Which rearend gear do you think???

04mustang70 · Mar 1, 2009

I have a 70 Mach 1 with a fmx trans, 351w with holley 600 aluminum intake, headers all mild hop up stuff. Presently have 2.90 rear gears in it. Im going to be buying a posi and gears. What would you suggest for a fun weekend cruiser? Want to be able to feel the difference. My gut says 3.70's but would 3.50's be better? What are your experiences? My engine is probably 300 horses. Thanks

66Runt · Mar 1, 2009

This ought to bring out a lot of opinions!
My thoughts are that if I had this car (a dream come true ) I'd either run the 3.70 gears, or 3.50s with a 2500-2800 stall converter... You know the more I think about it, the more I'd run the 3.70s and a high stall converter.
the 3.70s will bring the rpms up a bit high at freeway speeds, but would trap better in the quarter.
Truth be told, I'd convert to a manual transmission, and PJ that pony!

Good luck which ever way you go.

D.Hearne · Mar 1, 2009

Go with the 3.50's

DK832 · Mar 2, 2009

2nd that if you drive highways often. I have 3.70 but a very economical cam at part load + a quiet exhaust so its still fairly OK at 80.

04mustang70 · Mar 2, 2009

Thanks for your opinions I don't do any highway driving so Im still leaning towards the 3.70's. Is there really any big difference between the two gears?

iskwezm · Mar 2, 2009

i say 3.70s. there might be maybe a couple hundred rpm difference. I have 3.80's and so does my buddy and he drives the freeway once in a while, probably at about 65-70 MPH

latamud · Mar 2, 2009

I had 3.50's for about 5 years before swapping in 3.70s. I didn't get to drive it right away and sort of forgot about it. A week later I jumped on the highway and it was like a different car. I say either way you go you'll be happy. When I choose a rear gear I take into account my transmission gears, purpose of the car, rpms I want to hit, speeds I want to sustain and rear tire diameter. There are "mph" and "rpm" calculators out there that tie the math together to see where you stand.

zookeeper · Mar 2, 2009

What can you live with, RPM-wise? Here's a neat chart I found a while back, plug in your numbers and it'll tell you what RPM your motor will be turning. Then just make a decision based on how and where you do most of your driving.

Engine RPM Calculator

latamud · Mar 2, 2009

Here's the one i use
WHT Z06: Gear Ratio calculator
I like to get above 1500 rpm at 80 mph.

bnickel · Mar 3, 2009

personally i wouldn't go over a 3.50 without an overdrive. BTW, gear vendors just came out with a kit for the FMX....just FYI. i have a vintage hone-o-drive unit coming that i'll be using in my 69 cougar with an FMX. if that doesn't work i also have an AOD core out in the shed that i could build. my stang currently has 3.25's and i really like ratio a lot, it's a good all around gear for a non-OD car and won't buzz your brains out on the highway. with the hone OD unit i'll probably go with a 3.70 gear, if i end up using the AOD i'll stay with the 3.25 gears and use the wide ratio gearset for the AOD to get a better ratio out of the hole

blown65 · Mar 3, 2009

If your not doing highway driving then I'd do the 370's or even 390's. There is no point in going with an OD or a gear vendors box unless your doing freeway 70-80mph IMO.

ForceFed70 · Mar 3, 2009

I used to have the same combo 351W FMX and was running 3.73's.

I didn't do a lot of highway driving, but I hated it when I did have to. I wouldn't make any long trips because of it. It sucks to only drive your car around town.

If I could go back in time, I'd tell myself to go with the 3.50's.

jspagna1 · Mar 3, 2009

I have a C4 in my car and I am running 3.55's. If you had an AOD you could go 3.70's, but I would'nt. Just MO.

A66Auto · Mar 3, 2009

4 speed with 3.55 here. They are fun on the street. I have mangaflows and they do get kinda noisy on the freeway after a few mins. The gears are not that far apart, so you should be happy with either ratio after swapping out the 2.9 gears.

blown65 · Mar 4, 2009

jspagna1 said:
I have a C4 in my car and I am running 3.55's. If you had an AOD you could go 3.70's, but I would'nt. Just MO.

You can go much more gear with an AOD than that. Our 67 has 4.56 gears with an AOD and cruises at 60 around 2500 or so.

zookeeper · Mar 4, 2009

I had 3.89 gears in my '65 and at 60 mph, it was about 3200 rpm. I live in a rural area with several miles between towns and hated freeway driving because of those gears. Personally I would stick with 3.5 or so with a non-od trans and no lower than 3.89 with an od trans, you'll be a lot happier in the long run.

latamud · Mar 5, 2009

blown65 said:
You can go much more gear with an AOD than that. Our 67 has 4.56 gears with an AOD and cruises at 60 around 2500 or so.

Tire size makes a difference here. What are you running a 28" tall tire?

blown65 · Mar 5, 2009

latamud said:
Tire size makes a difference here. What are you running a 28" tall tire?

its about 26 inches.

Ford AOD 4-spd automatic

2.40, 1.47, 1.00, 0.67

.67 OD at 60mph with a 26" tire is right about 2500 rpm with 6% figured in for converter slip.

28" tall would be about 2300 with 6% slip figured in. Could have more slip but still its close for most purposes.

For comparison, my 65 when it had a 3500 stall converter at 60mph with 3.89's turned right about 3200rpm. Thats with 6% slip figured which is pretty much dead on to what I saw. With my 4.56 gears now though, I see 3800 at 60.

http://www.tciauto.com/Products/TechInfo/Calculators.asp#6

blown65 · Mar 5, 2009

From a 3.50@60 26" is 2878
3.70@60 26" is 3042

So you all are telling me that 170 rpm is a deal breaker?

jcp123 · Mar 7, 2009

If you were doing a DD like mine or just a nice little cruiser, I'd stick with the 2,90's or 3,00's like mine. For a weekend car as you said, go 3,70's!

Which rearend gear do you think???

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