Final drive ratio?

[regularjoe]

Ok, my 72' 240Z that used to race SCCA ITS is going back onto the road and I have to dump the welded rear end. I have a R180 that has a 4.3?? ratio out of the front of an 80 something pickup, does anyone have any experience with this ratio in a street/auto x car? I think I still would get plenty of highway speed while gaining quickness out of the tight corners. I have 4 other 180s and 1 200 I need to check yet if there is a particular ratio that comes recomended.

Thanks in advance,

Regular Joe

[Arne]

Joe, a 4.3 is dang low ratio for street use, especially with the shorter profile tires people often use these days, and very much so without a 5 speed. With a 4 speed you're going to be turning some pretty significant revs on the highway with that diff. Exactly how many depends on the tire size.

A stock 240Z has an overall ratio in third gear of 4.77 (1.42 x 3.36). So 4th gear with a 4.3 is going to be in the same ballpark as 3rd in a car with stock gearing. Top speed of maybe 100 at redline....

[regularjoe]

Joe, a 4.3 is dang low ratio for street use, especially with the shorter profile tires people often use these days, and very much so without a 5 speed. With a 4 speed you're going to be turning some pretty significant revs on the highway with that diff. Exactly how many depends on the tire size.

A stock 240Z has an overall ratio in third gear of 4.77 (1.42 x 3.36). So 4th gear with a 4.3 is going to be in the same ballpark as 3rd in a car with stock gearing. Top speed of maybe 100 at redline....

I was guessing at 110 mph for the top speed at red line. But just guessing, is there a list of the gear ratios for the 4 and 5 speed trannys? I have those other rears to check out, I think one of them is a 3.92 or something like that. I also have a couple of trans and if I remember one is a 5 speed, maybe that would make the 4.3 workable.

Thanks again,

Regular Joe

[Mike B]

I agree with Arne, 4.3 is probably too low for highway use, even with a 5 speed. I think most people prefer a 3.9 or 4.11 with a five speed. Here is a link to an online calculator that should be helpful http://www.geocities.com/z_design_studio/.

-Mike

[26th-Z]

The 3.9 final gear was standard for the F5C71A-5 five speed. We got the 3.364 final gear with the F4W71A-4. There are several final gear sizes in the competition catalogs however they are all listed as limited slip. This site may be of help http://www.geocities.com/z_design_studio/

But generally I agree with Arne, a 4.3 is dang low for street use.

[Mike B]

You type too slooowww Chris;).

-Mike

[regularjoe]

Thanks to all.

Great info to get myself "edumicated".

Joe

[rxsleeper]

Hey Joe, I probably have a 3.54 open R180 that I would be interested in trading you for your 3.4?. Shoot me a pm if you are interested. I currently am running a 3.9 or 3.7 for the local tracks I hit. A 4.3 I could use on the hillclimbs. BTW, I keep a locked 3.54, 3.7, and 3.9 on hand. I have been promised a 4.11 from another racer I just need to pick it up. If I ever get enough coins I will upgrade to a Quaife.

And to answer your question, With the 3.9 I see 7k rpm on the front straight at Nashville Speedway. It is too tall in my opinion for street use with the 4 or 5 speed. I would think a 3.7 or 3.9 coupled with a 5 speed would be a nice street combo. Enough grunt at the bottom and not too bad on RPMS in the uper gears.

Edited August 31, 2009 by rxsleeper

[Mike B]

With the 3.9 I see 7k rpm on the front straight at Nashville Speedway. It is too tall in my opinion for street use with the 4 or 5 speed. I would think a 3.7 or 3.9 coupled with a 5 speed would be a nice street combo. Enough grunt at the bottom and not too bad on RPMS in the uper gears.

What speed are you going at 7k RPM with a 3.9 diff? I assume that is with a 5 speed? Using the calculator I linked to with an 81-83 factory 5 speed, an 81-83 280zx 3.9 R200 diff, and 205/60/15 tires, it calculates 3165 rpm at 80 mph in 5th gear. With that setup, 7000 RPM would be about 177 MPH.

-Mike

[rxsleeper]

FWIW, I am using the OEM 4 speed per the class rules (ITS) and 225x50x14 Hoosier R-6's. I think it works out to about 122 mph. A couple of the other guys are running a 4.11, same tranny, and 225x50x15 IIRC and they pull me on the straight. I know this year the 2nd place car changed tire diameter to drop his rpms on the straight. I just don't remember exactly what size he went to. He was seeing 7100-7300 on the front straight.

[Galaxybj]

JFWIW - In 1972 I put the comp 5 speed in hls30-30855 using the gearing that the 2000 roadster had and the comp 4.44 lsd. Overall ratio in 5th was 3.77. I never felt that it was too low geared for the highway and got decent mileage too. I had engine work (cam, distributor, SM needles) done at FAR Performance so it was putting out more hp over stock, but not a lot more. I never minded the bit higher rpm's and enjoyed the performance. I look forward to getting her back on the road - maybe next year.