I wasn't sure If I should post this in Tires or Computer chips forum, so feel free to move it if needed.

I changed tire sizes from P255 70R16 to LT265 75 16.

According to all the tire calculators I should have gone from 30.1" tall to 31.6" tall.
I was going to recalibrate my speedo with my Superchips tuner, and when I measured the Tire height on the truck sitting on the concrete, The actual measured distance was 30.75". (I also got new rims that are 16 X 8 instead of the OEM 16 X 7's)

Now I know the tire squats a little from the weight of the truck sitting on them, so I just calibrated the speedo for the actual 30.75 measurement. Was that correct, or should I have plugged in the 31.6" figure that the calculators give you?

BTW, they are Nitto Terra Grapplers, and they are MUCH larger looking than my Grabber 255's. I wonder if those old tires were really 30.1? I might measure my spare to see.

 

I beleive you should have used the actual measurement of the tire before it was mounted on the truck.

 

Thanks for the reply,

If I did that though, and added the extra almost inch, wouldn't that give me a false reading on my speedometer? The tire aired up and under the weight of the truck is 30.75, not 31.6.

Will the tire gain height as it rolls under speed?

Not doubting your input, just confused

 

I'm basing my answer on the fact that tire manufacturers measure the tires before they are mounted on a vehicle. THis eliminates the variables. For instance, lets say you run your tires at 45 psi on your F-150. Now if someone else runs the same tire a 35 psi on an F-350 deisel, it is not going to measure the same height while mounted on the different trucks because of the difference is air pressure and the weight difference of the two trucks. I beleive if Superchips wanted you to measure the actual height of the tire while inflated and mounted on the truck, the instructions would say to do so.

 

Recalibrate the speedo to the actual height the tire manufactur says. The reason for this is because the whole reason for recalibrating your speedo is because your tire's circumfrence will be bigger. The circumfrence of the tire will not change with tire psi unless you start over inflatting and make them look like balloons. The tire squat you refer to is normal and this will not affect the actual circumfrence of the tire and that is why you need to recalibrate the speedo in the first place. Your tire is rolling farther with each revoloution increasing distance covered therfore increasing speed therfore giving you an inaccurate speedo reading. Hope this answers your question.

 

Thanks for your input

This is the Thread where Mike Troyer addressed the issue:

http://www.f150online.com/forums/sho...hreadid=147000

Long story short, you should measure the actual height with normal air pressure on the rim, not what the tire calculators say it measures.

 

Wow, the computer is running the speedo? I didn't know that. LOL.

I thought that it was still a cable coming from the tranny.

That is awesome though, no more having to switch out that little speedo gear!

 

badogg - that only applies to the newer trucks with the digital odometer. The mech od trucks still have the speedo gear.
~j

 

Most vehciles still use a vehicle speed sensor, but the computer reads it and then send it too the cluster (or sometimes the other way around) and the wholecluster is electonically controlled. Some applications are using the ABS wheel sensors to determine speed too. Anyway....

I slightly disagree with Mike T. on this. If you are going to use a measured tire diameter, at least measure side to side to avoid the flattened area by the contact patch. Better yet, validate your speed with a GPS and adjust accordingly.

 

rbraughn


I'm looking to do the same w/ my 01 screw 4x4 3.55 xlt, stock 255/70's goodyears are shot and want to put on some bridgestone dueler HL's 265/76/16's.

Was this an easy process or is it more worth it to get a local shop to recal.?

anything else you noticed in the change?

txs

 

Wow,

You brought this back from the grave didn't you?

If you have a Superchips Tuner, it is as easy as hitting an arrow on the tuner until the correct size is shown and then pushing the "yes" button.

Measure the tire on the rim, properly inflated, not under the weight of the truck, and then program that in and you should be fine.

The only thing I have noticed is if you check for DTC (trouble) codes I get a code that says "tire/axel out of acceptable range.

Someone said that is because the computer sees this new value and thinks something is outta whack, but it should be fine. It wont throw the Check engine light, you will only see it if you pull the codes with a reader.

 

Does anyone know how to re-calibrate the speedometer on a 2002 F350 7.3 Powerstroke? I have a 15" lift on this truck with 38" tires, so right now the speedometer is more than 10mph off at 60mph. I have a Superchip in this truck, but for this model you can't adjust for tire size. Will I be able to re-calibrate it and if so, how? Thanks in advance for any help.