Gear Question (4:10's or 3:55's)

[Carlwalski]

I'm getting the GV unit and I found my car had 2:76 gears, the rebuild shop out of town didn't check the gears, just rebuilt/restored it. So, I'm a little stuck.....I've been thinking; "Yeah, 4:10's will rock!" but when I found the gear I had (2:76:1) I thought, "Hmmm, if I get 4:10's my mpg will rise."
With the GV unit that would be a waste, right? The unit is suppose to help you gain.....

So, if I get 4:10's with the OD gear it will run like 3:20's in top gear.
If I get 3:55's with OD on it'll run like it is now, 2:76 (no loss in mpg and I gain a better launch).

Question: Is there a sure fire way to measure what rpm different gears do at 65-70mph etc?
Tyre size + gear ratio + engine/transmission = ? 

If I knew that by getting 4:10's with a 3:20 top gear it would run only 400-500rpm difference than what I have now 2:76...I'd probably go for the 4:10's.


Hope this all makes sense, my resto team are trying to convince me that 4:10's are too much, but they know at the end of the day it's my car, my money and my call.



Thanks,
Carl

[Theskrich]

I believe this is what your looking for http://www.geocities.com/z_design_studio/
I think the 727 annd 904 are both use 1rst 2.45, 2nd 1.45,3rd 1.00

[Oldschool]

Just a question for you NZ....   You are considering the overall MPG, and RPM at highway speed.  You are also wanting a better launch.  It is relatively easy to calculate gear ratios and how they affect top gear RPM, which in turn affects MPG. That's the easy part.  Since gears act as torque multipliers, this is the bigger mystery to me:  how much more torque can Lucy utilize for the launch and not blow the rear tyres out from under her.  If she will spin em some now, if you increase torque to the rear wheels by 1/3, she might launch slower due to the lack of traction.  I might do some research and see how much more gear you need on the bottom to accomplish what you want and then see where that put's you on the top end, RPM and MPG wise... Working from the bottom instead of the top.  Make sense?  Just my 0.02 ...        ....Oldschool

[Carlwalski]

Yip, thanks lads.....


OS - Yip, that makes sense. I have taken off once from the get go and she hooked up good with slight wheel spin (like 0.5 seconds) wheel spin. You make a good point and I have seen threads about low gears not being the best solution sometimes..... Still, over stock tyres 275's help and limited slip diff also helps plus the cars suspension is set up perfectly. I'll see but you brought some good info and advice to the table....cheers brother!!

[Topcat]

NZ, did you put frame rail connectors on your car? Would help keep body stiffer if you're gonna have taller gears.Would'nt want body flex... C pillar area is a concern. Figure while your under there, ya can add them too.

[Carlwalski]

Hmmmm, I don't think I'll need them TC...I have 400hp maximum and roughly 500-lbs-ft of torque.
Not quite top fuel power. lol Not quite anyway.  Factory cars with 4:10s didn't need them and with Lucy's set up as is (non-stock tyres/suspension) I figure she'll handle it well.

[Carlwalski]

Does this seem right to you guys? With 4:10s and a 3:20 curising gear with the GV and my 27.99 inch trye diameter it works out to be 65mph  @ 2,500rpm. http://www.speedworldmotorplex.com/calc.htm

I worked out that 2:76 gears run 2150rpm at 65mph so it looks pretty good, I only gain 350rpm's.

[Bamakx]

I have the ratio between the gear selections you are looking at with my GV.  I am running 3.73 and it gives great low end and good highway speeds.  For me I would not good to any other gear ratio.  I had to put subframe connectors on my car, it was cracking paint at the C-pillars.

[Carlwalski]

Thanks Bamakx....what kind of power are you throwing out mate?
I am thinking and have been for the last hour (lol) that maybe 3:73 or 3:91 gears might be the way to go.


Decisions, Decisions.......

[Carlwalski]

I made an rpm chart for anyone who needs it, it was made for me but it's quick reference for someone else.


Gear Vendors RPM Chart

Speeds below at 65mph (105kph)

3.55 Gear (2.76 OD) = 2150rpm
3.73 Gear (2.91 OD) = 2270rpm
3.91 Gear (3.05 OD) = 2380rpm
4:10 Gear (3.20 OD) = 2495rpm


Speeds below at 75mph (120kph)

3.55 Gear (2.76 OD) = 2480rpm
3.73 Gear (2.91 OD) = 2620rpm
3.91 Gear (3.05 OD) = 2740rpm
4:10 Gear (3.20 OD) = 2880rpm



 

[Bamakx]

It dynoed at the rear wheels at 492 hp and 515 ftlb of torque.

[Plum6Pak]

It dynoed at the rear wheels at 492 hp and 515 ftlb of torque.

What size tires are you running and do they hook up pretty good? I'll be running 560 hp and 590 fpt, with 4.10's I'll need some good traction to get her to hook up.

Does this seem right to you guys? With 4:10s and a 3:20 curising gear with the GV and my 27.99 inch trye diameter it works out to be 65mph  @ 2,500rpm. http://www.speedworldmotorplex.com/calc.htm

I worked out that 2:76 gears run 2150rpm at 65mph so it looks pretty good, I only gain 350rpm's.

Personally I like the 410's, I had them in my 69 383 RR, changed from 3.23's to the 410's and I couldn't believe the launch! Highway driving was the pits, I had to watch cars passing me like I was sitting still unless I wanted to have the engine wound out like an indy car. With this GV on it that would make highway driving fun again and still have the killer launch! IMO you'd have a blast with the 4.10's and the GV, there a lot of other opinions too be sure, but for me, that would be my choice. Although it may not be for you Carl, maybe you do more country road driving than in town. If that's the case, maybe go a bit higher gears.

[MEK-Dangerfield]

 

  I agree with Plum,
   Unless most of your driving is at 75 MPH, I think the 4:10's would be a killer. 

  Mike

[hemi71]

This is always a good topic...lots of opinions!

I'd go with the 3.91's with the GV. Good compromise with the OD, gives you bottom end pull and top end is still quite liveable with the OD on the GV.

3.73's might be good also, if i remember correctly, that gear set is not available in all 8 3/4 cases, so you should check into that if you are going to change the center section you have in the car now.

As far as frame connectors go, I really like em in a car as long as you are not concerned with the undercarriage appearing "stock". They really tighten up a car, so even at lower HP levels, thay make a big difference in handling, and body flex (with tha accompanying squeaks and noises) is substantially reduced. The 71 hemicuda clone i just sold, and my 70 I now have both have them.

 

[Bamakx]

I have a set of BFG drag radials and they hook very hard.