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Hi-PO Stang
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With the transmission gear ratios Ford puts in Mustangs now , I would think a 4.56 gear would be great for daily driving.
MikeHTally
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Would be a hoot. Make sure you have a direct connection to a refinery. 
BmacIL
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Well the 18+ manual gearing w/ 3.73 is taller than the 2015-2017 w/ 3.31, so it needs the gear.Would be a hoot. Make sure you have a direct connection to a refinery.
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Grimace427
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The GT350 can rev 750rpm higher so that isn't an apples to apples comparison.
Last edited:
Grimace427
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Think how fast your driveshaft will be spinning. Any imbalance will be amplified.
Brazos609
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Your two posts make absolutely no sense and are contradictory. First you are talking about running the engine to redline then immediately follow up with a warning about the driveshaft spinning too fast. The rear differential ratio has nothing to do with the driveshaft RPM. Changing the differential ratio affects the half shaft and tire RPM. The driveshaft RPM is determined by engine RPM and transmission ratios. If you're worried about driveshaft RPM you must never go above 50MPH because that would make the driveshaft spin too fast, right, or do you just shift into sixth gear at 20MPH to play it safe with driveshaft imbalance?
Grimace427
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The driveshaft is directly bolted to the rear differential. Your statement is 100% incorrect.
Cobra Jet
Well-Known Member
If it were me, I’d definitely do a 4.56 rear, no doubt.
For those of us who have had prior Mustangs and did gear swaps, going from a 3.73 to 4.09’s is like going from 3.08’s to 3.27’s - what’s the point.
When I owned my prior 96 Cobra (DOHC), that engine was capable of 7500RPM shifts - and it came from the factory with POS 3.08’s (most likely for EPA fuel consumption figures).
Anyhoo - I upgraded from 3.08’s to 4.30’s without any hesitation, purely due to the fact that if you mapped the gearbox ratios to possible RPM analysis, jumping from a 3.08 rear to a 4.30 rear would not impact daily drivability at all, nor would it make any dramatic impact on fuel consumption.
Sure it sounds like a very steep jump from a 3.08 to 4.30, but in reality, it was an excellent decision. Not only did it eliminate the car from bogging when it was in 4th and 5th gears at lower RPM’s, it also made the car a lot more fun. It made it actually “drivable” and believe it or not, my fuel consumption actually improved instead of decreasing. The reason being is the car was in a much better gear to RPM ratio at certain speeds that it was operating more efficiently.
The 1-3 shifts were REAL F-U-N when romping on it.... tire obliteration from 1-3 was cake... LOL.
But in all honesty, even with the current S550 transmissions and their ratios - you’ll have the best bang for the buck with 4.56’s in a N/A application. The car will operate more efficiently and the shifts are still docile enough for daily driving. Sure, if the vehicle owner is going to explore supercharging or turbo applications later and their interest is track/timed events after forced induction, then the 4.09 is a better choice.
Don’t fear the gear!
For those of us who have had prior Mustangs and did gear swaps, going from a 3.73 to 4.09’s is like going from 3.08’s to 3.27’s - what’s the point.
When I owned my prior 96 Cobra (DOHC), that engine was capable of 7500RPM shifts - and it came from the factory with POS 3.08’s (most likely for EPA fuel consumption figures).
Anyhoo - I upgraded from 3.08’s to 4.30’s without any hesitation, purely due to the fact that if you mapped the gearbox ratios to possible RPM analysis, jumping from a 3.08 rear to a 4.30 rear would not impact daily drivability at all, nor would it make any dramatic impact on fuel consumption.
Sure it sounds like a very steep jump from a 3.08 to 4.30, but in reality, it was an excellent decision. Not only did it eliminate the car from bogging when it was in 4th and 5th gears at lower RPM’s, it also made the car a lot more fun. It made it actually “drivable” and believe it or not, my fuel consumption actually improved instead of decreasing. The reason being is the car was in a much better gear to RPM ratio at certain speeds that it was operating more efficiently.
The 1-3 shifts were REAL F-U-N when romping on it.... tire obliteration from 1-3 was cake... LOL.
But in all honesty, even with the current S550 transmissions and their ratios - you’ll have the best bang for the buck with 4.56’s in a N/A application. The car will operate more efficiently and the shifts are still docile enough for daily driving. Sure, if the vehicle owner is going to explore supercharging or turbo applications later and their interest is track/timed events after forced induction, then the 4.09 is a better choice.
Don’t fear the gear!
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Brazos609
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The rear differential ratio and tire size determine the road speed at a given engine RPM and transmission gear combination. If you are going 4000RPM in sixth gear with a 27.7" rear tire and a 3.73 or a 4.56 rear differential ratio the speed of the vehicle will differ, not the driveshaft RPM.
Schwerin
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Unless you're tying to say that the change in gear will need higher engine RPM to maintain the same speed I've lost you. And even so if your shaft isn't balanced enough to take the few hundred RPM change then its already not balanced enough to be driving with.
CNC Lester
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I daily 4.09 in my 18 pp1 manual. First is a little short in traffic but u can start off in second without too much clutch feather. As for mileage, 21.4 average. 70 mph is under 2500 rpm in 6th gear. Stomp it above 3500 rpm in first and it's sudden, u will spin 275 ps4's thru first and into second. MT street ss will hook good on that roll.
I have long tubes, pmas, and lund tune. Also have pedal commander set at sport minus 3.
A blast to drive. Going procharger soon.
I have long tubes, pmas, and lund tune. Also have pedal commander set at sport minus 3.
A blast to drive. Going procharger soon.
Grimace427
Well-Known Member
You are wrong again. For a 3.73 gear ratio, for every rotation of the rear tires the driveshaft turns 3.73 times. For a 4.56 gear the driveshaft spins 4.56 times for every tire rotation. Engine and transmission is completely irrelevant. As long as the car is moving the driveshaft is spinning. The higher(numerical) the differential gear ratio is the faster the driveshaft is spinning.
Grimace427
Well-Known Member
It may not pose a real problem but it is always something to consider when you are spinning the driveshaft faster and faster. At some point you will hit the critical speed limit of the shaft and it will catastrophically fail. Most quality aftermarket shafts are rated to 9,000 shaft RPM which should be more than enough, but some aren't balanced to those speeds(certainly not the stock shaft) so it is questionable how long the shaft will tolerate high speeds.
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