What diff ratio do i have?
 

I have just put my '30CCPU on the road expecting the speeds to be about the same as my '29 Phaeton but it seems to rev its tights :D off. The speedo is reading way fast when compared to the GPS too.
I have seen somewhere, a method to find out what ratio is in there. As best I can recall, it involved marking a rear tyre at the bottom and the ground along side, jack up that wheel till it is just clear of the ground. take out the sparklers and turn the engine by the crank for 10 turns exactly. Have someone observe how many turns the wheel does It is usually 3 and a bit but it is that bit that is important. There was a diagram showing where the mark on the tyre ended up with the different ratios.
Does anyone have that info please.
I suspect mine is running a 4.11 ratio instead of the 3.78 I expected.

Re: What diff ratio do i have?
 

Here is the info I have;

Pull your plugs and get your hand crank out,
Put it in 3rd Gear.
Make a mark on the jacked up tire at 12 o'clock and crank the engine over exactly 2 revolutions. When you hit exactly two revolutions, have a friend grab hold of the tire so it cannot move, or set the hand brake or whatever.

See where the mark on tire ends up:

Between 11 and 12 o'clock, ratio is 4.11
Between 12 and 1 o'clock, ratio is 3.78
Between 1 and 2 o'clock, ratio is 3.54

These are from notes of the procedure that I have taken from another Barner's post.

Re: What diff ratio do i have?
 

Thanks Mike, That is what I wanted. Seems I was close except for the number of turns of the engine and I forgot to mention the top gear bit.

Re: What diff ratio do i have?
 

Turn engine 2 turns? :confused:

I would jack up one rear wheel, mark the bottom side of the tire, put the tranny in high gear, then count the exact number of turns the hand crank makes to have the rear tire turn exactly 2 turns.

Re: What diff ratio do i have?
 

I can see how either way would work. They use the same principle, after all. Being lazy, I'll opt for Mikes - less thinking required which I too often get wromg anyway.:rolleyes:

Re: What diff ratio do i have?
 

For this procedure you have to jack up the RH rear wheel, if you jack up the LH rear wheel the readings will be backwards.

Re: What diff ratio do i have?
 

Thanks, I add that for the future. :D

Re: What diff ratio do i have?
 

Hi Mike
Thanks for that info.
Do you know what it would be for an early 28 diff. 3:70:1
Thanks
Dave

Re: What diff ratio do i have?
 

This process isn't precise enough for you to tell the difference between 3.78 and 3.70. They'll look the same to your eye.

Re: What diff ratio do i have?
 

My math was backwards. See later response.

Re: What diff ratio do i have?
 

NK, you really didn't mean 200 times did you?:eek::eek::eek:

Re: What diff ratio do i have?
 

Sorry, my mistake. With one tire off the ground, crank the engine for 20 revolutions. If the ratio is 4.11 the tire will revolve 9 3/4 revolutions. If it is 3.78 the tire will revolve 10.58 times. If 3.70 the tire will revolve 10.81 times. And for 3.45 the tire will revolve 11.6 times.

Or, you can crank the engine to revolve the tire 20 times and count the engine revolutions. For 4.11 the engine will turn 41 revolutions. For 3.78 it will be 38 revolutions. For 3.70 it will be 37 times. For 3.45 it will be 34 and a half revolutions.

Re: What diff ratio do i have?
 

If a person can see the ring teeth through the fill plug then carefully count the number of teeth. The 3.78:1 is 9/34 and the 4.11:1 is 9/37 with the larger number being the tooth count on the ring gear. The pinion gears were 9-tooth.

Re: What diff ratio do i have?
 

Thanks Bruce

Re: What diff ratio do i have?
 

I read where you raise one rear tire, put transmission in neutral. Mark the drive shaft, mark the tire. Have someone turn the tire 10xs. someone else counts the revolutions of the drive shaft. 41 and a smidge is 4.11:1
Sound right ?