4.10 gears on a mildly built 318?

[zerfetzen]

Until I read in Paul Herd's Challenger & Barracuda Restoration Guide (p. 92) that '67 and '68 barracudas with 225 ci engines had stock gear ratio's up to 3.91 with their 7 1/4 axles, I thought it sounded crazy to put 4.10's in my '73 Cuda 318, and thought I'd have to settle for 3.55's, fearing to over-gear it.

A couple years in the future or so, however, I want to put a performance 440 in it and have 4.10's, so I don't want to buy 3.55's now, and 4.10's then, if I can just buy the 4.10's and save myself a couple hundred bucks.

For the mean time though, I drive it every day, and I do have to get on the highway, but only for a short time like a mile, and I always drive it slow in the right hand lane (after all the money I've put in it, I just fear a highspeed accident).

All I can say is I despise 2.76's, and think of them as grandma gears.  Is it possible (or uh, reasonable) to put 4.10's on this car?  It sounds like it has good torque for a 318, but I haven't dyno'd it yet.  What do you think?  Thanks.

PS
The 904 transmission has a low-end gear set on it.

[Carlwalski]

IMO that may be a "little" too much gear, you should drop a few cogs and buy a 3:91 crown & pinion.
What tyre size are you running in the rear? 4:10's do sound very yummy though. lol 

[zerfetzen]

At the moment, Hoosier 275/50/15's, which measure about 26 inches tall.  But I'd like to try 60's, at about 28" tall.

[Grancoupe]

I don't think its a matter of engine size that could be an issue. You need to keep the revs down. A taller tire will keep you from ending up in a rev range you may not want to be in. 275/60/15 should should keep you covered. Look at the bright side, you can always drop an extra 2500 for an over drive unit if you think its too high.

[ShelbyDogg]

It all depends what you do with your car.  If you street race (which is illegal I KNOW) put in the 4.10s.  I had 4.56s with my 440 in the 80s. for you know what. Gas was cheaper.  Then I'd pop in a 3.23 for cruises to the Mopar nats. every year. I had spare center sections for each gear.   Now in 2000, I sold the 3.91s and 4.56s and settled with a 3.55 for my best all around gear, and put in the 833 O/D for my trips to the Nats. Buy a whole spare 8-3/4 pumkin with your new gears.
 I decided I like driving 80 on the highway all the time and listening to my Sirius radio over my engine wearing out.

3000 miles planned for the Power Tour this year.
Rob

[Carlwalski]

At the moment, Hoosier 275/50/15's, which measure about 26 inches tall.  But I'd like to try 60's, at about 28" tall.

With 4:10 gears and you current 26" tyres you would be killing your engine with even 1-5 miles of highway driving.
You would be running at 3440rpm @ 65mph and forget about cruising at 80mph which equals 4240rpm.

With 3:91 gears and 28" tyres 65mph @ 3050rpm would be your best.
Which is a tad over a 400rpm drop, very good IMO while still getting that good launch.
Cruising at 80mph with 3:91 gears and a 28" tyre would be 3750rpm.


Hope that helps.
Cheers,
NZ440R/T

[MyMopar]

Aren't there 3.73 gears available for the 8 3/4?  That would be the best bet all around, better launch than 3.55 and less rpm than 4.10 and 3.91.
I ran 4.30's in my Charger and let me say 1st gear is useless and 3rd gear is good for around town local driving.  This is w/an auto. 
But if you are a stop light warrior than 4.10 would be perfect, if you do like to drive on the highway though, I would suggest something a tad higher.

[zerfetzen]

Thanks for all of your input, I appreciate it.

I think I will go 4.10's, bearing in mind that I will probably stay an insultingly low 55 mph (keep the rpm's low) for the brief time I'm on the highway.  But off the line, I'll set it up to eventually be a monster!

[Carlwalski]

Thanks for all of your input, I appreciate it.

I think I will go 4.10's, bearing in mind that I will probably stay an insultingly low 55 mph (keep the rpm's low) for the brief time I'm on the highway.  But off the line, I'll set it up to eventually be a monster!

