VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc 3.55/3.73

Today I took the '48 for a short drive to check the speedo with a GPS.
speedo GPS
45 54
50 60
55 66
60 72
Wondering, I was suppose to have installed 3.73 from a '53 std shift. Would this be right?

Russell,

I don't know but I sure like your results.

Thanks for the post,

Charlie

I'm in the same boat...Welcome aboard...Nice how them folks on the freeway let us tag along, yeh..? How did this happen...? I wonder...My Coupe was owned by a kid whom added "cherry bombs" to the jolopy so...

So far I have been doing my testing and break-in driving on a two lane back/side road. It seemed to me that when I got up to around 55 the old girl was really moving. So on "Black Friday" I bought myself a GPS just to check this out. I was quite surprise at the results, but happy.
That's why I started to doubt that it's 3.73.

Oh, shh...I miss read your post...No, with those numbers your right in line with 3:73 rear end gears...Now, it's time to look for the correct tranny speedo gear set...If you'd like to see the correct speedo reading...

The easiest way to fix your speedo problem is with a Speedometer gear ratio adapter. With your new axle ratio your speedo is reading 20% slow when reading 60 mph. You need to increase the cable speed by 20%. Adapters are a little pricey at $65 to $75 each, but much easier that tearing the transmission tail stock apart and finding the correct gearing. Adapters take one rev of the transmission speedo gearing and increase or decrease the speed. In your case you want an adapter that takes the 1 rev input and makes it a 1.2 rev output. The adapter should be marked 1.20. Google Speedometer Gear Ratio adapter.

So does this adapter fit on the outside of the transmission...?

Kevin, I've had this bookmarker for some time since I knew this was coming. adapter
Or this. adapter2

Thank you, Russell. What's that one about...We have to decide between 17.6% or 26 something %...? I knew this was coming...lol

Thanks Oldie. I had a brain freeze and didn't see the 20%.
My second link has a 20% increase as stock.
Thanks again.

I'm going to speculate that your speedometer reading was not accurate before the gear swap. You did not change the ratio by 20%.
The speedometer in my '36 1.5 ton truck is way off. It has larger than correct tires on the rear, but not enough to account for the error.

Mike

Installing a 3.70 in place of a 4.11 would make your speedometer about 10% slow.
50 would show 45. 60 would sow 54. 80 would show 72, etc.

I see an adapter for 11%...$62 dollars...

MPH to Kilometers...! Don't tell Jianis, after all that work he did repainting his speedo to read kilo's...

Gene, yes, I had gone back and looked at your post about this on the other site. You said 10% with 3.70 and 15% with 3.55. That's why I could not understand the difference in the above readings.

I think Mike must be right about the speedometer being wrong.

I think I'll just use the GPS for awhile and see if anything changes for now.

Assuming that you have a 6-volt system, what is your source of power for the GPS? I like the idea but as of now, have to take along the jumper or a 12 volt battery, etc.

Could one simply use a voltage increase gadget to increase 6-volts to 12 on the cigarette lighter and then just plug in your GPS?

Just wondering and thanks.
Charlie

You can buy a voltage inverter to run 12 volt accessories. My GPS has an internal battery and I use it when I have it in one of my old cars. The battery will usually last for a day at a time.
Don

You guys are correct, my numbers are incorrect and it's all about the placement of the decimal. Percentage of error is calcuated as the actual (72) divided by the indicated (60) times 100 which in this case is .12 or 12%. Knowing that the adapter output needed to be greater than 1,I read the number as 1.20. Sorry for that. The adapter you need should be marked as 1.12.

Russel

As Mike mentioned your speedo seams to be showing on the slower side
The best way to know your gear ratio is to remove the differential cover from your axle housing then you can see and measure the teeth of the ring and pinion gears and make the math. Of course to do this you have to have a new cover gasket and fresh oil to finish the job.
Another small chance is to just look through the filer plug opening and see if you can count the pinion teeth if they are 11 then you have a 3.727 if they are 10 that's a later 3.7(1953/54) If 9 teeth more ods is that it is the original 4.111.

