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Discussion Starter #1
'66 with the 200 c.i. straight six, auto.

I've got a diff rebuild in the not-too-distant future. That has me thinking about gearing options that would give me better highway speed at a lower rpm. I want this drive train to last longer than me and I'd also like to do some cruises that would involve freeway speeds, something I avoid now.

It's the stock diff but I don't know what the gearing is. I see charts with everything from 2:8 to 4:11.
Has anybody made a change, and if so, with what results?
 

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The decision may already be made, if you have and keep the stock axle then you will be keeping the stock gears. As far as I know no one makes any aftermarket gears for that differential. You would have to upgrade to at least an 8 inch to have an aftermarket selection of gears. On another note it’s probably already conservatively geared so I’d wager highway rpm relatively low.
 

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You could swap in an overdrive transmission, auto or manual, and run a taller gear for more oomph and have the OD for lower cruising rpm on the big roads. Or, you can just figure your ratio and go up to a 2.79 : 1 - ish.
 

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I see charts with everything from 2:8 to 4:11.
You must be looking at a Data Plate decoder chart that shows all of the gear ratios including the 8" and 9" rear ends. To the best of my knowledge there were only 3 different ratios offered in the 7.25" rear end used in the 6 cylinder cars. Those ratios were 2.83, 3.20 and an ultra rare 3.50 EquaLock.
There is absolutely no aftermarket support for the 7.25" rear end. If you want a gear ratio other than 2.83 or 3.20 you will need to find a '65/'66 V8 8" rear end with 5 lug axles and have the axle flanges drilled for your 4 lug wheels.
 

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If you've got the stock rear end, and I'm reading the shop manual right (page 4-31) you've got either a 2.8:1 or a 3.2:1 ratio with that 7-1/4" rear.

You'll need to look at the code on the door data plate or the little tag on the rear end if it's still there to figure out which. If you have the tag on the diff, that's in the back of the Rear End section, the code on the door tag is defined in the front of the Shop Manual.
 

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Your car is probably already set up from the factory to give low revs on the highway and good gas mileage. If you were to go below 2.8 gearing the engine would lug and the car would be underpowered. Lower gears do not necessarily increase the lifespan of an engine. Lugging is hard on engine bearings. In 1966 there were interstate highways with 70 MPH speed limits. A 6 cylinder automatic Mustang was and still is perfectly capable of driving on any highway, at the speed limit or faster, all day and all night for many thousands of miles.
 

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Discussion Starter #7
If changing diff gears isn't an option I'll learn to live with what I've got. Which is...
a stock setup except for the tires which are radials bigger than the spec bias ply.
According to the speedometer app on my phone I'm doing 60 mph when the car says 55. That works out to about a 10% error. (I hate math.)

I can't find any rpm redline figure for the stock configuration for the 200 ci with a C4. Given the tire/speedo issues I'd like to put a tach on it and figure out what rpm I'd be doing at 70 mph, and if going that speed is even a good idea.

I've read the guys who say they drove their car at 70 all day with no problems, but humor me. I'm overly cautious.
 
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