ABS tire size change - Dodge Cummins Diesel Forum
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post #1 of 31 (permalink) Old 01-23-2020, 12:26 PM Thread Starter
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ABS tire size change

Hey guys. I bought a smarty jr a month ago and last night I finally got into the abs part to change my tire size. From factory my truck came with 265/70/17 tire size. I currently have 285/70/17. I know it didn’t change my Speedo much going a size bigger, but I figured I have the capability to change the Speedo to read correctly, so why not.

When I got to the point to change the size, the smarty read the current size as 30.7 (set from factory). I googled a tire size converter and found that 285 is 32.7. Plugged that in and finished up. Then I googled what size 30.7 is Just to double check everything and found its closest to a 245. That’s quite a ways off from 265 and my Speedo was probably further off than I thought.

How can that be? Did factory set the size smaller than the door sticker size? Could the smarty have read it wrong? AnywAys, I feel like my speed at 60 is different now compared to before. Maybe I should have a sheriff radar me and pull me over to see how I compare to his radar? Lol


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post #2 of 31 (permalink) Old 01-23-2020, 02:53 PM
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Where I live in WA there are numerous speed limit signs with radar built in, generally they all only work under 40. But that's how I check mine to get close.

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post #3 of 31 (permalink) Old 01-23-2020, 03:46 PM
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Get a free GPS speed app on your phone . It will accurate enough . Or Google Maps/Navigation

Compair

I had to play with the Smarty settings like 4 times to get it correct / accurate . There is allot to the circumference of a tire thats rolling on the road.
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post #4 of 31 (permalink) Old 01-23-2020, 04:05 PM
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Smarty does the tire size in a terrible way. Best is to use a GPS app and dial it in that way either going up or down in size until you get it right. The biggest problem is, not all tires are the same height (different brands all same size can be slightly different height wise) and using the height is a poor way to do it.

Closest way to set the smarty is to measure your actual tire height and start there or go to the mfg website where they list size details in depth and find out what they list the tire height as.

That said, rev per miles is king and how they should have done it. How I've always done it for years on other brands with SCT and such. It's much more accurate.

Just interesting info, on a 305/55/20, size I run, the tire calculators will tell you the tires are 12" wide. MFG spec puts them all between 12.3-12.5" depending on the brand. Height also varies a few tenths of an inch as well.

So if you ever buy a new car, finally replace the tires and wonder why the speedo is now off a hair, you know why! The tire sizes are just as bad as shoe sizes. No exact specs, just sorta close ones.
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post #5 of 31 (permalink) Old 01-24-2020, 04:16 PM
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Quote:
Originally Posted by mhoward View Post
When I got to the point to change the size, the smarty read the current size as 30.7 (set from factory).
And here I thought someone had been into my settings before me when I went to adjust mine for the 315/70R17s I just put on the back. Mine showed up at 30.4" stock. I adjusted it to 34.4" and now my speedometer is off by about 6mph over my GPS confirmed speed at 70mph. (Previously it was about 3-4mph over with the stock 265/70R17s at 70mph) Now I'm curious if there's a speed bias build into the instrument cluster that displays a higher speed than the odometer is counting. Older cars and especially motorcycles are notorious for this.

I'm going to adjust it down to 31.8" and see if that fixes it. But if there is a displayed speed bias, it'll definitely throw off my fuel burn data. (At this point I may have to log the displayed distance traveled over a known run to see if it's off there.)


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post #6 of 31 (permalink) Old 01-24-2020, 04:24 PM
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See the info I mentioned, your tires probably are not that tall.

Further that I didn't even talk about, is usually the data supplied by the tire MFG relates to a specific size wheel (i.e. they may measure the tire mounted to a 9" wheel but will fit an 8-11" wide wheel which slightly alters the height depending on which way you go.) They usually mention what width wheel they measure on.

Further to that, most usually measure at max inflation on the sidewall with zero weight on the tire. So running a max 80psi tire at 45psi with weight over it is going to end up shorter than their measurement.

Why this is the worst way to set the tire size. Literally awful. Revs per mile stay the same regardless of PSI or wheel width, only changing as tread wears some which that wear can usually be accommodated for pretty easily.

