Rearend gears

[leach1295]

I have a 1972 Dodge Dart Swinger with a built up motor Rear end does not have the right gears for a good 1/4 mile track/ streetcar. What size should I put in? Has a positrac and a 8 3/4 bolt rearend. What gear size /ratio fir bith tracj and street. I think 4 11 would be to much. It is a three speed automatic. With current gears will do 130-140 on highway but need suggestions for both street and 1/4 mile track. Would appreciate any advise I can get. Thanks

 

[1968FormulaS340]

I like 3.91s, what size is your rear tire?

 

[leach1295]

I have 14's on the back

 

[67Dart273]

Thanks to the www (Al Gore's internet) this is much easier than when I was a young'in.

Now you can just jump on the 'ol www, look up your TIRE SIZE, plug in your RPM and AXLE RATIO into an ONLINE CALCULATOR, and presto bangee, you have some answers.

So Google stuff like "mph calculator", "tire calculator," etc.........

[ame="http://www.google.com/search?hl=en&safe=off&biw=1366&bih=617&q=mph%2C+rpm+calculator&aq=f&aqi=g1g-j1g-b1&aql=&oq="]mph, rpm calculator - Google Search[/ame]


So let's say you are running a certain tire size, you'll need that first:

[ame="http://www.google.com/search?hl=en&source=hp&biw=1366&bih=617&q=tire+size+calculator&aq=f&aqi=g10&aql=&oq="]tire size calculator - Google Search[/ame]


So let's just pick a size out of the blue--you'll have to plug in your own and you didn't tell us what that is

http://www.1010tires.com/TireSizeCalculator.asp?action=submit

says that size tire is 24.74" in diameter

so now we go to one of our RPM/ MPH calculators and plug in some stuff

http://www.wallaceracing.com/gear-speed.php

So with that tire size, a transmission gear of 1, and rear gear of 4.11, here's what returns:

Engine Speed (RPM) Speed (MPH)
8000 RPM 143.26 MPH
7500 RPM 134.31 MPH
7000 RPM 125.36 MPH
6500 RPM 116.40 MPH
6000 RPM 107.45 MPH
5500 RPM 98.49 MPH
5000 RPM 89.54 MPH
4500 RPM 80.59 MPH
4000 RPM 71.63 MPH
3500 RPM 62.68 MPH
3000 RPM 53.72 MPH
2500 RPM 44.77 MPH
2000 RPM 35.82 MPH
1500 RPM 26.86 MPH
1000 RPM 17.91 MPH

I don't know about you, but cruising at 70 while doing 4K doesn't appeal to me very much for everyday driving.

 

[Brad4406]

You need to include how tall a tire you plan to run and how often you plan to go to the track. Since it's an 8 3/4, you can get one of each; a street gear and a strip gear and just swap them out before and after you go to the track if you don't go that often.

 

[67Dart273]

I have 14's on the back

"14's" is not a tire size. Look on the side of the tire and get the whole number string. 205R 75 14 or whatever

 

[leach1295]

p24/60r15s is the full size on the tire. I plan on trying to go to the track at least once a week.

 

[leach1295]

I need to remember to check my typing before posting it is p245/60r14 my cursor moves places at times:|

 

[pettybludart]

Track once a week,go 4.10,s:cheers:

 

[67Dart273]

I need to remember to check my typing before posting it is p245/60r14 my cursor moves places at times:|

That would be 26.6" tall

4.11 would give you

8000 RPM 154.03 MPH
7500 RPM 144.41 MPH
7000 RPM 134.78 MPH
6500 RPM 125.15 MPH
6000 RPM 115.53 MPH
5500 RPM 105.90 MPH
5000 RPM 96.27 MPH
4500 RPM 86.64 MPH
4000 RPM 77.02 MPH
3500 RPM 67.39 MPH
3000 RPM 57.76 MPH
2500 RPM 48.14 MPH
2000 RPM 38.51 MPH
1500 RPM 28.88 MPH
1000 RPM 19.25 MPH


3.90 would give you

8000 RPM 162.33 MPH
7500 RPM 152.18 MPH
7000 RPM 142.04 MPH
6500 RPM 131.89 MPH
6000 RPM 121.75 MPH
5500 RPM 111.60 MPH
5000 RPM 101.46 MPH
4500 RPM 91.31 MPH
4000 RPM 81.16 MPH
3500 RPM 71.02 MPH
3000 RPM 60.87 MPH
2500 RPM 50.73 MPH
2000 RPM 40.58 MPH
1500 RPM 30.44 MPH
1000 RPM 20.29 MPH

and 3.55

8000 RPM 178.33 MPH
7500 RPM 167.19 MPH
7000 RPM 156.04 MPH
6500 RPM 144.90 MPH
6000 RPM 133.75 MPH
5500 RPM 122.60 MPH
5000 RPM 111.46 MPH
4500 RPM 100.31 MPH
4000 RPM 89.17 MPH
3500 RPM 78.02 MPH
3000 RPM 66.87 MPH
2500 RPM 55.73 MPH
2000 RPM 44.58 MPH
1500 RPM 33.44 MPH
1000 RPM 22.29 MPH

 

[Dusteriffic]

With current gears will do 130-140 on highway

Holy cow I'm surprised you still hve a license after acquiring this data. :burnout:

 

[leach1295]

had a cop friend check me with his radar

 

[rumblefish360]

Well, as you see, at the numeric number of the ratio goes up, so does your RPM to stay at the same speed. Also, to keep the abilty to run at high speeds on the Hwy. you'll need 1 of 2 things, an overdrive trans or the abilty to obtain higher RPM's from the engine.

Gas mileage will suffer with any change in a numericaly high gear ratio.

 

[leach1295]

Need that good jump off the line if I need to put a stall speed torque converter in then I will if that will help

 

[kursplat]

if i may add a piece of "been there, done that" advise, with the mods your happily throwing at this car, forget worrying about gas mileage 8)

also, what, no pics?


have fun