If you keep it at around 50-55mph on the highway for your brief 1-5mile trips you'll be fine.
With 4:10 gears you'd be doing 2705rpms @ 55mph with a 28" tyre and 2460rpm @ 50mph.
IMO go 4:10s (you seem set) and upgrade to taller tyres (28" - 275/60) to help ease the pain a little.


Cheers,
NZ

(PS: Let us know how you get on and what you choose)

[GoodysGotaCuda]

With 4:10 gears and you current 26" tyres you would be killing your engine with even 1-5 miles of highway driving.
You would be running at 3440rpm @ 65mph and forget about cruising at 80mph which equals 4240rpm.

With 3:91 gears and 28" tyres 65mph @ 3050rpm would be your best.
Which is a tad over a 400rpm drop, very good IMO while still getting that good launch.
Cruising at 80mph with 3:91 gears and a 28" tyre would be 3750rpm.


Hope that helps.
Cheers,
NZ440R/T

Sorry, but where did you get those figures? My daily driver, 318, low gear set in the trans, 3.91 with 28" tall 275/60/15s sits at 3700rpm at 70mph. So 65mph isnt only 3050rpm, and 80 isnt only 3750rpm. My big ol 5" tach stares me right in the face as i commute on the freeway to work every day....i KNOW 

Car screams, loves the low gearing and to rev. 4.10s with a low gear set would be some pretty crazy gear reduction. screaming pretty good on the freeway. 4.10 at 65mph with a decent sized tire will not be 'killing your engine'. remember there was a good portion of these cars left the factory with 4.10s, and were driven daily, everywhere. a decent motor can withstand that rpm for quite awhile. You may wanna up your oil change interval and you will be spending more time at the pump.

but i dont see and actual motor damage due to sitting at roughly 3500-4grand on the freeway. i do it everyday. 5grand? now thats screamin pretty good. and i do not sit and cruise over 4k at any time, just is a wee bit over my comfort level.

just my 2cents.
-Mike

[Carlwalski]

The calculation assumes the car's transmission is in drive or 1:1 ratio, no overdrive. It is spot on. 

[MyMopar]

formula's

mph= rpm x tire dia. / gear ratio x 336

rpm= mph x gear ratio x 336 / tire dia.

gear ratio = rpm x tire dia. / mph x 336

tire dia. = mph x gear ratio x 336 / rpm

I don't care what a tach says, these math formula's won't lie, unless you put in the wrong info.  With the formula's listed above you can find out what exactly you are running or where you would like to run.

so 4.10 gears with a 1:1 final ratio going 55 mph w/28" tire (we all know advertised height is more than actual) will yield a RPM of 2706, not bad, 65 would be 3198 RPM.

Again this is IF your tire actually measures 28" (which it won't) and you have a direct lock for 1:1.

[Carlwalski]

formula's

mph= rpm x tire dia. / gear ratio x 336

rpm= mph x gear ratio x 336 / tire dia.

gear ratio = rpm x tire dia. / mph x 336

tire dia. = mph x gear ratio x 336 / rpm

I don't care what a tach says, these math formula's won't lie, unless you put in the wrong info.  With the formula's listed above you can find out what exactly you are running or where you would like to run.

Bang on, that's exactly what I got but you can still round the rpms off and it still equates.
eg: 4:10s with 28" doing 55mph = 2705rpms to your 2706rpms and 3200rpm to your 3198rpms @ 65mph.

Bugger all difference just so people don't get confused. 

[GoodysGotaCuda]

The calculation assumes the car's transmission is in drive or 1:1 ratio, no overdrive. It is spot on. 

does converter slip come into play into your mathmatical formulas? These cars are non-lockup of course. 

[MyMopar]

does converter slip come into play into your mathmatical formulas? These cars are non-lockup of course. 

No, that is why I stated "Again this is IF your tire actually measures 28" (which it won't) and you have a direct lock for 1:1."

Manual trans. cars are closer to the formula output than the auto's due to the converter.  Also like I mentioned, a tires actual diameter is usually less than the advertised.  You can use the formula to figure out what your actual dia. of the tire is based on the 4th formula.

In any case, this is more of a tool and reference to help figure out where you approx. would be/want to be.