OnyxCobra

+1 easier solution would be to get a slightly taller rear tire.

Jfsram

Accurate enough for this topic. Original poster has a GT so it's a TracLok.
As long as both wheels are spinning the same direction during the test it will be fine.

Portmast would be correct if we are talking about an open differential and the opposite wheel was not taken into the calculation.

Even an open diff can be checked with either 2 revolutions of one tire or one revolution each tire vs the driveshaft.

I do agree with luck13. Not enough difference between 3.7 to 3.5 to warrant a gear swap.
If the rpms need a change, jump to 3.27 or 3.08's.

yoxtheox

mustangs are not cruisers!!!!! your either haulin *** or at stop light

1slomofo

+1

87 T TOP GT302

I've had 4 different gear sets,2.73,3.55,4.10 and now 3.73 in my 87 gt .In my opinion the 3.73 is perfect compromise of performance and fuel economy. There is some difference but not really enough in my opinion to change as it will cost unless you do it yourself.

Portmaster

It never ceases to amaze me! It may be a traclok but it's still a limited slip clutch type differential and even then is subjective to the condition of the clutches. Unless that carrier is under load it's not locked up tight. A little brake drag can screw up your calculations and even then its biased to one side. Then even with with a spool unless you have a degree wheel on the yoke and one on an axle you can be off. The only way to be 100% sure is do a tooth count on the ring and pinion. Anything else is just guesswork. I've seen this screwup so many times over my very long life in a shop it's pitiful. it's easy to remove the rear cover and do the count and only cost the price of a tube of RTV. If he drains the gear oil into a clean container there is no reason it can't be reused. 4 dollars for a tube of rtv and about 45 minutes to be 100% sure on the gear ratio beats the hell out of any other method thats not even close to being accurate.

Jfsram

I'm not advising on the MOST ACCURATE method of figuring out his gear ratio, just the easiest.

Your idea to the original poster to confirm the gears before setting out and swapping them is a good idea.

With the rear tires off the ground a 90% worn trac lok will still spin the opposite tire the same direction, the same amount.

I do not debating your method of counting teeth but one wheel revolution and counting the driveshaft rotations is good enough for figuring out your gear ratio for comparison.
No degree wheel required. 3 turns is 3.08's. 3 and 3/4 turns is 3.73's and so on.

Anyways, either method would work.