The Stovebolt Site Forums The Shop Area Driveline Another 3.55 question

Hi, I'm in Australia with a 1946 Chevy Coupe Ute. Front end and running gear made in Canada and the ute body made in Australia. Photo attached

I've done a lot of searching through the site and found lots of info on how to do the 4.11 to 3.55 diff conversion but very little on the actual benefits.
My high school math tells me that 3.55 is about 12% higher than 4.11.
Current top speed with my setup (216 engine, 3 speed transmission, 4.11 diff) is about 40 mph. It will go a bit faster but it's definitely not happy about it.
So, using my high school math again, if I change to 3.55, my new top speed would be 45 mph.
That hardly seems worth the extra expense and effort, so I think I may be missing the point.
I'd love it if someone could give me some real world examples of what changes I could expect if I went down the 3.55 conversion path.

Thank you
Bob

That doesn't sound right at all. My 1950 216cid, 3spd, 4.11 easily makes it to 65mph and comfortably travels at 50-60mph.

Before converting your rear end, you need to find out why your truck is so slow.

Correct diameter tires will help. When I installed 16" rims and 30" diameter tires, it made a dramatic improvement in driveability and speed.

Is your clutch slipping? Is your engine worn out? I'd do a compression test just to get some basic starting point information.

Engine, tranny and rear end are not rebuilt. Clutch is new.

Prior to new clutch, top speed was between 30 and 40mph depending on terrain. Top speed going up hill was 30.

Thanks Otto.
I've looked at the tyres being the problem because I currently have 15 inch wheels and the diameter of the tyres is only 27 inch.
But the tires on my Mazda 4x4 with 16 inch wheels are only 28 inches.
What tires do you have that are 30 inch diameter?

225x85 R16 are about as close to the original diameter you can get.

I edited my prior reply but you nmay not have seen it. What kind of shape is your clutch in?

My rims are 6" wide. The originals are 4.5" wide. If your rims are real wide, you'll lose some height. It doesn't seem like that's your biggest problem, though.

My current wheels are 7 inches wide and they do look a bit silly.
It seems pretty good otherwise.
Runs smoothly, doesn't burn oil, compression is about 105 on all cylinders

What units are you using for this measurement?

Pounds per square inch.

Ok, just making sure there wasn't a a metric conversion somewhere in there. That is a pretty reasonable number for these engines.

Running 27" tires vs 30" tires is another 10% speed gain.

When you say it isn't happy going over about 40mph, does it run out of power or are you just not happy with the RPM it is spinning?

40 miles per hour, 4.11 gears, and 27" tall tires only maths out to 2045RPM. That is pretty low for a max.

Engine runs smoothly, doesn't burn oil, compression is around 105 on all cylinders, clutch is not slipping.
I think the tyres are probably the main problem, especially if they should be 30 inch diameter, as mine are only 27 inch

It doesn't run out of power. Theres still plenty of pedal left, but it does get quite noisy. But currently I don't have any soundproofing on the firewall, so it makes a racket

Another question: how sure are you that your rear end is a 4.11? The 46 truck could have come with a number of ratios.

Given the ~80 years since it was initially titled and the custom work that has gone into it, who knowd what axle it currently has (unless you have verified it yourself).

Honestly, that sounds pretty normal. Adding some sound deadening would help and changing the rear end and/or tires would get your more speed for the same noise.

Not sure at all. Not sure how to tell. Everything else on it is standard, so I have no reason to believe it would have been changed

I think what I'm unsure about is when I look at the numbers using simple math, whether its 30 inch tyres or the 3.55 diff, the difference is only about 10 or 12 percent.
Which would increase my speed from 40 mph to about 45. That doesn't seem like much of an improvement given the expense involved.
So I'm wondering am I missing the point. I've read reports of people getting massive improvements and happily driving down the highway at 60 mph. Is there more to it than what the simple math calculation indicates?

