Apples

I see I didn't catch the last few posts... glad to hear it went well! and nice to hear them going that extra mile.

Only curious, since I used a Superchips FlashCal to adjust for my new tire size:
Why enter a measurement other than the actual tire size?

kwhite1109

Your guess is as good as mine. I know that I initially programmed it for 35" on the spot and my speedometer was off 6 mph at 65 if I remember correctly. I adjusted it down to 33.25 and then it was spot on with two gps apps and my handheld gps.

I think I have seen a few posts about why this happens with the hypertech but I am not sure where that was.

Apples

Well, sheesh. There you go!

SilverSurfer15

it takes the literal physical size of the tire, your standard 35" mud tire is typically ~33.75" mounted on a truck, they run smaller than advertised. Always have, some companies are worse than others, and you will probably see some saying "true size". But if you measure an unmounted 35, its probably going to come in around 34.25 with no weight or anything applied. Mount it on a wheel and slap it on the truck, even smaller as the weight is applied. Also, the tools aren't always super accurate either. You usually have to mess around with it, a good starting point is subtract 1" from your advertised size and go from there.

its 'better' to use revs per mile, as they the manufacturer should be able to provide you with that. But not all the tools will accept that.

The bottom line is you want the exact size of the tire as it is rolling down the road with the weight of your vehicle on it. The slightest .25" of change (different brands in the same size) will technically throw your speedometer off. Although minimal, thats why people used to say they were only 95% accurate...

Apples

That's what I was shooting for, but then I discovered the FlashCal only accepted .25" increments of tire diameter...

I should find center on one of my rear wheels (the PCM's speed sensor is attached to the tailshaft housing of the trans?), and measure that radius... X 2 for diameter... and see how close it is to TheTireRack's posted Firestone data.

Thanks, SS, for the reminder of what's really going on. Hahrr...

JB 1 TIME

Excellent information. I am going to do the exact same thing as the OP (3.31s to 4.10s) within the next couple weeks. Just need to find some shops around me. The only one I know of that would probably be able to do this (other than the mechanic I take it to for maintenance) would be 4 wheel parts. Anybody ever dealt with 4 wheel parts?

Newfordguy29

I've been shop searching too they all give the same answer "uhm never done that, Yukon says that isn't possible" yet I called ford racing tech support and dude didn't hesitate to tell me it'll bolt right in, f150 15+ and mustang 15+ have the same differential. Jus look up off road shops or differential shops until you find a winner

SilverSurfer15

I wouldnt recommend 4 wheel parts, I think this is a job where you want a mechanic who specializes in axles to do the work. Its not overly hard, but its tedious and there a ton of short cuts you could take. A smaller local place will probably be your best bet.

JB 1 TIME

Yeah that's what I was thinking. I found some shops just now that specialize in drivelines, axles, etc. I will probably go to one of them instead of 4 wheel parts. Most likely won't even go to my usual local shop because I don't know if they could handle the job correctly.

rts9364

Exactly. I've found a small place to do mine (4.10s) that seems like a good scenario with a respected installer. Just today we agreed he's going to source all Ford parts and got it set up, so I am taking mine in in another week or so.

Our local 4 wheel parts has helped me out a time or two now, but the franchise nature of their business is somewhat scary when talking about anything requiring skill; you just don't know what you are going to get.