how accurate is smarty's abs calibration?

[nickleinonen]

well, i got my new toy today , and loaded #2 in, as well as calibrating the speedo via the abs thing.



my truck came stock with the 265/70/16 michelins. the smarty showed that i had 30. 7" programed in the ecm for abs size



my truck has had 315/70/17's [34. 5" according to bfg] under it since day 4 of owning it. i had previously calculated my speedo was out by about 12% going by radar display trailers. . now i don't have a gps unit to do a accurate speed check, so i set the cruise at 60kph and timed myself for 1 minute a few times, and the distance counter is showing at 1. 0km over that minute, so it seems pretty accurate. . [smarty set abs to 34. 50"]



now, with that all said, what have other smarty owners with 315/70/17 or 35x12. 5x17 tires calibrated their speedos to. . ?



[i am going to borrow a gps if i can and check it myself, but until i can get that done, i want to know i am somewhat in the ball park on the setting]



oh yeah. . smarty = #2 feels like stock until you put the boot to it. small puff of smoke and off she goes

 

[JApol]

from what i have heard you can recalibrate your speedo with the smarty as the tires wear.

 

[Bob4x4]

The smarty will allow you to enter the tire height out to the second decimal place... ... ... so yes it is very accurate. There is enough variation between tire brands of the same size that the gps is the best way to verify.



Bob

 

[Gary - K7GLD]

I found with mine that the actual, physically measured tire diameter when entered into the Smarty was still off significantly from actual distance registered on my odometer - no big problem, it provided a starting point that soon got me to the setting that was accurate.



One interesting fact that surfaced during the test runs, and again while towing our 5er, was how much the added weight of the 5er changed the effective diameter of my 285 TOYO's, and threw the odometer off again...



And of course, Smarty users will remember that the changed ABS setting will revert back to it's stock setting if Smarty programming is returned to stock...

 

[JCasper]

And of course, Smarty users will remember that the changed ABS setting will revert back to it's stock setting if Smarty programming is returned to stock...

I thought that it was supposed to stay even after returning to stock.

 

[Bob Wagner]

no, the speedometer/abs is a vin lock feature

 

[testraub]

Thank goodness for that Silly me :--) I forgot to write down the setting before changing and I swap stocks back on in the spring :-laf I haven't played with my SMARTY much yet, but when the weather gets better... . Look Out Arlington! Oo.



Oh and Japol... You stay off these SMARTY threads til you get yours! hehehehehehe :-laf

 

[JApol]

Don't worry Testraub, I will have my SMARTY in due time. Quicker of Bob would good will one to me Oo. J/K

 

[JVolpe]

I used 34. 4" for my 315s and that seemed to be very accurate. It was also what tire size the Smarty read from the ECM. My dealer had reprogrammed my ABS RPM to 605 based on what I told them. The OD speed difference between the 265/70R/17 stock tire and the BFG 315/70R/17 should be around 8. 74% slow not close to 12%. Just using my favorite calculators.

 

[brods]

no, the speedometer/abs is a vin lock feature

So this means one has to run a performance program to adjust for tire diameter? Can the stock program be run with the smarty still vin locked?







If you want to be accurate, it its the tire's loaded radius that matters. This is the distance from the center of the hub to the ground with the truck's weight on the tire. 2 times the loaded radius gives the tire diameter that the truck electronics will "see". I don't know how much the loaded radius changes at highway speeds, may not be enough to be a factor.

 

[JCleary]

I went outside and measured the tire height on my truck. Front tire, BFG 315, maybe 35-40psi of tire pressure.



I plugged that number into the Smarty, and to my surprise the speed was correct according to the radar unit on the side of the road!



The first time I did it (darn vin-lock feature, did not know it would go back once returning to stock) I used a buddy's GPS. I played around with different settings until it was spot-on at 70. I did not bother to record the number.



Either way worked for me, but the easier was definitely measuring my front tire.

 

[nickleinonen]

The OD speed difference between the 265/70R/17 stock tire and the BFG 315/70R/17 should be around 8. 74% slow not close to 12%. Just using my favorite calculators.

that's what i thought it should be, but the stock programing had the tire size at 30. 7" [like a 245/70/17] and not somewhere in the 31. 4" range that michelin specs their 256's at. . no one at work has a gps i can borrow, so i think i will borrow a new garmin 60cx from costco for the week. . [yeah, i know, that isn't nice]

 

[nickleinonen]

well, i managed to borrow a gps from someone at work. . man was 34. 5" way out. . i set it a few times and now i have it somewhere i am happy with it. .



33. 75" set in the abs adjust, it reads pretty much spot on from 50kph to 80kph [within 0. 5kph], at 100kph on the speedo, it is off by about 1kph depending on where on the line it is for the needle. . at 140kph on the speedo, i am sitting at 138kph on the gps. . since 99% of my driving is in the city below 80kph, i am happy where it is. . if i drove more at hwy speeds, i'd adjust it down to say 33. 70" to be a little more accurate at those speeds. .



that's all with ±38psi tire pressure in the rear [and i should drop it another 1-2psi to get more even wear]

 

[Bob4x4]

When the taller tires ie 315's wear the height changes considerably from new.



Bob

 

[JCasper]

With my 35" Toyo M/T's I have mine set to 33. 75 right now. It is within 1 MPH at 60 MPH. That is GPS checked.

 

[brods]

Nick, if you have a chance, measure the distance from the ground to the center of the hub on your back wheel and multiply it by two. Does this come close to the 33. 75" you had to use? On paper it should, but we all know the real world usually throws some curves.