The fastback

Hello everyone, i am in the process of rebuilding my 1965 mustang fastback (this is my first build) I have a 351w with a tremec 5 speed tranny, And i have no idea what gear ratio to put in my 9". It is a restomod mainly used to cruise but still want to be able to run with power. Any help you guys could give would be greatly appreciated.

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Couper

If you're going to narrow the 9", look for an F100 or F150 that has what you think you want, including the ratio, then have fun with that,...see what you like or don't like about that ratio.

Many F150's came with 9", 31 spline, trac lock, between 3 ish to 4;11 gears.

The fastback

I have a 9" already, I believe it came out of a Mach 1, I just dont know much about rear ends. Id like something i could use to cruise and still be able to hit the track

03BottleFedGT

3.55 and 3.73 are good choices. still have good street manners and can still get up and go when needed.

69FB

I can vouch for the 3.73 since that is what's in my brother's 07 Shelby GT/SC. He's running a 5 speed and it has good cruising manners and pep. We just did the "Long Haul" for the Hot Rod Power Tour this year in it and it was great.

MustangBradley

I've got a Tremec and a 3.50 rear gear. It runs great on the highway, and around town, it runs at about 1200rpms at 50mph in 5th. If I had to do it over again, I think I'd get the 3.73s. The Tremec has a shallow 5th, so you can go a little more aggressive with the rear gear. Resist the urge to get a 4.11 if you aren't a street racer or strip racer.

Jonk67

You can get a good idea of RPM's at any speed in 5th (or any other gear if you know the ratios in your T5) with a gear calc.:
http://www.ringpinion.com/Calc_RPM.aspx

First figure out your tire ht. with the bottom calc. then plug that in with your 5th ratio and speeds you intend to drive at in OD to see where it puts your RPM. I shot for having my OD @70-80mph to be ~2500 as that is what I turn in my DD modern cars. Stepping from 3.50:1 to 3.73 won't make a huge diff at that speed but will help your off the line. I made a spreadsheet with columns for each variable to see what changing any of them did to the RPM's, that way I could do a good comparison with my known variables (tire ht., OD ratio).
Jon

69mach1377

Keep in mind it probably wider than a 65 rear end, so mind you backspacing on your wheels...

kalli

i run 3.5 with 5speed gearbox. couldn't be happier. 3.73 could be an option and probably great fun, but that would make my first gear a complete joke :-)
i'd say 3.5 or 3.7 where 3.5 is sensible and fun, and 3.73 the nonsense fun version
below 3.5 and above 3.8 I'd say is just wrong.
all of that assuming you have stock-size tyres in diameter. 225/60R15 or any other wheel with that outside diameter ~25.6 inch

check for your rear tyre size or just post (225/60R15etc) and come back to us
http://www.miata.net/garage/tirecalc.html

oh and post the gear ratios of that tremec. or just go with what jonk said: http://www.ringpinion.com/Calc_RPM.aspx

Kalli

boogerschnot

I would go with kali on this one. Ive got 3.70s in my nine inch with an aod.. First gear with a five speed would be very quick. With the aod i can just punch it and let the car shift but a manual would be a pain in my opinion.