I am (was) thinking about swapping my 3.5 diff for a longer one. This morning though I was checking photos of my car and discovered a tag on the diff that suggests I already have a 3.07. Still I am feeling that the car is revving quite high at highway speeds but the tach isn’t really giving me any more information other than that the car is running. The needle is jumping up and down covering a corridor of at least 500 revs.
Is there any other way to gauge the revs correctly and / or to determine if the tag is telling me the correct ratio?
look up the appropriate ratios for Jag diffs ie 43/13 for instance
jack up the rear wheels, and mark the tyre and driveshaft with chalk
rotate the wheel and mark off on a piece of paper each rotation, and as well for chalk mark on driveshaft
I advise rotate the wheel the full count and tally up at the expected ratios, the chalk marks will show a complete rotation when you have the right count, ie 3.07, 3.31, 3.54 but whole numbers, trying to count 3.07 vs 3.54 turns is not sure to be accurate
Many thanks, Tony. The ratio as per tag should be 43 divided by 14 = 3.07. Does this mean the driveshaft needs to turn 3.07 times for one full turn of the axle shaft?
Yes, but its important to turn the wheel the full 14 turns, and mark of each turn with a stroke on paper for wheel and propshaft, dont lazy out and just try 3.07 turns
check the other ratios so you can double check what happens at 13 turns for instance
If you turn the Road Wheel 14 times and the diff has a ratio of 3.07:1, the Drive Shaft will turn 43 times. Therefore, you will have to keep close tabs on both the Road Wheel and the Drive Shaft for a relatively large number of turns.
I find the following to be quite definitive in determining which of the popular ratios, 3.07, 3.31, or 3.54 is being used.
Have a helper count the number of revolutions of the Drive Shaft (Pinion Shaft of the Differential), while you rotate the Road wheel three rotations. Use chalk or some other means to mark. an Index Mark on both the Road Wheel and the Pinion Companion Flange, close to their circumference and have some static item as a Start Point alignment reference.
Once you have rotated the Road Wheel three accurate revolutions, the number of turns of the Pinion Flange (Drive Shaft) and the correlation with the differential ratios are as follows:
The number of full and partial revs are far enough apart that any one count can’t be confused with that of any other. If you used the 3.31 ratio as the standard of 10, revolutions less than 10 will be a 3.07:1 ratio, revolutions in excess of 10 will be a 3.54:1 ratio.
I think you may have a speedo that’s not reading properly: at 75 mph, with a 3.07 rear end, my Jaguar was turning just shy of 3000 RPM. That is not too high number.
I would think first you would want to confirm the tachometer accuracy. Simply hook a diagnostic tach to the engine.
Interesting it seems you are implying you would like to swap diffs because the possibly inaccurate tach says rpm’s are too high rather than it seems busy on the highway.
Tom
Thanks to all for your help and advice. I will check the function of speedo and tach as proposed. Maybe I have a ‘fake’ 3.07 diff (certainly way above 3000 revs at 75MPH).
I have the 3.07 in my car. Book says 25mph/1000 rpm and it operates at exactly that. I have the correct std size tires (Vreds) on the car. If you have anything else obviously you could have some other kind of performance.
I have to ask if the tach is original to the car, or at least the correct tach for 3.07 gears? I can’t recall now but I recall someone posting the correct tach numbers for each gear set. The number will be something like 1312 for example if I recall. Maybe someone here can post the correct tachs for each gear set?
The tachometer measures the engine speed. The speedometer measures the road speed and is directly affected by the final drive ratio. So, there is one speedo variant for each final drive ratio (actually several, as there are other variables such as whether the dial is annotated in MPH or KMPH etc.)
