph / 62 mph, according to the GPS the speedometer shows 116 kph / 72 mph.
Can someone point me out the best closest speedometer drive gear?

 

[giantpune]

Eventually got my AOD installed. The speedometer (in kph) is over by 16 kph. While driving 100 kph / 62 mph, according to the GPS the speedometer shows 116 kph / 72 mph.
Can someone point me out the best closest speedometer drive gear?

Do you know which speedometer drive gear you have now? If not, you can check it by just removing the 1 screw and clip that holds the speedometer cable into the back of the transmission. Then pull the cable out and the gear is stuck to the end of it.

You can count the number of teeth. Since your speedometer is too high, the cable is spinning 16% too fast. You would slow it down by using a speedometer gear with 16% more teeth, or as close to that as you can get.

 

[Renxus]

Do you know which speedometer drive gear you have now? If not, you can check it by just removing the 1 screw and clip that holds the speedometer cable into the back of the transmission. Then pull the cable out and the gear is stuck to the end of it.

You can count the number of teeth. Since your speedometer is too high, the cable is spinning 16% too fast. You would slow it down by using a speedometer gear with 16% more teeth, or as close to that as you can get.

Mine is 18 teeth. Adding 16% will make it 20,88. So the closest round count would be 21 teeth. Will order and see it from there.

 

[Renxus]

Mine is 18 teeth. Adding 16% will make it 20,88. So the closest round count would be 21 teeth. Will order and see it from there.

The formula worked like a charm. I changed the gear to 21-teethe and it is dead on with the GPS now.
Thanks giantpune!