Jianis

Oldie, don't mean any disrespect,but, you had it right the first time. It is 60/50=1.2. Move decimal 2 places to right=120% or 20% increase. 50+20%=60 or 55+20%=66. My speedo is 20% slow. Pretty sure.

So, I guess we'd go with the "speed up the rate" with the 20% adapter for $79...(?) I don't want to open the transmission up...Just screw on an adapter...

Hey Kevin, If my math and way of thinking is correct, then yes. But the price is $62.00. Look above the selector box. Going 60 on the GPS but showing 50 on the speedo would mean I need to increase the speedo by 20%. The 20% increase works for all the speeds I have indicated in the first post. 45 x 20% = 9. 45 + 9 = 54. Just what the GPS indicated. OR do what I am doing for now. A "sticky note" on the windshield.

I was looking at the adapters...I thought we needed the other/ "speed it up", one (@$79) rather than the "slow it down", for $62...I thought I'd made a mistake the first time with the $62 adapter...Kevin

Were you thinking of getting one, soon...?

I'd like to have one, just to see how fast I'm going w/out having to think about it while I'm driving...More distractions I don't need...But, I'd prefer someone else ( w/GPS perhaps )...Giving the rest a "thumbs-up" before diving in... :D: Thanks, Kevin

Look one more time at the adapters. Speed-up #54RA2 $62, Slow-down #54RA $62. Custom $79. Pretty sure.
Yes, speed up for me. Find someone with a GPS and check yours to be sure of the adapter % you need.

The Filling Station sells a 6v to 12v thing ($60). It is 1 3/4" x 1 3/4" small. 6 volt wire in, 12 volt wire out. Connect to switch and lighter, done.

I bought one and that was what I intended to do. However. I looked at the label on my new GPS, Garmin 50 Nuvi $99, and found that it is 5 volt. The lighter adapter that came with it indicates it works on systems from 10 volt to 30 volts. I said to myself, what the heck, try it! I first plugged the adapter into the GPS. Then the lighter. The GPS came on in about 2 seconds. "Booted up" and worked fine. I switched it off and the screen came up, said battery charging.

Went through the connect, disconnect several times.

It may "Fry" the next time, who knows. I think the adapter would go before the unit. Hoping!!

Santa also went ahead and sent me one of thoes portable jump start things. It has a lighter socket I could use like you said.

Dear Russell, I was referring to you...lol ...If'n you purchase the adapter (speed-up/20%)...and check it w/your GPS to insure it is the correct one, and your satisfied with it...Then I'll buy one, too. Pretty sure we have the same ratio...

Oooooooooooooooh!!

Russell,

Thanks for the information. Are you saying the GPS works on your stock 6-volt system just fine?

Charlie

Hi Charlie, yes, so far. Please understand, I am not telling anyone to do this. I don't know about any other brand or model GPS. Just the one I have.

I tried to check the voltage at the plug end of the adapter but my hands are not steady enough to get inside that tiny space. I will try again this weekend.

Most GPS unites run on 5.7 volts because the uSB plug on the cord that comes with the 12v adapter is a reducing voltage adapter, most accessories that use a uSB connector are 5.7 volts the same as a computer.
I use a 6v to 12v converter in my 29 and 53 chevys to run my GPS and CB, also will charge my phone.
Al

anty one that has an old Car or truck probably has a need for a good battery booster -air compressor -trouble light set.

I have one that has a 12v outlet (cigarette lighter receptale, an air compressor, a floresent trouble light, jumper cables, and also a 300 watt 120 VAC invertor, I haul it in all my old cars on trips and have a couple of outlets for my GPS, Dixie air horns and CBs and walkie talkies, sit it in the rear floor boards and you are ready for about anything from a flat tire to a dead battery, day or night.