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Discussion Starter · #1 · (Edited)
A few things I can't make sense of:

When I run the DR's on the rear, which are a fair bit shorter than my street setup, I've noticed that the 2-3 shift happens at an indicated 71-72mph, versus 80mph spot on, in street trim. What I'm wondering first off is whether that's a math error in the computer, and I'm still shifting at 80mph? or am I actually shifting at 71mph?

This brings up another question I've been wondering: how is the speed calculated? In the old days it was a cable driven by a front wheel - I know it isn't anymore, but does it read off of all four wheels and calculate it, or is it front or rear exclusively? I'm guessing it's not rear only - otherwise burnouts would indicate speed. EDIT: as Bob points out, though - on the Dyno speed is registered, and that's rear-only...

I suppose the question comes up if your front and rear tires are different height - which tire size do you use when you set the wheel size with the Predator?

:spock:
 

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A few things I can't make sense of:

When I run the DR's on the rear, which are a fair bit shorter than my street setup, I've noticed that the 2-3 shift happens at an indicated 71-72mph, versus 80mph spot on, in street trim. What I'm wondering first off is whether that's a math error in the computer, and I'm still shifting at 80mph? or am I actually shifting at 71mph?

This brings up another question I've been wondering: how is the speed calculated? In the old days it was a cable driven by a front wheel - I know it isn't anymore, but does it read off of all four wheels and calculate it, or is it front or rear exclusively? I'm guessing it's not rear only - otherwise burnouts would indicate speed.

I suppose the question comes up if your front and rear tires are different height - which tire size do you use when you set the wheel size with the Predator?

:spock:
:panic:Junior WHATCHA doing looking down at the speedo while going down the Track !!! :blah::tinfoilhat:
 

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Speedo works on the dyno.So it reads off the rear but Junior your situation seems to indicate the front also.
The 8 MPH drop in speed at the shift corresponds with the approx 10% change in rear tire height.
Interesting.
 

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Sidebar question: What does running shorter tires do for your MPH if anything at the track?
It dropped .3 of a sec off my ET. Going from a 20 inch NITTO's to 17 inch M/T ... I went from 11.8X to 11.5X
Better hooking from tires ?
 

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Discussion Starter · #7 ·
Speedo works on the dyno.
Oh, sure enough.

So it reads off the rear but Junior your situation seems to indicate the front also.
The 8 MPH drop in speed at the shift corresponds with the approx 10% change in rear tire height.
OK, meaning that I am actually shifting at 71-72? It does feel like the shift comes earlier, but I didn't want to make too much out of that without something other than "feel".

Interesting.
To you too? :mrgreen:

Sidebar question: What does running shorter tires do for your MPH if anything at the track?
I can't say, because my ET's and trap speeds were about the same as with street trim - except when they were far worse - I had a lot of trouble getting the DR's to stick. I think I'm having trouble heating them up enough.

But the first couple runs last weekend with the DR's were really close to the previous week with street trim: 13.4 @ 105.9
 

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OK a few things,

Smaller tire mean effectivly more gear so with the 2.82's with a short 26" tire gives you the effect of having about an 3.09 to 3.12 ( I think something very close to that ) rear gear vs the 28.5" stock tire.

That means that for the SAME speed you will be turning MORE rpm the Idea is to get our cars into their happy place in the fatty part of our tq curve, that is also why we raise the shift points so that when it shifts we are dropped right back into the fat tq curve.

I noticed when we did the street cruise at Atco with the DR's on the car and a 30 mile loops run, my indicated speed was RIGHT, and I know it is right with my 20's as I use a very accurate GPS to read my MPH. The computer will learn to some degree if it sees something differnt from front to back ( running the 275 40 17's in teh rear and the 245 45 20s on the fronts )

Typically at the track I am crossing the lights at an indicated 120mph ish with my real speed being about 112-115mph. When all I am doing is running WOT and return road it never corrects.

So yea you are travelling slower MPH whe you shift when using the DR's

it reads off the rear wheels.

-Robert
 

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Discussion Starter · #10 ·
OK a few things,

Smaller tire mean effectivly more gear so with the 2.82's with a short 26" tire gives you the effect of having about an 3.09 to 3.12 ( I think something very close to that ) rear gear vs the 28.5" stock tire.

That means that for the SAME speed you will be turning MORE rpm the Idea is to get our cars into their happy place in the fatty part of our tq curve, that is also why we raise the shift points so that when it shifts we are dropped right back into the fat tq curve.
And the opposite too - for the same RPM, you're at a lower speed. So that part makes sense.

