Engine/Transmission (1998.5 - 2002) SMARTY and Tire Size Question

[Mick2500]

OK,

I've read everything I can find on the Smarty (Nice work Marco) :-laf I have 315X75X16 tires on my 2001. 5 and the speedo has been off by 10% forever. I just correct it on the fly and figure the 10% difference at fill ups.

Will the Smarty allow me to adjust for my 315's? 34. 6" is the overall diameter and 601 revolutions per mile.

Also, I have been running a TTPM ever since they came out. My HO runs very smooth and I have some 275's in the closet just waiting for the first valve adjust to put them in. Would the Smarty work better WITHOUT the TTPM installed? I'll keep my boost elbow either way.



Thanks,

Mick

 

[darkhorse]

OK,

I've read everything I can find on the Smarty (Nice work Marco) :-laf I have 315X75X16 tires on my 2001. 5 and the speedo has been off by 10% forever. I just correct it on the fly and figure the 10% difference at fill ups.

Will the Smarty allow me to adjust for my 315's? 34. 6" is the overall diameter and 601 revolutions per mile.

Also, I have been running a TTPM ever since they came out. My HO runs very smooth and I have some 275's in the closet just waiting for the first valve adjust to put them in. Would the Smarty work better WITHOUT the TTPM installed? I'll keep my boost elbow either way.



Thanks,

Mick

If you have an accurate way to measure you distance traveled (GPS or a measured course) you can use the Smarty to recalibrate your odometer. By using the even numbered programs you can use Smarty's fueling & let the TTPM do the timing. But you could sell the TTPM & use the odd numbered programs to take full advantage of Marco's remarkable work.

 

[darkhorse]

darkhorse,

Thanks! all the 5 star Dealers I've talked to tell me I can't adjust for anything over the 275X75X16 tire size. Guess they lie a little. Guess I'll just have to take a gamble on the Smarty with some of my tax refund.



Mick

I don't think they are lying, Mick. They just don't know what's possible. Give Bob Wagner a call. He can answer all your questions better than I can.

 

[Bob Wagner]

you measure from the ground to the top of your tire, says its 40" you enter 40. 00 into smarty and you are done.



any tire height between 25" to 45" in . 250 increments

 

[Gary - K7GLD]

A related question:



My truck in original OEM form. according to the door sticker, was equipped with 745 size tires - but mine had 765's right off the dealer's lot. As far as I can tell, my MPH is right on the money, according to several roadside radar traffic controllers - but my odometer consistently reads about 10% FEWER miles than actually traveled!



How can that be, is it possible due to different sensors for MPH vs distance traveled, for differing results to be displayed?

 

[Mick2500]

Thanks Bob!

 

[Bob Wagner]

Hey Mick, It was good to see you posting again. Need to keep in contact with the old time guys :-laf

 

[Mick2500]

Hey Mick, It was good to see you posting again. Need to keep in contact with the old time guys :-laf

I apologize Bob, I've been putting a LOT more miles on my Beemers than my Cummins lately. I think I'm almost to 12K on the truck. LOL



I talked a couple of folks (where I used to work) into buying Cummins and just had to get back in here to see how they are doing. :-laf



Mick

 

[Mad Bomber]

A related question:



My truck in original OEM form. according to the door sticker, was equipped with 745 size tires - but mine had 765's right off the dealer's lot.

Do you mean the 245's V the 265's? That was a tire upgrade option. My door sticker also shows the 245's but the 265's is what's on there

 

[Marco]

but my odometer consistently reads about 10% FEWER miles than actually traveled!

Gary,

If your odo is 10% off means that your speedo is also 10% off.

Speed / time = distance. Fact is that it's almost impossible to find a correct setting for the speedo (thus the odo) without the help of an GPS that reads the actual speed. Don't ask how I know... .



Marco

 

[Gary - K7GLD]

Gary,

If your odo is 10% off means that your speedo is also 10% off.

Speed / time = distance. Fact is that it's almost impossible to find a correct setting for the speedo (thus the odo) without the help of an GPS that reads the actual speed. Don't ask how I know... .



Marco

Thanks for the info Marco - I was sorta hoping there were separate sensors for the speedometer and odometer - apparently not. I can't figure why the 10% difference between the 2 - I have pretty well verified the difference, and guess the need for speedometer accuracy takes priority over the odometer.

 

[DonS]

you measure from the ground to the top of your tire, says its 40" you enter 40. 00 into smarty and you are done.

That didn't quite work for me, but was good for a first approximation. Measuring that way (level on top of tire, measured from ground to level) gave me 32. 5" with my BFG AT/KO LT295/75R16 (55psi, front). I entered 32. 50" into the Smarty, but was still off by about 2% (I guesstimated the speedo at 49 MPH, while the GPS said 50).



Going to 32. 75" got it a bit closer: 0. 8% based on a 440 mile drive (440. 5 on the odometer, 437 on the GPS).



