Best Diff Gear for Road Course in a 5.0?

[DPE]

(Some may berate me for cross posting from MF, but in a week I didn't get much response so I figured I'd try over here.)

So I tracked my 2010 GT a number of times, and it had the 3.73 which I would almost consider mandatory given that motor doesn't have tons of torque down low and you've only got 5 gears to play with. However, I'm looking at buying a plain 5.0, perhaps at the end of this year, and am curious what folks think of the gearing on track.

From looking at the gear/mph splits, it sure looks like a 3.73 would mean a whole lot of shifting and quite a bit of 'throttle management' in the lower gears to keep from lighting the tires. I won't be on R-comps; probably tweeners like NT-05s or just plain summer tires. I know lower is typically equated with faster, and it may well be, but it kinda seemed like a 3.55 would give the car a little more range in each gear and on the tracks I run reduce the number of shifts by a few per lap. I'm not afraid of shifting or having to manage the throttle, don't get me wrong, it just seems like a 3.73 might be overkill for a car with that much engine and not exactly an overabundance of traction. I dare say a 3.31 may even be just fine, as that retains essentially the same mph at redline in each gear through the first two gears and then goes slightly lower than my 2010 GT did in 3rd and 4th. And that car worked fine with 100hp less....

So for those who have tracked their 5.0s, anyone on anything other than a 3.73 have comments? Any 3.73 guys feel like it's perhaps a bit too low? Anyone think I'd be a dumbass to get anything other than a 3.73?

The car will be used on track several times per year, but will also go on long road trips etc. All purpose car, not a track toy. But I'm more interested in track impressions right now, just to get a feel for it.

 

[fun4me]

The 3.73 have upgraded discs. For the steet, you basically shift in to 2nd immediatly in city driving. For the track, you stay in the power band. The my82 is geared differently, so the 3.73 will feel completely different from your 2010. I'm sure someone will calculate the equivelent ....
Another thing you have to calculate is the tire size, the stock tires range from
27-27.7", most sticky tires are under 26", that will make the gears even taller.

For a DD, I'm glad I went with 3.31.

 

[Norm Peterson]

There is a lot of unknowns for anybody not running on those tracks or with a 5.0 car. But if you get your axle gearing at least to the point where you aren't making upshifts just before your braking zone (which will be longer assuming that the extra power bumps your peak straightaway speeds), the number of shifts per lap goes back to being how favorably the transmission gears are spaced. Closer spacing means better use of the powerband but does occasionally require an additional shift.

It is possible to estimate speeds (and hence gearing) based on the longer or longest straights, corner speeds, and what gear you want or need to be in numerically. That's one approach. It is not possible to do so in the sort of time frame suggested by the length of time it takes to read a forum thread reply, and would require some specific track data.

I still think I'd err on the side of the numerically smaller of any two ratios that you might find yourself choosing from.


Norm

 

[DTL]

3.73s are fun, but they're a lot of work. With the MT-82 and a 26.5" sticky tire, you're doing a LOT of shifting on most road courses around here. On the street, with the stock height tire, I usually shift 1-3-5, just to keep my arm from falling off from fatigue! I've been considering trying some 3.55s that I have laying around, but until I start making a bit more power, the 3.73s will be fine.

 

[DPE]

Thanks DTL; that's what I was thinking may be the case. In response to the other posts, I have it all laid out in a spreadsheet as far as mph at redline in a given gear with my preferred tire size etc. (for each of the three axle ratios). I'm not looking to optimize gearing for certain road courses, as I have no need to set lap records while doing track days in my street car. Rather, I was curious as to whether the 3.73 was overkill to keep an engine in the 'powerband', when said engine has over 400hp and a rather broad torque curve. It sounds like that may be true, at least to one person .

I had an '06 Subaru STI awhile back, and that car had absurdly low gearing; felt like it was geared great for a VTEC engine perhaps, but not for a relatively large turbo four with a broad powerband. I was shifting constantly on most any track, and most corners were taken a gear higher than what I consider 'normal'. Also turned more rpm than necessary at 80mph on the highway. Rather avoid repeating that situation.

