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Sat Jun 20, 2020 10:33 am

4:12 vs 3:88 Not what you think!
Vehicle: 1963 Ragtop Bug-Mild 1776/205r15 Rear Tire /EMPI 5 spokes

At the risk of avoiding the whole 3:88 vs 4:12 debate which I have read plenty about on this site which BTW has been a big help. I had a question arise.

I was leaning towards the 3:88 over the 4:12 as in Texas we do alot of highway driving where unfortunately 60MPH means 80MPH and 70MPH means 90+MPH! I don't plan to cruise at 75-80MPH but would like to be able to cruise comfortably at around 70MPH without the motor screaming.
Recently I l was told that a 4:12 with a .82 4th will make a difference over a .89 4th gear. I believe the current motor will pull the 3:88 with eventual plans to go bigger (1904/1915) down the road. Any real world input is welcome, Thanks!

Sat Jun 20, 2020 10:53 am

First thing. - Are you really using 205R15" rear tyres ??? They are HUGE!
Even with the stock 4,375 transmission you are then doing 70 mph @ about 3200, - and you want to reduce rpm further (?)
I wouldnt recommend that, especially not with only 1776 displacement.

Please confirm that you have written the tyre dimesnsoin correct.

Sat Jun 20, 2020 10:56 am

build more motor. big motor works less. use big oil pump if you go with highway trans....I use big pumps in everything anyway( 30mm). there are many diferent ratios you can come up with and diferent tire sizes. you need a real base line to do comparasons... 388 with .93 gear isant a lot of diferance than a 412 with .89, a 388 with .89 might be what you want or possiably the 412 with .82 thats up to you.I like plenty of power so the motor dont have to work hard and you can use 3.88 gears better with various 4th ratios& tire sizes. I like tall tires too. and dont forget about the gear drop between gears as well as crusing speed, will 3rd be too much and 4th not enough.... figure out your balance and then throw the dart and see what you get.

Sat Jun 20, 2020 11:29 am

What the hell is a 205r 15?

Are you not leaving out the rest of the tire size?
Please dont tell me your leaving us to assume if it is a 50/55/60/65/70/ or 75 series tire

The biggest problem with gear ratio questions, is the poster does not give their actual tire height in their question, and everyone gives answers based upon assumptions on tire sizes they use on their own car, but this is no help to the poster.....

Sat Jun 20, 2020 11:31 am

How tall are your tires? It doesn't make a damn how wide they are. There is a huge difference between a 205/60-15 and a 205/75-15.

My personal opinion is stay with stock gear ratios for anything under 100 HP.

4.12 with .82 4th has a lot more zip in 1/2/3 where you do all you stop light street racing. 3.88 with .89 4th gear you don't have the big gap between 3 & 4 gears but it is a tiny bit slower in 1/2/3 gears.

Sat Jun 20, 2020 12:21 pm

A basic 1776cc, 40idfs, engle 100 cam, can cruise at 4000rpm, as long as you don't do anything to screw it up.

Once you get into stroker cranks then it may need to be geared higher.

Sat Jun 20, 2020 2:11 pm

68Comeback wrote: 4:12 vs 3:88 Not what you think!
Vehicle: 1963 Ragtop Bug-Mild 1776/205r15 Rear Tire /EMPI 5 spokes

At the risk of avoiding the whole 3:88 vs 4:12 debate which I have read plenty about on this site which BTW has been a big help. I had a question arise.

I was leaning towards the 3:88 over the 4:12 as in Texas we do alot of highway driving where unfortunately 60MPH means 80MPH and 70MPH means 90+MPH! I don't plan to cruise at 75-80MPH but would like to be able to cruise comfortably at around 70MPH without the motor screaming.
Recently I l was told that a 4:12 with a .82 4th will make a difference over a .89 4th gear. I believe the current motor will pull the 3:88 with eventual plans to go bigger (1904/1915) down the road. Any real world input is welcome, Thanks!

Had a ‘71 Ghia that came with a 3.88 and .89 4th. Put a 1915 in it and had a blast. Then I got a ‘70 Ghia, put the 1915 in it and hated it. Turned out it somehow had a 4.37 in it, so out that came.
I run 195/50/15 tires so I’m starting out handicapped. I went 3.88 with an .82 4th and couldn’t be more pleased. I was a little concerned with the hole between 3rd and 4th, but the only time it’s annoying is on a 35mph road. Mostly, I don’t even notice.
With my tiny tires I can run 72mph at 3000 rpm. That’s via Waze GPS. My speedo says I’m going 90.

Sat Jun 20, 2020 2:46 pm

If you intend to pair it up to a more powerful engine, another point you might want to consider is 1st gear. I've got a 4.125 gearbox (tires are 175/65/R15), and the problem I've been having is that with my mini-stroker, 1st gear is now so low as to be practically worthless. It's gotten to the point where if I have any forward momentum going at all, or if I'm on even a slightly downhill slope, I don't bother with 1st and just take off in 2nd. I've got the original 3.875 gearbox with a busted reverse, and as soon as I get around to fixing it, it'll be going back into the car.

The 4.125 with a 3.80 1st gives a final drive of 15.675. The 3.875 with a 3.78 1st gives a final drive of 14.648, more than full point taller. As mentioned, final drive in 4th gear between the two is roughly the same.

