- 3,020
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OK, I am going to forward this and see if they get the adjustment theu want for 2.87 gearing.
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A0 High Speed Gears - Speedometer Dip Switch Adjustment
- Thread starter dronsen
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spankybear
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Has we come to a consensus on the dip switch settings for the 2 high speed gear setting(3.07 and 2.87)?
- 1,811
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Not really, but I detailed HOW you come up with one that works, in a post above. So you can start with my settings, and see if you need to adjust.
- 3,020
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This should be very simple. Hook up a scope and find pulses per mile. Set up a benchtop pulse generator to the calculated PPM you want and start flipping to find the pattern of the switch settings.
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"Simple" is not the same thing as "quick and easy". Lots of things are conceptually simple, but a pain to actually do.
- 463
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- Seville, OH
I thought the 3.07 setting was figured out?
Will have to figure out 3.07 with 48" tires...
Will have to figure out 3.07 with 48" tires...
coachgeo
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you can use this calculator to determine true on the ground final gear ratio with different size tires. (3,07 upgrade and the 3,90 the OEM gear ratio, is with stock tires size) also, remember to double those ratios above, in the calculator (example 6.14 instead of 3.07) to account for the 2:1 reduction at the wheel ends.
Gear Ratio Calculator
Easy to use gear ratio calculator. Use our gear ratio calculator to find the correct gear ratio to run when changing tire sizes to keep your stock gearing.
somewhere in here I put in a thread the calculations of 3:07 upgrade with the 14r20 size tire ... and speed at each gear in the tranny .. was good while back.
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Why do all that? You just take the percent change of the diameter and that will be your percent change in speed too.you can use this calculator to determine true on the ground final gear ratio with different size tires. (3,07 upgrade and the 3,90 the OEM gear ratio, is with stock tires size) also, remember to double those ratios above, in the calculator (example 6.14 instead of 3.07) to account for the 2:1 reduction at the wheel ends.
PS- ignore the "new ratio needed" calculated by this site. It is only the "effective ratio" your looking for.Gear Ratio Calculator
Easy to use gear ratio calculator. Use our gear ratio calculator to find the correct gear ratio to run when changing tire sizes to keep your stock gearing.tiresize.com
somewhere in here I put in a thread the calculations of 3:07 upgrade with the 14r20 size tire ... and speed at each gear in the tranny .. was good while back.
Stock tires are 46.4" tall. If you get 48" tires, divide 48" / 46.4" = 103.4%. So when your speedometer reads 58MPH, you'll actually be doing 58MPH * 103.4% = 60MPH
Probably not even worth adjusting it, for 2MPH.
coachgeo
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thought he was looking for overall effective ratio in this case..... not speeds. (btw the 14r20 at 49"-ish height; with 3.07 gear has an effective ratio nearly the same as having stock tires and the more rare other ratio available for these axles (2.87) )
spankybear
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there are 2 setting people have said in this thread for an A0 truck with 3.07
this, 1101101001 and
this, 1101001001
or decimal 873 and 841
or...
hex 369 and 349
spankybear
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Well I tried this setting, 1101101001 and speedo was showing about 10%is slow... I then tried 1101001001 and this was showing the speed within 1-2 MPH on the GPS phone app... Then I tried to see how the odometer was reading. Using the mile markes on the side of the highway the odometer was showing 1.1 miles for every mile marker I passes... so about 10% fast. I am tempted to try the first setting to see if the odometer reads correctly . Has anyone else tested their odometer?
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This is one of the things I wonder about in the discussions about fuel mileage. I have a suspicion that the LMTV odometer system (meaning it could be the fault of the odometer, the sender, or other parts in the system) is fairly inaccurate.
People report mileage numbers that are just not possible, according to physics. There are a whole bunch of reasons they could be getting those numbers (e.g. just guesstimating optimistically / not calculating, speedometer settings have been messed with by GI mechanics, running different tire pressures changes tire diameter and thus actual distance traveled, etc.), but there are enough people who say they are actually trying hard to calculate MPG, and then get wildly different (and often physically impossible) numbers, that something else is going on too.
spankybear
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Yes sir... It seems that people will only check the speed and not the distance. The strange thing is that the GPS speed and the speedometer is the same why is the odometer off? I am going to play around with a stop watch and mile markers and see if that agrees with GPS and speedometer. I have been wanting to make some videos for youtube. Maybe I'll start with this?
