So by your theory, if I load my truck, my speedo should be off from when it's not loaded. I just don't think THAT little difference would make a hill a beans to the speedo, the gear ratio or the ECM. If it were so critical, you couldn't just plug in a tuner and correct the speedo. My speedo is right on the money and I set it up for the tire measurement from top to bottom.

This isn't rocket science.

The calcs everyone has listed are easy ways to get in the ballpark of where your speedo will be, height and width when comparing two size tires. Once you decide on tires and size, if over or under sized a programmer will take care of the rest without all the math.

Let's agree to disagree...

 

No, that's not what I'm saying. What I mean by loaded is under the weight of the vehicle, regardless of load. When you start to jack up your truck, the tire is going to raise up a bit before the tread lifts from the pavement. That distance is what I'm referring to. But you're right, if you want to use the inflated tire diameter off of the truck to make your speedometer adjustments, go for it. You'll only be reading less than your actual speed. I'm just attempting to share some knowledge. Please forgive me.

 

Great point. I had not thought of this until you mentioned it. That is

The difference between loaded and not loaded is very small. The big difference is from off the truck to on the truck.

The bottom of the tire is flat on the pavement. The radius would be measured from the center of the tire to the pavement. That distance is shorter than from the center of the tire to the top of the tire.

Loaded or unloaded your splitting hairs, but if the tire is low on air the radius will change, and the revolutions will change along with the radius.

 

I think it's great that everyone is here and passes on information, I just simply disagree that the difference is that great and therefore can't make a difference. It's nothing personal, I just respectfully disagree.

I know what you mean by loaded and Loaded is Loaded. The tire is loaded by the the truck and then by the payload...by your theory, they should both change the rolling circumference. You can't use one load and ignore the other. Both are minuscule to circumference so I don't buy it and that's ok. I just think you're theory is bogus.

I never said anywhere I measured the tire OFF the truck. I take measurements for the Tirepaq while the tires are ON. This IS loaded with the truck weight and if I add a 1550lb load in the bed, by your theory, my speed will be off because I just loaded the tire more and changed it's circumference. I ain't buyin' it.

 

This is my point exactly! Loaded is loaded no matter what the load is and none will change the tire enough to make a speedo off. Unless they are all flat then you would base it on the rim height and speed would not be an issue anyway.

How much does your truck weigh!? Using Windsors theory, let's say your truck weighs 4000lb, divide by 4 and the average load on each tire is 1000lbs, now add 2000lbs to the bed and you just added another 1000lbs to each rear tire. Let's say for each 1000lbs the tire radius is reduced 1/4". If his theory is correct, the speedo is changed two fold. Not having any REAL numbers that could mean anything, but I have never seen my speedo off when hauling a load. How about wreckers, they haul half the weight of the vehicle behind them on just the rear tires. Will they get a ticket for speeding or going too slow? My point is, it would take a huge amount of ANY load to change the tire enough to throw off the speedo. It just isn't enough to change it.

Again, I agree to disagree...

Windsor's theory is the DEFINITION of splitting hairs.

sau4u, any of the calcs will get you in the ballpark and a tuner can adjust your speedo etc., so don't worry about being SO critical about this and that. There is no two speedo's that are exactly the same anyway but are within a range of acceptability. Ford and B.f. Goodrich are not designing and building Swiss watches here, with precise gears. There are variables and they know it.

 

It's never easy to communicate these concepts over the internet, but I think we're saying the same thing. Payload can alter the radius, but even a quarter inch of compression from an unloaded vehicle to a fully loaded vehicle only translates to a 1.5% difference. However, if you have a payload that is heavy enough to reduce the radius that much, it's a lot of weight and you probably should be airing up for that load anyway. That 1/4" would only translate to about a mile per hour difference at 70, and you'd never notice it. Most speedometers aren't that accurate anyway. The truth of the matter is the radius will change with speed to some degree as well, granted only slightly.

My point was that using the manufacturer's published diameter to program your speedometer will yield inaccurate results. The published tire revolutions per mile date, while not perfect, will yield better results. Ford uses revolution per mile data as a parameter in the PCM, ask any tuner worth his salt.

 

Here in lies the problem.

The 1/4" was just an out of the air number, I have no REAL numbers and with hundreds of manufacturers making tires with different tread and different height, there are enough variables that up to and including tread wear, you could reprogram till the cows come home and I doubt you'd see any noticeable difference in speed read at the speedo.

I use the "tape measure from the ground to the top of the tread while the tires are on the truck" method to set my Tirepaq and I'm sure it's as accurate as any. In fact, that is the way SC states in the user manual to do it.

 

why does bluejay get his user name in bold?

 

I'm guessing because he's a moderator.

 

Couple that with the fact I'm Cajun and can't use my hands to "s'plane tings lik dat" I lose half of my communication skills. We're using the same basic method, just different approaches to get the same end result.

 

...

 

You make a good point. I'm just trying to put it into perspective. The foot print of the tire will flatten more with the first 100 lbs than it will with the last 100 when it's already loaded with 1800lbs. For example, the axle may drop an inch after the tire touches the pavement until the truck fully rests on the tire, but may only drop an additional 1/4 inch when the cargo load is added.

I've never measured this, maybe I'll try it some day for fun. My opinion is that the inch is significant, but the 1/4 inch is splitting hairs. I agree with windsor when you load the initial curb weight of the vehicle, but then agree with you when the cargo is loaded in.


I see, I thought you measured off the truck. I though the tire manufacturer measures a tire off the truck. Your method sounds like a good method. So I got off my but and measured my tires.

31" Ground to top of tire ( 15.5" radius)
14 3/4" Ground to center of rim (14.75" radius)
32" Front to back (16" radius)
31.9 Tire calculator for 275/55/20 (15.95" radius)

Sorry, I don't have 1400lbs to load my truck. I think your method works because it is easier to measure to the top of the tire than to find the center of the wheel. If your measuring your tires on the truck with a full cargo load, then your already applying 50% of windsor's theory.

That puts us 50% in agreement.

 

I measure mine on the truck and no cargo load at all, only the truck.

I think we've beat this horse enough.

Like I said before, let's all agree to disagree, even by your estimate of 50% (which incidentally, I think is closer to 30%) heehee! . Cheers