Jump to content

Recommended Posts

I have driven a manual for so long, I'm just not use to the higher RPM's of an automatic. I thought some of you could shed some light on an issue I'm dealing with. My '73 Mach-1 has a mild 4v Cleveland with a C-6 and what I believe to be 3.73 gears (275-40-17 rear wheels). My problem is that I tach at just over 4000RPM in the freeway which is most of my driving. For the daily drivers out there, what gears do you run? Or better yet, what is a typically accepted RPM range to "cruise" at in the freeway?

I know this must sound stupid but my previous car had a T-56 and my '68 has a TKO-600. I haven't not had an overdrive for 20+ years. Thanks a lot everyone.

Mike

Link to post
Share on other sites

I would say 3.00 to 3.25 are optimum for what your looking for. If you drive more on the freeway then I would go with the 3.00. If more in town, go with the 3.25.

 

Jeff

Jeff

1972 Q Code Convertible

 

Link to post
Share on other sites

I"m in the same boat.I don't have a tach at the moment or get on the hi-way much ,but I have a C6 w/ 2.75 gears and want to go lower.I have a 460 that offers a lot of torque at most of my rpms.It seems to high a gear ratio for me.I was going to shoot for 3.50"s ,but other big block guys said I might like 3.25:s better for all around cruising.A 351 w/ a lot of hiway driving is a bit different than me though.

1_11_01_14_4_00_47.jpeg

460 , Edelbrock Performer carb. + intake , 204 / 214 cam , Pertronix Flamethrower 2 billet dizzy + coil , Crites long tubes , C6 .

 

 

 

 

 

 

 

 

 

 

Link to post
Share on other sites

My car had 2.75's from the factory which might sound great for hwy but i had to really kick it to get any life. There no middle, only very lazy or roaring. Changed to 3.25's and its a different car. I can get more performance with less effort. Easy to get up into the 3k's where the engine comes to life. Much more fun.

Link to post
Share on other sites

There is another thread running tonight that is asking about the same question. My car has a 2.91, runs great on the highway, but it is very hard to break loose when fooling around. It's like having a car with overdrive.

 

Sorry I didn't think about this last night...

 

http://www.7173mustangs.com/thread-trans-gear-vs-rear-gear-vs-rpm-vs-mph

 

This is in the WIKI section complements of Don65Stang.

1_24_09_13_4_29_06.png
Link to post
Share on other sites

I am running stock 3.00's in an 8.8". I have a very mild 302 that I built for economy because my Mustang is my daily driver and most of it is highway driving. My C4 is new and right now if I had to guess (because I took my cheesy tach that was sitting under my dash off), I am running about 3200-3500 rpm at 80mph. My new idea is to do a coyote motor swap with a new 6 speed automatic and have a 9 inch with 3.55's in it, but then it probably wont be a daily driver any more. hope the info helps.

1_12_09_14_10_32_45.png

 

- Nik

Link to post
Share on other sites

It sounds like 3.25's are the way to go. I was thinking 3.50's initially, but I don't think it woukd be all that different than what I have now. I really miss my 5-speed..

Link to post
Share on other sites

It sounds like 3.25's are the way to go. I was thinking 3.50's initially, but I don't think it woukd be all that different than what I have now. I really miss my 5-speed..

 

I have a 3.5 in a non-daily driver. I drive it to work on the freeway about once a month. You probably wouldn't like it on a daily driver.

 

3.25 is a nice compromise.

'Mike'

73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

 

Pics of modifications included in: [button=http://www.7173mustangs.com/thread-1973-convertible--3335]My Garage[/button]

 

Link to post
Share on other sites

Oh ok. Thanks will_e.

71_resurrection, have you considered building up the 8.8? My '95 GT had a very mild 383 stroker with a Vortech S-trim. It made a very smooth 500rwhp. I had a factory 8.8" housing with forged 31-spline axles and a Torsen differential and it was great. Much easier than swapping for a 9", you already have it and you can use any of the disk brakes for an 8.8". Just a thought. The only this u woukd do differently is have the axle tubes welded (factory ones are pressed in).