There are just WAY more variables than a tire calculator height gives you.

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post #7 of 31 (permalink) Old 01-24-2020, 04:56 PM
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Quote:
Originally Posted by nosympathy View Post
See the info I mentioned, your tires probably are not that tall.

You're absolutely correct. My tires are most certainly not that tall. As for the tire size being a "terrible" metric to use, it's the same as revs/mile as far as the computer is concerned. Just another variable that gets converted into whatever format the abs computer uses to do its speed calculation. (Distance/Time)

I can play around with finding the "correct" tire height to enter into the computer for the tires that I'm running. My concern is if there's a differential between what the computer "knows" as far as distance traveled vs what it displays as an instantaneous speed.


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post #8 of 31 (permalink) Old 01-24-2020, 05:19 PM
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Originally Posted by skymasteres View Post
Quote:
Originally Posted by nosympathy View Post
See the info I mentioned, your tires probably are not that tall.

You're absolutely correct. My tires are most certainly not that tall. As for the tire size being a "terrible" metric to use, it's the same as revs/mile as far as the computer is concerned. Just another variable that gets converted into whatever format the abs computer uses to do its speed calculation. (Distance/Time)

I can play around with finding the "correct" tire height to enter into the computer for the tires that I'm running. My concern is if there's a differential between what the computer "knows" as far as distance traveled vs what it displays as an instantaneous speed.
While it is a metric that gets converted to whatever the computer wants, like I said the revs per mile never change regardless of multiple factors vs the tire height does, so revs per mile would be a one and done entry and tire height isn't. I explained that all pretty clearly lol.

You could probably figure out what the smarty is calculating revs per mile at with a little math, then figure out what the correct revs per mile is for your tire and work backwards.

Most car computers I've messed with always calculate speed by using revs per mile. How the car MFG usually program it as it's the easiest way.

Smarty is taking whatever the stock revs per mile is for the OE tires and coming up with 30.7 which means nothing unless you can find the OE tire specs for revs per mile and calculate that way.

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post #9 of 31 (permalink) Old 01-25-2020, 10:23 AM
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the ecu monitors the wheel sensor counts and converts that number to speed. in hpt it took several adjustments to get it nailded down to +/-0.5 mph. torque has a 1 digit resolution, better than most gps apps which allowed me to zero in on that resolution. of course as your tire wears down, counts go up.
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post #10 of 31 (permalink) Old 01-27-2020, 07:45 PM
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Quote:
Originally Posted by steve05ram360 View Post
the ecu monitors the wheel sensor counts and converts that number to speed. in hpt it took several adjustments to get it nailded down to +/-0.5 mph. torque has a 1 digit resolution, better than most gps apps which allowed me to zero in on that resolution. of course as your tire wears down, counts go up.
Steve have you noticed anything by the way of a speedometer vs odometer bias in your logging? i.e. Higher displayed speed vs distance recorded?


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post #11 of 31 (permalink) Old 01-27-2020, 10:58 PM
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Something else about speedometers are there is a tolerance built in to them from the manufacturer. I work for a factory that made dash clusters for Honda, Mitsubishi, and Chrysler (that included Dodge Ram at the time). I was in the Quality dept. and I performed QA checks on finished lots. The ECU sends a signal to the cluster and it converts that in to a MPH. Speedos had a +/- spec they were allowed to deviate. We checked them at 30, 50, and 85 mph or KPH depending on where the destination country was. The higher the speed the broader the spec got. At 85 the spec was +/- 9 MPH. You could be 1 or 2 off at 30 MPH and then 9 MPH at 85. Makes you feel really great about spending $75K on that brand new Ram, right!
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post #12 of 31 (permalink) Old 01-27-2020, 11:17 PM
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Quote:
Originally Posted by nosympathy View Post
Revs per mile stay the same regardless of PSI or wheel width, only changing as tread wears some which that wear can usually be accommodated for pretty easily.

There are just WAY more variables than a tire calculator height gives you.
Sorry, but the revs per mile is based on tire inflation and load, both of which affect the number.

Still, revs per mile is, like you pointed out, far more accurate than the overall diameter.

Maybe if we start telling people that the brain is an app, they will start using it?

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