I noticed when we did the street cruise at Atco with the DR's on the car and a 30 mile loops run, my indicated speed was RIGHT, and I know it is right with my 20's as I use a very accurate GPS to read my MPH. The computer will learn to some degree if it sees something differnt from front to back( running the 275 40 17's in teh rear and the 245 45 20s on the fronts )
But which will it base it off of, I wonder - I mean, it has to have a known starting point in order to adjust it - let's say you throw some tall front runners on and some tiny DR's on, and then drove around town for a few days - how could it determine what the actual speed is, with no outside information? It has to calculate it based on the programmed tire size, no?

Typically at the track I am crossing the lights at an indicated 120mph ish with my real speed being about 112-115mph. When all I am doing is running WOT and return road it never corrects.

So yea you are travelling slower MPH whe you shift when using the DR's

it reads off the rear wheels.
Exclusively? And I assume not through the TPMS - it must have a magnet on the hub/axle or something, right? There is that ESP receiver that's inserted into the knuckle - is that it? Reads a signal off the hub, or something?
 

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Uses the Toner rings for the ABS system off the Axle Hub.

I think it logs wheel speeds, it has a known good value, and in lm case the known good was the front wheel speed on teh 20's and the rears changed on the 17" DR's

so it used the known valve, fixed the DR rear value and bang all good, if you change all 4 you will not get a correct wheel speed ( in my opinion thats why guys that did the wheel and tire swaps never had a correct odometer reading )

-R
 

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Exclusively? And I assume not through the TPMS - it must have a magnet on the hub/axle or something, right? There is that ESP receiver that's inserted into the knuckle - is that it? Reads a signal off the hub, or something?
Sure the speedo sensor isn't in the tranny? I don't know about LXs, but the speedo cable on my Dart (and on my daughters' '95 and '98 Neons) are connected to the tranny. In the Dart, with different rear gears and different tire diameters, I have to install different speedo gears.
 
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Discussion Starter · #13 ·
Uses the Toner rings for the ABS system off the Axle Hub.

I think it logs wheel speeds, it has a known good value, and in lm case the known good was the front wheel speed on teh 20's and the rears changed on the 17" DR's

so it used the known valve, fixed the DR rear value and bang all good, if you change all 4 you will not get a correct wheel speed ( in my opinion thats why guys that did the wheel and tire swaps never had a correct odometer reading )

-R
Bear with me, because it's close but I ain't there yet - but it's got to be calculated between both front and rear then. And here's the thing: it can see that the front wheelspeed and the rear wheelspeed don't match, so it adjusts one - but how does it know which one to adjust?

Let's say you left the rears stock and threw on some 26" fronts - how would it know that the "known" is now the rear, and not the front?
 

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I could be wrong but isn't the speed read from the tranny? Or am I thinking about the odometer?
 
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Discussion Starter · #16 · (Edited)
Jr I think its a check from last known good run and adjust from there.
But see, how would it know that anything's changed? Let's say for argument's sake that you put 26's all around - the speed would be off, but how would it know that it was - if it's simply reading wheelspeed, then it will calculate vehicle speed based on the programmed tire size. Since the tire size has changed, the calculations will be off, but it would have no way to know that they were. See what I mean? You'd have to tell it that you're running 26's now.

Now if it does see that one set of wheels is now traveling faster than the other, it may try to adjust the readings, but it would have to know which set has changed - and I can't see how it could tell which one changed, unless there's a third input somewhere.

Is there something you are digging for inparticular ?
No, just something that I've been wondering about, and really getting curious for it to make some scientific sense to me... so far I can't point to anything that I can wrap my head around that makes sense. Any of the explanations so far have too many variables or questions.

I'm leaning towards the tranny thing, though - anybody have any way to test it, or find a reference somewhere?
 

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OK previous run you had a rotional speed of X at each corner,

You shut down the car

Swap outthe rear tire to a 26" tall

The computer starts and see Y speed at the rears and X speed at the front, it knows that X is right from the previous runs ( its reading the NSNSNSNS in teh toner rings for the ABS/Traction control that is why 05 cars have to pull fuse 16 or 17 to kill that )

With enough miles it can figure out a correction factor.

I woudl assume that it could do that front to rear or rear to front.

Now you throw 26" all around you are now seeing Z speed, but because that is teh sme at all 4 corners it dont care, so your speedo is now off.

At least thats is how I think it works.

-R
 

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I don't know if this helps but when we changed my gears and it was the incorrect ratio I asked why we couldn't just change the tire size to make the computer happy and Diablo said there is a driveshaft speed sensor or something like that that computes the speed.
 
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