New Axle ratio?

[RoadRacer]

I have a 3.27 8.8 LSD rear from a 2002 Mustang GT (junkyard edition), and wanted to ask what people think about changing the ratio?

I autocross once a month, but are likely to circuit race too next year (not race race, just do more open track days). As such I've been thinking about acceleration. I've heard people say my car is low revs doing autocross in 2nd. (Of course they may not be used to large v8's with redline at 5500)

I'm doing about 2000rpm at 80mph in 5th, ~2500 at 100mph, so I feel I have room to sacrifice some of that while getting faster acceleration. I've never done this, so are there any good rules of thumb for ratios?

3.55 is next one up, is that as simple as calculating (2000/3.27)*3.55 = 2171 to find my new revs at 80? That doesn't seem like a big change.

3.73? Would be 2281

Even 4.10 is only 2500rpm at 80 (if my math is right)

Obviously want to balance driveability (my daily driver, literally) with increased performance.

[Jim1855]

Engine matters.
I ran 3.73 for about 2 years switched to 3.27s and never looked back. Old TKO-600 w/ .63 OD.
Useable 1st gear, 2nd ran hard. No Auto-X but ran 3rd & 4th on road tracks to 140.
427w at about 540 hp, 440 at the wheels, Mustang dyno.
Choose 3.31, Torsen T2 for this build. Same motor, lighter car.
Check out Scuzzy's post on trans gearing/speed. His calculator is very good.
Jim

[RoadRacer]

Thanks Jim, no idea about my SBC internals, bought it secondhand, but it seems fairly mild, ~300hp. Looking for some upgrades that will help until I can get the dream 383.

[JimLev]

What is the diameter of your rear tires?
I saw this formula a while ago.

MPH x diff ratio x 336
———————————- = Engine Speed
Tire Diameter

[J R Jones]

What is the diameter of your rear tires?
I saw this formula a while ago.

MPH x diff ratio x 336
———————————- = Engine Speed
Tire Diameter

This equation may work with 1:1 transmission gear but does not account for overdrive which may be 5th and 6th.
jim

[Bob Cowan]

I've used 4.10, 3.55, and 3.27. Of the three, the 3.55 was the best compromise for street and track. I was planning to try 3.83 next, but that didn't work out. I would certainly go 3.55 for street use, and would certainly consider 3.83 for autocross or open track.

One thing to keep in mind is the huge difference between 4th and 5th gear. It's about 1,000 rpm's. With a lower gear like a 3.27, I often found I could not find the right cruising gear while on the back roads. On the track I only used 5th gear on the cool down lap. even with 500+HP, it was like throwing out a parachute.

[J R Jones]

I see the priority to have a comfortable RPM at highway speed, and that makes 5 and 6 speeds advantageous.
You might consider what maximum speed you intend to achieve? What is the advantage of gearing for 180-200 MPH on the street? Do you think you will go that fast on the track? Maybe, it ain't for the faint of heart.
We raced SBF in SCCA Nationals and the tracks had differing top speeds. To take advantage of our RPM range we geared to see ~7000RPM in 4th gear (high) just before the braking point on the high speed straight.
At the extremes we ran a 4.56:1 for 100-110 MPH to 3.5:1 for 165 MPH. Once rolling 1st was never used. Obviously one has to adjust driving style when the acceleration off corners is explosive.
jim

[NAZ]

Determine what your primary goal is for this car and adjust everything to maximize performance around that goal. If this is primarily a street car that gets driven on the highway 80% plus and your goal is comfortable cruising, gear the car to meet that goal. If you mostly drive city streets and seldom hit the freeway, you can get away with a numerically higher ratio and take advantage of the quick acceleration that gives you. If autocross is more important than comfortable cruising, gear it so that you can carry second gear (or whatever gear you decide) from the tightest turns to the fastest turns so you never have to shift but once during a run. If you want to maximize 1/4-mile performance, gear so you are on or near the rev limiter at the second trap lights. And that's where I geared mine as my primary purpose for my hot rod is drag racing and street driving is a secondary activity.

