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
Which rearend gear do you think???
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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.
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.
DK832
New Member
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.
iskwezm
10 Year Member
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
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
Founding Member
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
Engine RPM Calculator
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
Founding Member
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.
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.
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
Member
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.
blown65
Founding Member
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
Founding Member
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.
blown65
Founding Member
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
Founding Member
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?
3.70@60 26" is 3042
So you all are telling me that 170 rpm is a deal breaker?
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