Automotive gear ratio is most easily explained as the number of turns of the drive wheels in relation to the number of turns of the engine.  Basically , if the engine is considered the input and the drive wheel is considered the output, then the input gear or pinion meshes with the output gear or ring gear to drive or rotate it. The number of teeth on each gear determines the ratio.  For example: A rear end pinion gear may have 15 teeth that mesh with a ring gear that has 45 teeth; mathematically (45/15) this yields 3.00 ratio. This means that the engine will turn 3.00 times for each turn of the drive wheels. Most manual transmission RX-7's came with a 4.10 rear ends, while most RX-7 automatics came with a 3.90 rear end.  A ratio of 4.10 means the engine turns 4.10 times as fast as the wheels.  A 4:10 rear end will help with low end torque which is great for a Rotary powered RX-7 because a Rotary engine does not produce a lot of torque.  However, a 4.10 rear end at high speeds means that  the engine RPM's will also be high because the engine is turning 4 times faster then the wheels.  Example:  A stock Rotary powered engine with a 4:10 rear end and stock tires going about 80 mph on a freeway would be cruising at about 3,500 RPM.  Since a Rotary engine redlines anywhere from 7,000-8,000 RPM's (depending on  the year) 3,500 is a comfortable cruising RPM for a Rotary engine.  Looking at the charts above, a GM Borg Warner (B/W) T-5 (5-speed) gearing is similar to the RX-7 5-speed gearing, so, at first,  it sounds like a good Hybrid RX-7 choice - but is it the best choice?  Since the B/W gearing is similar to the Rotary gearing, this means at 80 MPH you're still doing about 3,500 RPM's, but now you're doing 3,500 on a V8 engine that probably Redlines at about 5,500 RPM's.  This is why you want to choose a transmission with an overdrive,  let's say you install, a GM TH350 (Turbo 350) that has a 1.00 final gear and you have a  4.10 rear end, now at 80 MPH, you'll be spinning at about 4,500 RPM's (yikes!).

REAR END UPGRADES

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By changing your stock taller rear end gears (numerically lower) with shorter gears (numerically higher), you can significantly improve the acceleration of your vehicle. When selecting the right gear ratio for your particular application, you must take into consideration many factors like your current and future modifications, as well as what type of driving you do most, along with rpm findings showing where in the rpm range it went from one gear at max shift to power range of the motor in the next gear. Typically vehicle equipped with taller gears, i.e. 2.73, 3.08, etc. trade slower acceleration for better high-speed performance. On the other hand, cars equipped with shorter gears, i.e. 3.73, 4.10, etc. will accelerate much quicker in the 1/4 mile but have a lower top speed. Gear ratios of 4.10 and higher should be reserved for the normally aspirated vehicles strictly set up for drag racing. Enthusiasts running high horsepower turbos, blowers, and or nitrous applications do not need as much gearing. These drivers generally limit the rear gear ratio of their vehicles to 3.73 and lower.

Since automatic-equipped vehicles are at a disadvantage to begin with, they seem to benefit much more by gear changes. This will in many cases bring the automatic cars up to par with the stock stick cars. In addition, the automatic enthusiasts will have the added comfort of not missing shifts and easier "hook up".

When upgrading the rear gear ratios on a stick-equipped vehicle, some drivers may initially have a harder time "hooking up" during racing or spirited driving. These drivers may have to adjust their driving technique and or purchase stickier tires/slicks to obtain proper traction.

The bottom line is that upgrading your rear gear ratio by two steps you can decrease your 1/4 of a mile times by approximately 3 tenths of a second. This is equivalent to finding 30 horsepower hidden under your hood. I think you would agree that hidden horsepower is always a good thing!!!

guys with V8 powered datsun z's tend to break everything back there.


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rear end could also mean the whole rear drivetrain/suspension

guys with V8 powered datsun z's tend to break everything back there.

some swap to an r200 differential and CV shafts which are several times stronger then beaf up the suspension

some convert to sold read axel

some use corvette c4 rear ends.

im sure its the same situation for an FC.  car was never designed for the size and shape of a powerful v8's powerband, much like the datsun z's
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holy crap, triple post
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Stock Turbo II FC drive lines will handle a 3 rotor so a  V8 is no problem. 
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-Fox-N-It2+Dec 1, 2004 - 1:30 PM

lol, i doubt a 3 rotor will make anywhere near the kind of low end torque a good v8 will make.

more likely to break something on the launch than when traveling through the powerband. that is if you get traction.
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