I use modeling software to establish theoretical BEST gear ratios and shift points. And the first step is to use a speed at RPM calculator. I developed a calculator that gives me not only RPM at speed but torque to the axles and uses torque converter multiplication and slip variables to calculate torque & terminal RPM. I run a 28" tire on the track (which is 29" at 150 MPH) and a 29" radial tire on the street with a 3.90:1 rear gear and 1:1 high gear. The engine runs 3600 RPM at 75 MPH on the freeway and 8000 at 166 MPH terminal speed at the track. At 7500 RPM I can run 1st gear (1.80:1 ratio) out to 84 MPH. Great for maximizing performance at the stoplight, horrible for cruising the highways and almost tolerable on city streets. It's the quickest street legal car in Flagstaff but you can't stand more than about 20-minutes and you need a break. Obviously, comfortable cruising is not a priority to me when it comes to this car.

So if you really want your hot rod to be civil on the road, gear it somewhere closer to 3.55:1 with a ~25" tire or if you primarily run the city streets, you may be OK with 3.73:1 ratio but you better be willing to make some compromises if going with a shorter gear as even with overdrive, the RPMs will likely be higher than your daily driver unless it's a 1968 F250 Camper Special with 4.56:1 gears.

[bobl]

If 5500 rpm is your red line you won't be able to run as low a gear as would make it optimal for autocross. You want to be able to stay in second. Figure out a ball park top speed for most autocrosses you'll be running and gear for that. I run a 4.10:1 and love it, but have a 7000 RPM red line. You might be able to run a 3.73:1. That would get you to around 60 mph in second depending on the trans gear ratio. As far as hwy driving it really depends on where you drive. I'm always on back roads, so very seldom run over 70. I found the 4.10 to make for a lot better cruising. I can leave it in 5th at anything over 40 mph. FYI, switching from 3.55 to 4.10 reduced my fuel mileage by 1 maybe 2 mpg at the most.

Bob

[RoadRacer]

Thanks for all the guidance..

Yeah, I autocross in 2nd, and as I said 80mph is 2000 - I have 27.4" tall tires (355/30r19).

Given that starting point, and the fact I would be happy even at 2500 @80mph, I'll likely go for 4.10, and get some extra pep in my 2nd gear autocross, while not throwing the baby out with the bathwater so to speak. I drive ~1,000miles/month. most of it at 80-90.

[RoadRacer]

I did some calculations.. using Tremec's great calculator: https://www.tremec.com/calculadora.php

and.. once I plugged in my numbers (TKO600.64, 5000rpm, 3.27 gears, 27.4 tires), yes 5th gear would be fine, even better, but I'd be completely screwed for autox! I'd literally run out of second gear.

Today, second gear gets me to 66mph, which is a pretty good speed for autox. With 4.10 I'd accelerate faster.. but second would only get me to 53mph. Not enough.

After speaking with some racing folks, they think I have perfect gears, but just not enough power LOL.. good answer! But seriously, they've persuaded me not to change gears, because I'll be screwed for autox if I do, instead I should save that money for engine mods, like new aluminum heads, full roller, etc.

It was interesting, and I'd have been screwed without that calculator.

[NAZ]

Whatever works best is what we do.

But, what about 3rd? I ran an autocross course once with a NASCAR Featherlite Tour car set-up for 1/2 mile bull rings and 2nd was way too short but 3rd was perfect. Started in 2nd (1st was like an old truck granny gear) and quickly bang 3rd and never shifted again. I'd get it on the rev limiter on the one sweeper they had set-up.

[RoadRacer]

Well, sure, but then that starts affecting it being roadworthy . I'd like to have 5 usable gears!

Everything is a compromise...

[sread]

Put a 3.55 in it and spin it a little higher - 5000 rpm is nothing - even if it has a stock cam should be able to turn 5700 in it - that would give you 70 mph in 2nd and a 2300 cruise at over 80 in 5th. Should put you right in the meat of the power curve coming out of corners in 2nd gear...upgrade the cam and valve train and it opens up even more possibilities.

[glastron351]

Here's my 2 cents (for what its worth). I have a 408w with 26.6" tires and 3.55 gears. Trans is a TKO600 with the tal 5th gear (.64 I think). Currently in 5th gear I am about 1700-1800 rpm which gives me a highway speed of about 60-65 mph (62 is the limit in Canada). I find the car lugs a bit and it doesn't really come into the bottom of the power curve till about 2100-2200rpm which puts me at about 72-75 mpg. Usually we can get away with about 6-10moh over the speed limit without too much hassle from enforcement so I am always running that line. Plan on trying some 3.73's next year which should bring my rpm up about 300 revs and allow me to cruise a little more comfortably at the start of the power curve...Seems minor but 85% of my driving is on the highway so to me its all about the little adjustments to make the car as comfortable as possible....

Again...just my experience for what its worth...good luck!