cegha

Alright guys, this has went on for years, but I drive so little I never cared. I wanted to get this fixed before an upcoming trip.

Basically I have Steeda Ultra Lite 18x9.5" wheels with Sumitomo 275/40/18 tires on all 4. According to rev per mile calculators I need to set my car to 756.8. However I remember I tried setting mine that low & it read horribly wrong on my speedometer. I have my revs per mile set at 771 currently & it reads almost spot on. I had it down at like 765 for a while now & it was slightly off, & I just bumped it up this week to 771 & it almost reads spot on but still not quite.

I measure my speedo against a Passport 9500IX radar detector that I have set to the speed measurement GPS feature as the default "screen" so to speak. I try to travel at a steady speed & glance back and forth between the 2, but still doesn't quite match up. Seems like speedo may now read a little faster on lower speeds & reads accurate or slightly under at higher speeds.

Main thing I want to know is why the hell can I not put in revs per mile a tire calculator is telling me & have it work? I have a Diablosport tuner that I do the edits in. Can anyone explain or help me figure out what is wrong.

jpplaw

The calculator is showing you the revs per mile for a perfectly round tire, which your tires are not (once they are on the car). The calculator doesn't factor in the contact patch.

Mike@DiabloSport

How much of a difference are we talking about here?

There is no science to this, ideally you edit revs per mile until the speedo is 'correct'. If its high in one spot and low in another, thats a Ford problem thats not going to be corrected with tuning unfortunately.

Thanks

cegha

It appears at least to ME that some spots it appears under & some spots it appears over. Haven't really had it on interstate past 60mph to see difference at higher speeds yet. Mainly only driving to job & friends houses less than mile from where I live.

I will try to get readings at 20, 30, 40, 50, 60, 70 on the dash & let you know what my radar detector tells me i'm doing via GPS. I also have a Pioneer Avic D3 with the wire hooked up for the digital gauges to work, which has a speedo as well. Will compare all 3.

The S197 speedo dashes are very close together so it could be me. Assuming it's not just me, what would make it read under in one spot but over in another, that is a Ford problem? What would Ford have to do to fix said problem?

Norm Peterson

Generally, there is about a 3% to 3.5% difference between the revs per mile that you calculate based on a round and perfectly rigid wheel. Think in terms of the wheels on a railroad car here. For some tire makes/models, the "%error" is a little higher than average, for some it's a little lower.

For a 275/40-18 and not knowing its make, model, or actual OD, I'd start by setting about 780.

Better still is to go to the tire specs page for your tires at TireRack.com or similar sites and find out what your tires' mfr posts for revolutions per mile. Use that information before trusting any calculated number too closely.


Tire distortion is why tires need more revolutions to go a mile, and it cannot be calculated directly from the tires' rolling radius either. Mostly, it is due to the tread compressing circumferentially as it passes through the contact patch.

It's too bad that none of the common "tire calculators" identify this effect.


Norm

TheLockNLoad

You could always lay a cloth tape measure on the ground, drive on to it then measure the circumference on your tire (this will take into account the "flat" contact patch), once you have the circumference in inches you just need to divide that into 63360 (inches in a mile) and that will give you the RevPerMile.

Hope this helps.