The numbers above don't quite "work", so I'm not sure what's really going on. I can only imagine that part of the discrepancy is due to the tire's shape varying with speed - during my initial 32. 50" test I never exceeded 50 MPH, while the 440-mile trip at 32. 75" was mostly above 70 MPH.



In any case, the initial "ground up" measurement was a bit too small. Fortunately, it's trivial to tweak the numbers with the Smarty!

 

[Marco]

The accurancy of the speedo varies with the actual speed!

I have 33'', setting the ABS to 32'' the speedo is dead on at 60MPH, at 70 it's a couple miles too slow, at 80 three, at 90 five... .



I know it sounds odd but that's the way it is.

That would explain what you're seeing Don, at 50 dead on, at 70... .

 

[DonS]

The accurancy of the speedo varies with the actual speed!

I have 33'', setting the ABS to 32'' the speedo is dead on at 60MPH, at 70 it's a couple miles too slow, at 80 three, at 90 five... .



I know it sounds odd but that's the way it is.

That would explain what you're seeing Don, at 50 dead on, at 70... .

I'm aware of the fact that speedos are usually only accurate in a small speed range, but (for some reason) I'd usually associated that phenomenon with fully-mechanical devices. I guess there's still the electro-mechanical interface at the needle, even with our mostly-electronic speedos, so my "always accurate" expectation isn't realistic.



Perhaps that's one (the only?) benefit to digital instrument panels.

 

[Mundgyver]

If you want to take the hassel out of it try one these.



http://www.atrol.com/era.htm



I have one and it runs dead on with my GPS. I run 285/16's but this box will allow you to run any size tire you want. It is a 4 wire hook up. Ground, Power, Signal from speed sensor in, Signal to ECM out.



These work very well.

 

[DonS]

If you want to take the hassel out of it try one these.



http://www.atrol.com/era.htm



I have one and it runs dead on with my GPS. I run 285/16's but this box will allow you to run any size tire you want. It is a 4 wire hook up. Ground, Power, Signal from speed sensor in, Signal to ECM out.



These work very well.

That device is similar to the "TruSpeed" in principle, I suppose.



With a Smarty, the hassle is gone. No wiring, no DIP switches. Just plug the box in and enter a number. If the number is wrong, enter another one. For me, the fact that there are no wiring mods is a big plus.

 

[Mundgyver]

That is nice, I did not realize that the Smarty could have just that parameter entered. I know I am planning on getting one when I get my taxes back. I bought the Abbott when I changed out my original factory tires at 35,000. The Abbott was the only thning that would work at that time as I did not want to have DC touching my ECM.

 

[darkhorse]

I'm aware of the fact that speedos are usually only accurate in a small speed range, but (for some reason) I'd usually associated that phenomenon with fully-mechanical devices. I guess there's still the electro-mechanical interface at the needle, even with our mostly-electronic speedos, so my "always accurate" expectation isn't realistic.



Perhaps that's one (the only?) benefit to digital instrument panels.

Wouldn't the discrepancy be the result of the tire diameter increasing slightly with the speed?

 

[Mundgyver]

If you put on a brand new set of factory size tires and measure your seedometer reading against a GPS you will see that when the tire reaches it's half tread left mark, that the speed indicated on the speedometer and the GPS will match almost dead on. New tires have a bigger diameter than old tires and tire manufactures compensate by tread depth against wear rate. This is one reason that the police generally won't pull you over if you are 5 mph over the limit. The mechanics of the tire size when new are constantly changing as the tire gets older and your speedometer is reading the rotations as the tire continually gets smaller in diameter. If the tire is new and thus larger, rotation speed is slower but foward speed at a given 60 mph indicated is faster than 60 mph. As the tire wears smaller the rotation speed of the tire increases as your forward speed slow. Hence we have about a 5 mile an hour difference between new and old tires. This effect can be tracked with a GPS. does this make sense.

 

[DonS]

Hence we have about a 5 mile an hour difference between new and old tires. This effect can be tracked with a GPS. does this make sense.

It makes sense, but I don't see how the difference could be 5 MPH at a "real" speed of 60 MPH. If the speedo went from 60 MPH with new tires to 65 MPH with worn tires, while actual speed is 60 MPH in both cases, that means the tire diameter is now 12/13 of what it was when new. A new 33" tire is now 30. 5" tall. That's some serious tread wear - 1. 25". I guess if you had some big mud tires and drove them down into the belts...



The same holds for Marco's speedo that is "dead on" at 60 MPH, but reads a "a couple miles too slow" at 70 MPH. If the speedo is reading 68 at an actual speed of 70 MPH, that's nearly a 3% error appearing with only a 10 MPH speed increase. If it was all the result of changing diameter, that 10 MPH increase would have to increase Marco's 33" tire diameter by almost 1 inch. A good part of this error has to be in the speedometer itself - somewhere in the path from the ABS pulse train to the pointer position.