Link to post
Share on other sites

I got 3.25's in mine stock.....Perfict for in town driving and the higher stall vert and 4v wanting some rpms..And still be able to hit high enough top end speeds...I have max out the speedo on my mach 1 a few times...On the freeway i was a hard to beat till i ran out of gears..lol..And good enough launch in town to roast the tires down the street at will...I have learned to love 3.25's...i would not change them out for anything..."Unless i had a more modern trans with a taller gear"

Link to post
Share on other sites

I have 3.25's in my car and it does about 3000 rpm at 70mph on the freeway. I wouldn't go any lower.

It is inexcusable for scientists to torture animals; let them make their experiments on journalists and politicians.

 

Henrik Ibsen

Link to post
Share on other sites

Well, 3.25's it is. Thanks everyone. I appreciate you info. Now time to change the intake. The guy I bought the car from put 4v heads, an Edelbrock Victor intake and a fairly mails cam. I'm thinking an Air Gap intake should give be better low end response with the higher gears.

Link to post
Share on other sites

There's a gear/rpm/speed calculator at ring-pinion dot com.

You didn't say what speeds you run on the fwy but if you're over 4K rpm, you're

either normally running 80mph or you don't have 3.73 gears (maybe 3.92?).

If you're normally over 80mph, I would recommend 3.00 gears instead.

I have 3.00 in my 71 with a 351c and about the same diameter tire. It still smoked the tires fine (when the c6 was there), but you're only running 2830rpm at 70mph and 3234 at 80.

(with 3.25 you'll be 3066rpm at 70 and 3504rpm at 80, still pretty high rpm at 80).

1971 Mach1

351C-4v

C6 is history-->>now TKO-500

Link to post
Share on other sites

My car came with a 3.25 standard. A previous owner did a lot of highway driving so

traded for a 2.75 posi. This has a toploader and on a recent trip to Yosemite I got

20 mpg and that is loaded down with gear from sea level to above 7000 feet.

And the 2.75 still has plenty of pickup around town. Average rpm at 70 mph is

around 2700 with 14" tires.

 

mike

spacer.png

Link to post
Share on other sites

I'll also say that the 3.25 rear in my car is a great around town & interstate use gear for a stock trans. I think it is perfect in my car. The 3.50 would be fine with infrequent interstate use or a 3.00 or even 2.75 with mostly interstate duty.

spacer.png

Link to post
Share on other sites

To be honest, I really don't know how fast I drive. I typically pretty much go with traffic unless I'm passing. But I'm thinking 3.25:1 is the way to go for my use, especially when I toss my single plane and pick up an Air Gap intake.

Link to post
Share on other sites

To be honest, I really don't know how fast I drive. I typically pretty much go with traffic unless I'm passing. But I'm thinking 3.25:1 is the way to go for my use, especially when I toss my single plane and pick up an Air Gap intake.

 

If you follow traffic in The Bay area that is about 75.

 

mike

spacer.png

Link to post
Share on other sites

My car came with 3.25s, and with the 235/60/14 tires, it seemed to tach out about as much as it does with the current 3.70s and 275/60/15 tires. I will likely keep the 3.25 third around in case I decide to take a long trip with the car. The 3.70s are okay for quick freeway jaunts, but it is revving above 3,000 at freeway speed (60ish).

1973 Mach 1 Q code 351 4V, 9A paint, standard interior, 3.50 rear, C6 trans.

Link to post
Share on other sites

Sure, I'd be up for that. Let me figure out what I do have first. I'm actually getting ready to start the police academy so if I can't get this done in the next week or two, it may be an extended project. I don't want you to think I'm flaking. PM me if you like and we can discuss it further if you like. If you're not in a rush, you can just have mine. I have a one-legger now but am thinking of a truetrac as well.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...