Is there any other settings/switches on the speedometer. The odometer is how the speedo should be set. That is a direct pluses to distance so you don't have to worry about how fast you are going... just a distance.
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There are no other switches that are exposed. I've never tried taking it apart to see what's inside.
Make sure to do your tests over longer distances and average. Doing it over only one mile is too easy to introduce error.
I'm not sure why it would be different, but it just seems to not be a very good system. I have a few of the [real LMTV] speedometers that I have played with on my truck, and they all seem to behave strangely. On mine, I even get slightly different results when the headlights are on (because of voltage change?).
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I have 2 different apps that use GPS for speed and they don't always match, so I would have to use mile markers over a longer distance to get an accurate reference.
A 14x20 tire is 1.427 x the diameter of the 395/85 R20, or a 150.79" circumference vs a 105.68" circumference, or a 42.68% increase of travel per revolution.
This would be in addition to the 3.07 gear change....
My point of the post was to get an idea of the switch positions for that combination.
A 14x20 tire is 1.427 x the diameter of the 395/85 R20, or a 150.79" circumference vs a 105.68" circumference, or a 42.68% increase of travel per revolution.
This would be in addition to the 3.07 gear change....
My point of the post was to get an idea of the switch positions for that combination.
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It isn't clear that it's that easy.
In my post previously, I describe the process of how to adjust it through trial and error. The least significant changes happen with the left switches (lowest numbered switches), and the most significant changes happen with the right switches (highest numbered).
However, there could be a handful of reasons for this though, and it hasn't yet shown able to just calculate a decimal number from this, adjust it for tire size difference, and then turn that back into switch positions...
- Spankybear's post (#30 above) is likely incorrect, because he's using the more common Big Endian encoding of the bits (left to right), but with the most significant changes being on the right it suggests the bits are instead encoded in Little Endian (right to left). I showed in my post previously that the most significant bits (biggest numbers) seem to be on the right side.
- We don't know what the number means, even if we encode it in Little Endian. It could be a multiplier, a divisor, a raw pulse count, etc.
- We don't know if the number is added/multiplied to an abitrary base number. For example, let's say the number calculates out to 500, but is always added to 1000, to make 1500 pulses per mile. If you change tire size 50%, you should need 1500 * 150% = 2250 pulses per mile. So the original 500 number would need to be changed to 2250 - 1000 = 1250. There wouldn't be an obvious relationship between 500 and 1250... it wouldn't make immediate sense that a 50% change in tire size results in a 150% change in the switch number. I've seen some speedometer programming tables that appear to work like this, where all the switches set to 0 corresponds to some base speed, and then turning on switches adds on from there.
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Code is beyond my level of electronic expertise 
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What I'm talking about with Little/Big Endian is which way the digits go.Code is beyond my level of electronic expertise
Take the normal number 750. That's Big Endian. 700 + 50 + 0. Biggest (most significant) digit is on the left.
Little Endian would be if we flipped the digits around... 057. It still means 750, but the biggest digit is on the right... 0 + 50 + 750.
The switches on the speedometer are labeled 1-10, from left to right. It appears that flipping switch 1 makes a small change, and flipping 10 makes a big change. This suggests that it's Little Endian, and if so, the numbers Spankybear calculated above aren't correct.
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I Googled Ametek 900 speedometer calibration.
There is a 20 page document that might be helpful if we have some of the numbers required to get to get to the chart.
If the stock tire size is used 33.64" diameter and the 3.07 axle ratio N calculates out to 246.
Then this chart is supposedly the switch settings...
There is a 20 page document that might be helpful if we have some of the numbers required to get to get to the chart.
If the stock tire size is used 33.64" diameter and the 3.07 axle ratio N calculates out to 246.
Then this chart is supposedly the switch settings...
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Is your speedometer an Ametek? All those I have are Medallion.