1995 e350, help calibrating speedo please

[gtbensley]

Hi guys, I recently bought a 1995 e350 ambulance with a quigley 4x4 installed and a 7.3 diesel. Love the van and have been doing some basic maintence and up keep type of work since I bought it. One thing that has been bugging me is the shift points are off due to it having larger tires on it. Through this link
http://www.ford-trucks.com/article/...EDOMETER_ON_9296_FORD_FSERIES_AND_BRONCO.html

I can see its possible to change the vehicle constant with the cluster. Now I thought I had it figured out and changed it once......I definitely did not due my math right and now my speedo reads way to fast. Can someone please help me determine what the constant should be that I am changing to?

Current tire size is 305/65/17 and the constant the cluster showed was 1020.

Thank you for the help.

 

[just kev]

I'd love some info on this as well. I'm sure my slow speedo is due to a combination of gearing and tires but it would be really nice not to need to add 5-15 mph do determine my speed.

 

[gtbensley]

It's very easy to change in a older ford, I just suck at math and can't figure out what the value is that I need to assign to the cluster to make it reedd correctly.

 

[ujoint]

How did you change it the 1st time?

 

[gtbensley]

I know the procedure......its not that part that I am having an issue with. The constant which is the number I need to put in the cluster is where I am falling short and have no idea how to reach that number. I posted a link in the first post that has the directions but I didn't have luck with all the math.

 

[just kev]

I know the procedure......its not that part that I am having an issue with. The constant which is the number I need to put in the cluster is where I am falling short and have no idea how to reach that number. I posted a link in the first post that has the directions but I didn't have luck with all the math.

I would assume the constant is a multiplicative number (not additive as the shift isn't an additive constant)

If you can give me 5 data points (say speedo speed at 20, 35, 45, 55, 65 and actual speed at those points (easy to do, just get some
to drive next to you while your talking to them on the phone and have them record the actual speed)) I'll give you a good multiplicative constant (making the assumption that it's simply linear).

What I need to know is how do I put said constant in the computer/speedo system (on a 97)

Does any one know what the computers actual formula is?? EDIT I'll read the bronco link info right now, sorry I missed that

kev

 

[just kev]

I just read the info from the link if you give me your diameter (i.e are they 33's 35's, etc) i can figure it out for you (or I can put the
math in here and you can do it)

 

[just kev]

Ok I hunted down the tire diameter for the 305/65/17's it's 32.61 inches. So in one revolution of that tire we'll sweep out about pi*diameter/12 ft
(remember that the circumference of a circle is given as pi*D, and we're dividing by 12 to go from inches to feet).

so 3.1415926535*32.61inches/12 inches = 8.5373 ft. Next we know that there are 5280 ft per mile so 5820 ft / 8.5373 feet = 618.4625.
So the tire will rotate roughly 618 and a half times per mile.

According to the link you posted above the constant for your speedo is given by tire revolutions per mile * .0135 = 8.3492.

Note this does not take into account any gearing changes for your vehicle and claims to be good for a 92-96 ford f series or bronco.

Hope this helps and makes a little sense.

 

[gtbensley]

Just kev, thank you very much for laying that out for me...yes it makes sense to me know, although I am not sure I could duplicate the finding myself. From all the research I have done I was coming up with around 9.0 but couldn't verify that was correct. I will go program the module and see where that gets me.

It looks like your 97 is slightly different due to it being obdII I know a dealer can use IDS to set your tire size on that.

 

[just kev]

No problem - i'm new here so it's nice to actually be able to contribute something.

There is one 'mistake' with my calculation. According to the link the revs/mile (my 618.5) number above. Is really supposed to be "revs/mile @45mph" which is some sort of modified number that I don't know how the tire peeps calculate. What your link suggests is to go to your tire manufacturers and get the 45 mph revs/mile number

If you did that and we call that number x then your constant would be x * .0135.

One other thing is that if we look at the end of the info in your link it shows the constants for a specific BFG 32 and 33 inch tires being 8.86 and 8.59 so one would expect the actual number for you to be around 8.7 but without the weird 45mph number from the manufacturer that's all I could do

 

[gtbensley]

It's a hell of a lot closer then 10.20 which it started as and the 6.87 I somehow managed to land on!! Rpm of the tire for one mile should make no difference if its 45mph or 60mph I would think. The tire is still rolling over the exact same distance correct?

 

[just kev]

I totally agree that it shouldn't matter but I wonder if it's some weird standard that has some fudge factors built in (friction precise, tire geometry, tire flex??, etc). In an ideal world my number is correct (or so I think

 

[gtbensley]

Well thank you again for the help. I really apreciate it.