These old engines live pretty happily at higher RPM than some of the newer ones. Try removing the fan and take a drive, and you'll probably discover where a lot of the noise is coming from. Very few people run them above 2500 RPM, and they'll easily turn another thousand without doing any damage. I drove a 1958 Chevy Del Ray coupe with a 235, a 3 speed, and a 3.70 rear end, with 14" tires. It had no problem running 70 MPH, and I could push it to 80 whenever I felt like it.
Jerry

I've made no "improvements" at all to my truck and I drive 55 mph all week long for work. Is it as quiet as a new truck? Heck no.

Are you saying that your truck will not exceed 40 mph at all or are you hesitant to push it beyond 40mph?

My truck has the stock 4 blade fan. It doesn't seem exceptionally loud to me, though. I don't own any modern vehicles so driving weird, old, noisy cars seems perfectly normal to me. I drive the '39 Packard 60-70 mph on the freeway. It, too has a 3 spd with a 4.11 rear end. It has about 53 more horsepower than the AD Chevy, though.

Switching from 27" to 30" tires will be the same as switching from your 4.11 rear gear to a 3.70.

I've just taken it for a drive with larger tires borrowed from my 4x4. Not a lot of difference in speed, to be honest.
It is noisy, but there's almost no sound proofing so I'm thinking it may be a perception thing. I haven't been game to push it too hard because of the noise and wondering if it'd blow up.
So, I think I'll install some proper sound proofing and see what it feels like. That's definitely the cheapest option, anyway.

Thanks everyone

I assume you are using a GPS or something similar to measure speed with the changed tires?

It sounds like the soundproofing is probably your best option. I don't know what parts availability is like down there, but making your own isn't too hard. If you want some advice on that I recommend starting a new thread over in Interiors.

Yes, using the GPS on my phone.
Lots of good sound proofing products available here and I’m happy to install it myself. It’ll need doing regardless of any thing else I do to it, so it’ll be interesting to see, or rather hear, the result

The fact that you are going about the same speed for the same amount of discomfor with the larger tires makes me feel fairly confident that the issue is road and/or fan noise. Soundproofing it relatively inexpensive and will be beneficial whether it is a final solution or not.

Sounds like you are off and running on this one.

Good luck and please come back and show us your solution amd how well it worked (or didn't work).

Don't worry about blowing up the engine by revving it up to drive 55 MPH. If it blows up, it needed to be replaced anyway.
These engines are noisy. That does not mean it is suffering.
30 inch tall, narrow rims and tires will help you reach 55 MPH at a lower RPM. I suggest you try that first before you go to the trouble and expense changing to 3.55 gears.
That Ute is beautiful and needs to be on the road and often!

Hi everyone, it's been a while and a lot has changed with my ute. I'm now much happier with it.

I added soundproofing and carpet which cut out a lot of the noise.
I confirmed that my diff is 4.11 by counting the teeth.
So far not going any faster but a lot quieter.
Realised the Heat Control Valve in between the Exhaust and Intake manifolds was stuck open. These things might work in cold places but in Australia I was effectively boiling my petrol (gas).
I couldn't get it to work properly so I'm using a clamp to hold it shut.
Instantly got another 10 mph and it doesn't sound like its working so hard.
Realised my Vacuum Advance wasn't working at all.
Fixed that and adjusted the timing, and it will cruise at 50 mph quite comfortably without sounding like its about to explode.
And it runs a bit cooler too.

Thanks for all your advice.

Bob

That's awesome, Bob. I knew you get at least 50mph out it. It makes it much more useful to use as a regular truck.

Putting taller tires on my 62 C10 definitely helped get a better cruising speed. From 25.5" tall, to almost 29" tall.

The 235 engine found a nice spot at just shy of 50mph on the rural county roads. Limit is 45 and everything seems to smooth out just above that so 👍👍

There is plenty of power left beyond that too. These old engines are surpringly torquey 👍

What you lose when installing the 3.55 gears on an otherwise stock AD, is engine braking. Of course you still get engine braking, but the difference can bite you coming down the back side of steep hills and mountains. Don't ask how I know.