The Stovebolt Site Forums The Shop Area General Truck Talk Odometer correction factor after rear end change to 3.55

Is there a conversion factor that I can use to correct my odometer after I have changed the rear end from 4.11 to 3.55?
I will eventually get a gear adapter but, until then I am trying to figure my gas mileage (which it is pitiful) and I am trying to see if I a have a bigger problem.
Many thanks as always.

do the math 4.11 x (-14%) = 3.53

Bill

yes these trucks are nearly criminal in their fuel consumption. You might be able to use a GPS to determine mileage, which would probably be more accurate with or without a conversion factor. The other factor would be whether or not you have a GPS at your disposal.

Thanks to both.
I have a GPS that I used to check my speedometer so I know how much it is out.
To convert my odometer reading do I reduce the mileage by 14%? If it reads 100 miles is that really 86 miles?
Sorry to be so thick.

sorry, if you've got a GPS, can't it do the math for you? can't it tell you how far you've gone as well as how fast?

Bill

Using 50mph as an example your driveshaft (and transmission output shaft) is turning 14% slower with the 3.55 to run 50mph than it did with the 4.11. This means the speedometer drive gear on the output shaft is turning 14% slower affecting both the speedo and odometer. To speed the speedo drive back to the 4.11 speed you need a drive gear with more teeth or a driven gear with less. Usually you have to change the drive/driven gear as a set to get the percent reduction or increase one needs. We wait till the rear end choice is made and tire size selected then use the GPS to find the error and correct accordingly. google speedometer drive/driven gears and select from one of the many suppliers. Some have charts to plug in your gear and tire specs.

Thanks coilover, I understand now. It's simple when you get your head around it.
Also, red58, I played with my Garmin and although it took some digging I found the feature you were talking about. I don't know it did that!

never stop learning, especially about the high tech stuff you grab onto

Bill

Make yourself a little chart of speedometer and actual speeds by multiplying the speedometer speed by 4.11 and dividing by 3.55 to get the actual speed.

For example:
40 = 46
50 = 58

I don't want to change the subject, but I would like to add my experience, which might be of interest to those considering adaptors.

I ordered an adaptor from JC for my 1954, which has the stock 3.9 rear. Because of tall tires, I was reading way low on the odometer compared to actual (GPS). Actually, when speedo said 40, I was actually going about 50.

Now here is the strange thing - With the new adaptor, I am 100% spot on at 40 mpoh. However, above that speed, there starts to be a difference, which gets bigger as I go faster. For example:

40 speedo = 40 actual
50 speedo = 48 actual
55 speedo = 51 actual
60 speedo = 55 actual
70 speedo = 62 actual

I thought the correction would be constant. What am I missing here? Thanks.

Harry

Harry, did you check the speedo that way before the adapter? the adapter correction isn't deviating, the speedo is, a spinning magnet is what makes the needle move, not direct drive gears - your speedo is as accurate now as it was before

Bill

You should plot a graph of the two speed readings and plot them on paper. I think you will find two lines with different slopes that intersect at 40. You should plot the readings below 40 too. This will show you how the error factors in. The two lines should intersect at zero.

Harold46 has the correct formula for the original post.

Ah, I see your point. No, I did not carefully check for different ranges at different speeds. Never thought it would matter. When I get a chance, I will pull out the adaptor, and do just that. Thanks, Bill.

folks need to keep in mind that aside from these rigs having 40-60 years of use and abuse, they weren't supplied with scientific grade instruments I'd be surprised if any year speedo was very accurate over it's full range even when new

Bill

A general rule of thumb for mechanical speedometers: Under 40 MPH, a little behind the actual speed. Look out that can get you some tickets. Over 40 MPH, a bit optimistic. You are not going as fast as it says you are. There is an adjustment for the spring tension. If you see a lot of slower than indicated you can increase the tension and make it better sort of. If you see a lot of faster than indicated you can reduce the tension. But to do this you need to pull the speedo out and take it apart. Put it back and try again over and over till you get it close. If you know it is behind when slow and ahead when fast just adjust your self it is easy sort of.

I stopped worrying about mileage 40 years ago. You can't do a whole lot about it. If you start spending money "improving" gas mileage, with this and that part, you will never get that money back. There are many things in life to be concerned about. Mileage is not one of them on an old truck. Drive it the same speed every one else is driving and when it gets low on fuel, put some in. How much was the 3:55 rear end? How much was the GPS? How much is the adapter? What mileage are you getting/Were getting with the 4:11? How many miles do you drive per year? What is the price of gas in Canada? You don't usually buy an old truck to gain fuel economy over your other vehicles.

Bartamos your points all well taken. I personally do not normally worry about gas mileage or prices with my other vehicles and fill them up when they need gas.
But, because I have never restored an old vehicle to this degree and because he was not running smoothly I wanted a rough guide to see when I was at.
I have been logging my mileage and speed with my Garmin and will make a determination after about 100 miles.
For my own use here in Canada I like the idea of making a little chart for speedo readings - which by the way is all over the map.

On another note, I think that my '51 and my 2 hybrids pretty much cancel themselves out as far as my carbon footprint goes!!

Yes you are leaving no trace from gas except when you have personal gas! You need to figure out why it's not running smoothly, but I don't know what smoothly is on that truck. You should get 16-26 MPG according to reports on the internet. I don't know what that is in KPL.