 

[DTL]

If my Mustang was a Daily-Driver, I'd definately be an advocate of 3.31s or something similar. With the 3.73s it's a workout to drive, turns a ton of RPM on the freeway and gets crap fuel mileage. When you stand on the throttle, however, they're badass. One small tip I might pass on is this: On a road course, (if you're on street tires) don't get too excited with the throttle at corner exit in any gear lower than 4th. I was intentionally NOT downshifting to 3rd at a recent NASA event in a couple of corners due to the wheelspin on the way out. With the 3.73s, even in 4th gear the motor makes enough torque/power from 3500 up to pull you out of the corner. It makes it easier to get back after the power as well. I got snap oversteer a couple times in 3rd gear, right in the power band and the car picked up a consistant .7 sec on a 1.20 lap leaving it in 4th. Just something to try.

 

[SoundGuyDave]

With the 3.73s, even in 4th gear the motor makes enough torque/power from 3500 up to pull you out of the corner. It makes it easier to get back after the power as well. I got snap oversteer a couple times in 3rd gear, right in the power band and the car picked up a consistant .7 sec on a 1.20 lap leaving it in 4th. Just something to try.

Excellent advice right there! Any corner where you REALLY have to feather the throttle at exit is a good candidate for leaving it up a gear and seeing what that does to your exit speed, particularly if you're near redline before you're absolutely straight at exit.

My read on the MT82 ratios vs. the 3650 leads me to believe that you should really treat the MT82 as a 5-speed, with an additional granny gear at the bottom. More than once I've caught myself calling the wrong gear to a student... Okay, drop to third... NO, I MEAN FOURTH!!!

 

[DPE]

Sounds like good advice here. Thanks. Of note, with a 3.31 rear end on a 5.0, the mph at redline (7000rpm in the 5.0, 6500rpm in the 4.6) is similar to a 2010 GT with a 3.73 rear end in first and second, and gets progressively lower in 3rd-4th-5th (and then is quite a lot taller in top gear). I didn't take issue with my 2010s gearing, and in fact found it to be just about right in most situations on track or on the road. Maybe just a bit much rpm at 80mph on the highway. Given the 5.0 has 100hp more and a much broader torque curve, it seems like the car should be quite happy with a 3.31. I'd consider a 3.55 as well, but for the intended use it sounds like a 3.73 would indeed be overkill. Fun I'm certain, but a little too low for me.

 

[barbaro]

Since the 5.0 has variable cam timing and a flat torque curve you will be in the power band most of the time with 3.73's but you will shift more no doubt about it. But I tend to agree with you that for a road course 3.55 would be better. I have though about it myself.

 

[Vorshlag-Fair]

I own two 5.0 GT 6-spd Mustangs (2011 with 3.31 and 2013 with 3.55), both track cars, plus I have driven several customer's S197 5.0 with 3.73s. I ordered the 2011 GT with the 3.31s after much calculation and it was the right call for both autocross and track. The 2013 GT was purchased as a base GT I just happened to find cheap on a Ford dealer's lot, and it still has the 3.55 gears, for now.

As fun4me noted above, the gearing in the Getrag MT82 is completely different than the typical Tremec T56/TR6060 of the 5-spd used in the 2005-2010 GTs, so the "right" rear gear raito needed is also different - yet Ford stuck with the same three old S197 ratios... 3.31, 3.55 and 3.73. Weird.

Anyway, if you want to autocross in a class where you can only use the factory ratios (FS, STX, ESP) you MUST pick the 3.31 (or swap to it later), as this allows 2nd gear to really stretch out there to 70 mph and beyond, avoiding 3rd gear shifts on 99% of the courses you will likely see. And for a road course it is still rare for me to use 2nd gear (out of the 8 courses we've raced this car on only 3 have ever needed 2nd), so there is just a lot less shifting. We tend to use 3rd and 4th or 3rd-5th gears on road courses and 2nd gear only in autocrossing.



In my 2013 GT with 3.55's (above left) I'm already noticing I'm shifting a lot more and using 5th more often, and almost never use 2nd gear. And on the 3.73 cars I've driven (above right, 2 weeks ago), yikes! Yea you would NEVER use 2nd gear and I was in 5th a LOT more often. So it effectively becomes a 3 speed gearbox with 3.73s.

I think even drag racers are shunning the 3.73 ratio? On the street it is a bit of a mess. Again - I strongly recommend the 3.31, if you have a choice. Otherwise... well at least FRPP 8.8" rear gear sets are cheap.

Cheers,