Mods.....This is Superchips Tuner related


Mike,

Just an update on the earlier thread about measured tire height and calibrating the speedo.

At first I measured the LT265 75 16's on the truck at 30.5 instead of the normally reported 31.6.

Then it was said it was probably best to measure off the truck but on the rim aired up, so I re-measured it that way at 31.25", and re-calibrated the speedo accordingly.

Today I checked it against a GPS reading and at 70 MPH, the GPS said 66.5. Well I wasn't sure of the GPS accuracy, so I tested my dads Chevy that is stock, and at 70 MPH, the GPS read 69.8, so it appears to be pretty accurate (the GPS)

Sooo I guess I need to work my measurement back down closer to my 30.5" reading to get it closer.

BTW I still havn't had my buddies hit me with the radar. There's not too many places in town I can get to 70 legally

 

Hi rbraugn,

Something isn't right, that doesn't add up -

The info you plugged into the Micro Tuner based on the actual measurement of the new tire on the rim, inflated to normal pressure but off the truck would have yielded an accurate speedo calibration with the Micro Tuner *if* the original factory speedo calibration was right (or close enough) *and* if the new tire height was correct - which we now know wasn't..........

The Micro Tuner converts that tire height you enter into the actual equivalent revs per mile spec (the way the PCM actually does this) - one of the reasons why it uses 1/4" intervals, so it does the revs per mile spec change right on the money, based on having a 1/4" height resolution (interval).

If this is the gentleman I talked to about this just today by phone, then this new tire was supposed to be 32.0 inches by it's P-metric sizing as we discussed over the phone. But with your measuring it off the truck and getting 31.25", obviously it's 3/4" shorter - meaning it's circumference (the *real* dimension that matters) is actually off by a whopping 2.356 inches from it's "generic" P-metric sizing indicator! If you'll remember from our conversation, the info you got off the vehicle's original tire size from the sticker in the door frame did NOT agree with the revs per mile that's in your trucks' original PCM program -

This data looks like it's confirming that it's a combination of the tire height not being exactly what it's generic P-metric size indicators tells us, along with the original factory revs per mile spec not being correct according to it's original tire size as indicated on that driver's side door frame sticker (maybe it got the wrong sticker??) -- or your Dad's Chevy speedo is off - assuming of course that the GPS gear you are using is actually accurate. If it was made in the last year or so, it should be pretty accurate, but if it's older than that, it could well be suspect.

All in all, given what I found in your stock programming when we spoke on the phone (this has to be the same "Ron"), sounds like you are now basically having to "fudge" the speedo calibration to get it right due to it being improperly calibrated from the factory and the new tire height not equaling it's P-metric size indicator for it's circumference - and we have seen both happen before.

Funny how all this works, eh? Makes it frustrating trying to dial in accurate info, as obviously that isn't working, so you're just going to have to fudge it until it agrees with the GPS data.

It's all in the details................

Just keep plugging in numbers until you get it like you want it, basically - that's all you can do!

Good luck,

 

No mike, I never spoke to you on the phone. This is a follow up to a thread I started a couple of weeks ago. http://www.f150online.com/forums/sho...hreadid=147000

I went from 255 70 16's to 265 75 16's. I would agree that the speedo on the truck may not have ever been accurate to begin with, but the fact that the GPS is only .2 MPH off from My Dad's 2003 Chevy makes me think the GPS it is pretty accurate. I guess I could also check my Avalon against it also.

Remember under the weight of the truck I only get a measurement of 30.5" even when measuring the rear tires under the empty bed. But I went ahead and plugged in the 31.25" value for my re-calibration (measurement off truck on rim).

When I get off tonight, I will put my odometer in the test mode and see what the digital readout on the speedo says compared to the needle.
I guess the only way to really know is the certified Radar, or have them pace me in the patrol car as the speedo is supposedly certified also.

It's not really that big of an issue, but I do want it to be accurate for my MPG estimates.

 

One thing that might help--measure side-to-side rather than top-to-bottom on the tires. By measuring side-to-side, you still get an accurate diameter (and therefore circumference) while taking the load on the tires out of the equation.

Grim

 

Hi rbraugn,

Gosh, I must have sounded like a horse's patoot - I thought that you were the same person I had just talked to on the phone (obviously).

Just hours before seeing your post, I had spoken with a nice fellow named "Ron" on the phone yesterday (Saturday) about this very same issue - only now that I think about it, his aspect ratio was just a tad different from yours on his new tires. At any rate, since that *wasn't* you, my response must have seemed a bit "out there" - my apologies!

I'll re-read & do this again.............................well, no luck - I got a 404 error when I tried to pull up that link, so I can't go back and re-read that earlier thread.............

Using genetic P-metric size calculations, your original tires had a revs per mile specification of 692.1, and the new tires have a revs per mile specification of 657.3 - a significant difference (I wish I had your vehicle's computer code, as that way I could check to see what it was calibrated for from the factory, but no big deal, it really doesn't matter).

The circumference of the original tire shows as 94.4 inches, and the new ones show 99.39 inches, for an approximate height difference of about (in round numbers) 1.5 inches. It also shows the speedo should now read about 57 mph at 60 mph (before recalibrating, and assuming it was right to being with of course - and they usually are off a couple-few % when new) for close to a 6% error rate.

But of course, as always, once again we have to deal with the *actual* tire size, not the indicated size by the generic P-metric calculations.

So.....given all of that, I would have started off with a height of 31.5" in the Micro Tuner. What you plugged in was only 1/4" different than that (31.25"), yet you still ended up with significant error (according to the GPS, which at least agrees with your Dad's speedo).

All I can really tell you is that if you trust that GPS (and if it's not more than about 12-18 months old, I'd go by that for now at least), then simply keep adjusting the Micro Tuner's height entry until you get it agreeing close enough with the GPS - and then whenever you get a chance, have a buddy paint you with his radar & see what's what. We suggest having him paint you at a true road speed of 50 mph, and then see what the vehicle's speedo indicates - this just makes the math easy for everyone, you can do this at any speed, but I don't like doing it below about 40-50 mph in general. Speedo errors are linear percentage-based errors (as you know), so if it's off 3.2 mph at 50, just for example, then it will be off 6.4 mph at 100 (for a 6.4% error).

Grim made a good point (thanks Grim!) about measuring side to side instead of up & down - while it seems an obvious no-brainer (D-oh!), I've never done that. Actually, I generally don't use the Micro Tuner to make any tire size changes on our vehicles, as I do custom tunes on them & thus I just go right into the program and change the actual revs per mile spec myself.

The bottom line is, just keep playing with it until the GPS agrees - and then see if you can get a buddy to paint you with radar, and then compare.

Good luck!