Sat Jun 20, 2020 9:19 pm

Quote: How tall are your tires? It doesn't make a damn how wide they are. There is a huge difference between a 205/60-15 and a 205/75-15. I just fitted 205/65 tires to the rear of my '77 Beetle, which has its original 3.88 final drive. I chose that size because I have 5.5" wheels in back, and the 205/65 has the same outer diameter (25.4") as the 165/80 tires I chose for the front on 4.5" wheels. I've only taken the car on a 10 mile highway drive with the new tire set, but as usual it feels comfortable at 75 mph. Engine is a near-stock 1600 dual port with single 34-PICT 3 carb, though I really have to get cracking to reassemble a mild 1776 cc with dual Dell'Orto 36 DRLA's.

Here's a shot of my new front and rears; you can see how close the diameters are. The 205/65 replaced 185/65. I figured that the extra 20 mm tread width only has the tire about 3/8" wider on inside and on the outside than the 185's.

Sun Jun 21, 2020 7:28 am

The 3.875 box with the 0.93 4th is geared about 2% taller than a 4.125 box with a 0.89 4th. If your rear tire is P205/70R15 then you would be right at 3200 rpm at 70 mph with 3.875/0.93 setup.

If doing a lot of freeway or highway driving I found the smaller 3rd to 4th gear spread was nice for that 45+ mph run up to cruising speed. The smaller gear spread keeps the engine rpm up between the gears and the taller R&P allows more speed in 3rd. It sure made the heavy super beetle convertible with a stock engine feel a lot more comfortable for freeway onramps and winding highways.

The actual tire size and the engine setup (cam, carbs, exhaust) would make recommendations more meaningful.

Sun Jun 21, 2020 7:53 am

Alstrup wrote: First thing. - Are you really using 205R15" rear tyres ??? They are HUGE!
Even with the stock 4,375 transmission you are then doing 70 mph @ about 3200, - and you want to reduce rpm further (?)
I wouldnt recommend that, especially not with only 1776 displacement.

Please confirm that you have written the tyre dimesnsoin correct.

In my 67 hardtop, I drove it with a 1776, 205/15s (can’t remember aspect ratio), a 4.12 and .082 4th. My rears are 25” tall. I cruised at 60 or 65 gps mph at 2900 rpm in 105 degree weather from LA to Rialto (1-1 1/2 hours each way). Might have been 65 at 3,000. No overheating problems whatsoever. Standard pulleys.

Sun Jun 21, 2020 10:35 am

So We still dont know the original posters tire height yet?

Sun Jun 21, 2020 10:37 am

It is all about engine cooling. 3200~3500 rpm is the ideal engine cooling speed.
That is what my engine builder says.

Sun Jun 21, 2020 11:51 am

All engines are a little different. The more you focus on frictional losses, the slower your engine can cruise at. On the street, I would also lug it around at 40 mph in 4th. I have crazy low spring pressure a gentle cam profile, and it's very quiet. The idle speed is a good indicator of friction. I was trying to set best lean idle and even at 400 rpm it was still spinning smoothly, so I just set my idle mixtures at that point.

Sun Jun 21, 2020 2:13 pm

Appreciate all the feedback great stuff! For clarification the rear tire is 205/60r15 pic attached hopefully Thanks again!

[img][/img]

Sun Jun 21, 2020 2:53 pm

Give a few details about the engine. Cam, heads and carbs.

With 205/60 I'd suggest the 3.88 with .89 4th gear. That is the stock gearing from a 71-74 Ghia.

Sun Jun 21, 2020 3:54 pm

You have a 24.7 tire height.

I have a 25.3 tire height and went with 3.88 and .82 4th for freeway luxury driving and a mild little 1679. 3rd to 4th is a big jump, but its a freeway bomber and its worth the compromise. 75 to 80 avg speed. You have to be honest, if you drive 70 on the freeway with folks that are averaging 80+ as the norm you gotta cruise in the slow lane or get run over. Gear it right and you can drive with the flow of normal traffic.

Sun Jun 21, 2020 4:11 pm

A lot of people want to gear it higher because the engine is so "loud"
Unfortunately it's not QUITE so simple.

What it really is is that the farther open the throttle, the louder it is, period. Test that yourself!
so, your not going to make it quieter by gearing it higher. Just going to have to open the throttles more in order to achieve the same speed :roll:

My car is quieter at 75 than it is at 65, once I get over 3600 rpm, it's "on the cam" and "on the pipe", the throttles are not open as far.

BUT< BUT is true you want to gear it for the powerband of the engine.
If it's setup for LOWER rpm torque then DO gear it higher. If you've got a wild cam in it, then don't.

Wed Jun 24, 2020 8:15 am

A lot of people want to gear it higher because the engine is so "loud"
Unfortunately it's not QUITE so simple.

What it really is is that the farther open the throttle, the louder it is, period. Test that yourself!
so, your not going to make it quieter by gearing it higher. Just going to have to open the throttles more in order to achieve the same speed Rolling Eyes

My car is quieter at 75 than it is at 65, once I get over 3600 rpm, it's "on the cam" and "on the pipe", the throttles are not open as far.

BUT< BUT is true you want to gear it for the powerband of the engine.
If it's setup for LOWER rpm torque then DO gear it higher. If you've got a wild cam in it, then don't.
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Thanks not to concerned about the noise the sound of dual carbs humming along is fine for me!