This is a very long post about changing rear end gear ratios.

We get a lot of questions about that on the forum and I thought this could save a lot of people, or at least me, time by just putting it all down once.

Below, I start out with the basics, then go through performance gaps and other considerations, and go through some recommendations for different situations.


Basics: A Review of Things Most People Know About Changing Gear Ratio[/b]

A numerically higher rear end ratio (say 3.73:1 instead of 3.42:1) provides more torque multiplication in every gear. For example, late C4s through C6s with M6 transmissions came from the factory with a 3.42 rear end ratio. Changing to a 3.73 in these cars will mean 3.73/3.42 = 1.096 times as much torque multiplication to the rear wheels in every gear – a gain of right at 10%.
In first and second gear, the car will feel like it suddenly gained 30-40 HP (although in truth it gained nothing). At the drag strip it will launch quicker. Around town it will feel much peppier.
There are downsides to this change: at a steady 70 mph cruise in 6th, RPM will increase from about 1750 RPM to around 1950 – it’s a noticeable increase in noise and can be tiring on a long trip. Fuel economy drops about a mile per gallon.
Still, this particular gear change (3.42 to a 3.73) makes sense to performance enthusiasts: the downsides (and the cost of the new parts and labor) are minor compared noticeable increase in “useable power” and a quicker launch at the track

Changes in Gear Speeds and Top Speed[/b]

Sticking with that 3.42 to 3.73 swap for a moment, the top speed in each gear drops because of the change in rear end ratio. The table below is one we’ll use a lot in the next few pages (yeah . . . this is that long). The table below lists the top speed (at the red line) of the car in each of its six gears.

Completely Stock ’02 Corvette ZO6 with M6 and 3.42 rear axle ratio.
6500 RPM red line:
Speeds (mph) in each gear are –
49, 70, 102, 145, 174 (205), 140 (255)

Here, I list the actual speeds the car reach in each gear. For lower gears (1st -4th) this is at engine RPM limit, but in 5th and 6th gears the car does not have enough power to pull to the red line, so I list the speed it would reach if it could in parenthesis. A C5 needs about 480 RWHP (about 540 flywheel-rated HP) to hit 205, and about 875 RWHP to hit 255.

Incidently, the transmission gear ratios installed in various M6 Corvettes over the past 15 years have varying by up to 15% depending on year and model, as do the tire sizes by a smaller amount. I am using the values for my ’02 ZO6 when it was stock – so these speeds-in-gear may vary a bit from those for other model Corvettes. But all the conclusions I reach pretty much apply not just to all M6 Corvettes, but all Corvettes, automatic and manual, and just about any car with a transmission and rear end, for that matter.[/i]

But back to the comparisons. Look at this same stock ’02 ZO6 with a 3.73 gear:

Otherwise stock ’02 Corvette Coupe with M6 and 3.73 axle ratio:
6500 RPM red line
Speeds (mph) in each gear at the red line are –
44, 64, 93, 133, 175* (187), 150* (233)

The top speed in each of the lower gears drops by the ratio 3.42/3.73 (about 10%). The asterisks indicate that I’ve estimated the speeds in 5th and 6th gear based on some engineering approximations. I see no significant change in real top speed (reached as in the stock car in 5th), but a gain[/i] in 6th due to the better mechanical advantage.
[/b]
Performance Gaps [/b]
[/b]
Maybe these changes in the top speed in each gear seem insignificant – “There’s no change in top speed -- so you shift at a lower speed -- big deal.” But there is a downside: while the 3.73 car has more pep and torque at low speeds, there are speed ranges where it will have significantly[/i] less power available.
One of these is from 64 to 70 mph. With a 3.73 gear installed, the driver has to shift into 3rd at 64, while the stock 3.42 car can stay in second until 70. Even when one takes the greater torque multiplication of the 3.73 ratio into account, in this speed range the stock (3.42) car has between one quarter and one third more power. It’s in second gear and right at the top of its power band, compared to the 3.73 car: than in third and at an RPM far below it.
Of course, from 70 mph on up, both cars will have shifted into 3rd and the 3.73 car will once again have its 9.6% advantage. But there is another gap from 90 to 102 (the 3.73 car has to shift to 4th, the stock car is still in 3rd), and another from 133 to 145 (5th vs 4th). And technically speaking, yes, there is a gap at the 1st-2nd gear shift point (from 44-49 mph), but I can attest from personal experience that you’ll never notice it – both cars are essentially traction-limited in first and it just doesn’t matter.
Whether these gaps matters to you or not depends on your driving. If you do a lot of passing in the 55-70 mph range (say on two-lane back roads) the 3.73 car will give up something you will[/i] notice in day-to-day driving: in most highway passing situations it won’t be worth downshifting into second to pass. Of course, it’s a ‘vette, so it will still be plenty fact. But the 3.42 car, on the other hand, just owns the road in these speed ranges.
But the real point is that a change in gear ratio isn’t without some complicated downsides: the 3.73 car has no more HP than the 3.42, just an ability to apply its power better at some speeds, and less well at others. A car owner has to decide if the change – what he or she has to give up and what they gain, is worth it.
If you average this particular gear change’s advantages and disadvantages over all speed ranges, it actually nets out to zero: no surprise because there is no net increase in power, just a shift in where it can be applied better (and worse). But because most owners drive more much more often at lower speeds than higher, and because the 3.73 ratio can launch a stock car better at the strip and thus turn in lower ETs, this is a very popular mod.

Going Too Far[/b]
[/b]
If changing from a 3.42 to a 3.73 is good, how about going further? Rear end ratios are available only in certain combinations, but there are numerous choices. Factory ratioa used on C4 through C6 models include 2.56, 273, 3.15 and 3.42. They are all interchangeable. Aftermarket ratios like 3.92 and 4.10 are available for C4, C5, and C6 cars. Traditionally, even tighter ratios like 4.56, 4.88, and 5.12 were available for C3s, and I assume those could be obtained for C4 through C6 models, too.

But one can go too far. Here is a clear example of going way too far:

Otherwise Stock ’02 Corvette ZO6 with M6 and the modified rear axle ratio of 4.88:1:
6500 RPM red line
Speeds (mph) in each gear at the red line are –
34, 49, 71, 101, 143, 174 (178)

A 4.88 rear end increases torque applied at the rear wheels in each gear not by 9.6% as the 3.73 did, by a whooping 42%. However, it reduces speeds in gears so much that it brings the whole transmi

 

Is there anyway to tell if this has already been done to your car?

 

You can figure out what RPM your car should be at a steady, say, 60 mph and check the tach. An '02 ZO6 should be at about 1750 at 70 in 6th with the stock ratio, about 1925 RPM with the 3.73. A standard C5 has different transmission ratios and would be a bit different RPM at speeds - somewhere I have the ratios and will look it up.

 

Man,.......did I just get an education!!!
Thanks Lee!!! [sm=hail.gif]
That's goin' in my "hard copy" file!!!

 

Thanks Lee! Looking foward to seeing the c5 ratios. In the meantime I will try and look it up myself.

 

According to the numbers I have, a standard C5 transmission, rear end, and tires ought to do:
3rd gear - 2000 RPM = 34 mph
4th gear 2000 rpm = 43.7 mph, 2750 = 60 mph
5th gear 2030 RPM = 60 mph

Of course, you need to make certain your speedo is correct. But if you get these revs at these speeds then you have the stock ratio. If you get about 10% higher, you have the 3.73.

 

Thatwas a great explanation, thanks for taking the time to share that with us. PG.