why do different tire sizes confuse the PCM?

M

MikeP

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#1
The only thing I can understand being goofed up is the speedo.. Aside from the actual engine-to-road gearing being changed, which may effect the load on the engine, why does it matter? If you put oversized tires on your truck, all I can understand happening is the load increasing on your engine, due to the gearing change, but this increased load would just simulate, say, towing a trailer.. If the speedo is reading incorrectly, there's no other sensor reading road speed that would conflict with the speedo, so what difference does it make?
I've heard the argument that the ABS would be confused.. Why? All the wheels are spinning the same speed relative to each other and to the speed sensor.. who cares how much ground is actually being covered?
Also, I had an argument with a guy once, I wanted to change out my 15" steel wheels with 16" aluminum wheels, he said no way can it be done.. Okay.. who the hell cares what wheels are on it, as long as they are correct for the car (OEM) - The tire sizes were different, thus, the outside diameter of the tire was IDENTICAL.. therefore, the car had no idea there was a change. Speedo would still be correct.. I did it anyway, and of course, no problems.. Mike.
 
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#2
Excuse my quick unclear attempt to explain time lis imited so are my tallents at communicating complex ideas

engine rpm and the transmission output speed may conflict with the wheel speed..... the ecm/tcm will not agree with wheel/ABS speed? the modules EXPECT to see 1000rpm and 60mph.... if they now see 1000rpm and 62mph they default as one of the 2 is incorrect.

The use of increased rim size chaanges the geometry and would adversly affect braking, especially the ABS as it is calculated to a specific diameter, load weight etc.
 
M

MikeP

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#3
I'm not trying to be argumentative.. Really, I'm not.. But, the engine RPM is solidly coupled to the speed sensor which is solidly coupled to the wheels speed sensors. The only thing changing is how much linear distance is covered per wheel revolution. So, regardless of tire final outside diameter, it should all be the same relatively to each other, the only thing changing is how much ground you are covering, but the relationship between engine rpm, tranny output rpm, and axel rpm (abs sensor, therefore) should remain the same.

On the second note, the wheel size, I don't get that either..

Anyone else?

Nick D should have some insight..
 
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#4
After rereading and drawing a little picture ......I see your question, you are corrct the "axel" speed would be the same SO how would the modules ever KNOW the diameter of the tire???


As for the ABS the calculations it works on (programed) would be compromised. But should ONLY degrade its effectiveness not create a conflict.......all 4 wheel speed the same should be no problem.

Will be interesting to figure this out.
 
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MikeP

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#5
Yes!! Now you see my confusion.. I know first hand what it can do to your speedo and your fuel mileage and you "get up and go", I put 31" tires on my 1986 Kblazer when it came from the factory with girly tires... Wow, what a dog.. but it looked great.
Anyone?
 
M

MikeP

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#6
And while we're at it, regarding ABS, if you could lock all 4 wheels at exactly the same time, would the ABS sense the skid? Or would it just think everything (all 4 wheels and VSS) are stopped? Or does it recognize that in a split second everything went from 40mph to 0mph, and therefore we must be in a skid?
Something to try on ice, in a big empty parking lot.
 
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#7
MikeP said:
And while we're at it, regarding ABS, if you could lock all 4 wheels at exactly the same time, would the ABS sense the skid? Or would it just think everything (all 4 wheels and VSS) are stopped? Or does it recognize that in a split second everything went from 40mph to 0mph, and therefore we must be in a skid?
Something to try on ice, in a big empty parking lot.
Would help to know what vehicle all these mods are done to. The ABS would release brake line pressure to any wheel that has a sudden "drop out" in speed til it spins by itself again. This applies to 4 wheel abs systems. The WSS for each wheel would then read the speed when wheel speed picks up. If it tries to lock itself, the cycle repeats, 10 to 30 times a second depending on the system used. When the wheel turns too slow to generate enough voltage (usually below 5mph) the system is offline, so to speak.
As far as the wheel size deal...the PCM will see the load via MAF sensor, Map sensor, TPS, and other parameters. If the loads do not correspond to "predicted" strategy, the ignition advance, shift points, and several other things go out of whack. Transmission may get boosted line pressure to prevent slippage, and that line pressure may eventually wear out the innards prematurely, or break parts inside. $.02
 
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#8
The reason a vehicle is a dog when you put a bigger tire and wheel that is a larger wheel circumference is load. The engineers who set up the vehicle programmed the pcm to give the engine enough power to turn the trans in whatever gear its in with enough torque to turn the rear end with the proper sized tires to move the vehicle at the speed you want to go. Simple example if you have a standard trans and put it in second gear and try an take off from a stop it's a real dog. It's because you changed the gear ratio. Same thing happens when you go to a bigger tire You change the final gear ratio to the road. Hence it's a dog. It's usually OK to change to a slightly bigger tire but it may change your fuel economy. As far as abs It will work with bigger tires of the same size, but it will not act the same as you said a larger tire covers more ground so when the abs sensor which is mounted close the the center of the wheel senses a locked up wheel or a wheel that is slower that then rest and applies the abs to work the reaction times are not programmed for the larger outer ratio of the bigger tire so it will not work properly.
Thats why they change the ratio in the rear end to compensate for the bigger tires to give you more torque or build up the engine to give you more horsepower. Hope that helps you some. Greasemonkey :)
 
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#9
I think Kev explained it pretty well.

The PCM takes the engine speed (rpms), the transmission output (VSS) and the wheel speed sensor (ABS) information and compares them to the parameters that are set for that vehicle.

If the values add up wrong, the PCM thinks that the vehicle trans may be shifting worng, that there is wheel spin or a number of other things.

On the newer vehicles, with stability control, this becomes even more of a pain.
 

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#10
If you have all wheel drive and the tires are not only the same size , but unevenly worn, it will create havoc with the system...

If you have two tires on the front or rear, and they are different sizes, they will NOT turn at the same speed....
 
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NickD

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#11
Not sure if you are having a practical problem or a bar room discussion on tires. All I know about tires, with the huge increase on Homeland Security spending on additional police cars, if you put on larger tires without recalibrating the speedometer, you will get a speeding ticket even though yours says, you are driving the limit. Even if you are only going 1 mph over the limit, the law is like a pane of glass, and once it's broken, it's broken, so I have been told.

Also know as your tires wear, driving the same indicated speed, it will take you longer to get there. Also know that even with identical sizes but different brands, the diameter of the same sized tires can be over an inch difference. Because of this difference, when shopping for tires, I carry a tape measure as a rule, even checking the same brand tires to make sure they are all reasonably close with the same circumference.

To check my speedometer, can take it to my Ford dealer that has a dynamometer and pay 30 bucks for a speedometer check only or do it for free on the interstate using the miles post. Holding the speedometer at a steady 60 mph, should do a mile in 60 seconds, but more like 64 seconds, so actual speed is 3600/64 or 56.25 mph. Will even do a ten mile average for greater accuracy. Dividing the measured time by 60 gives the precise ratio of increased tire size you need to go the correct speed, or 64/60 equals 1.06667 so if your old tires have a diameter of say 26 inches, that time 1.06667 tells you, you need a tire that is 27 3/4" in diameter for a correct speedometer reading. Of course the ratio and wheel size has to be the same to fit the rim, but play with the sidewall height. In one case found the correct diameter tire that was two sizes over the recommended size of the label, so had to argue with the guy to put them on. He had me call the tire manufacture, and explain why, and their application guy, said fine.

Like to stick with name brand tires, while the local co-op guy is nice and all that, if I have a failure on a new tire out of town, I am screwed, have to bring it back to him and you sure can't drive 2,000 miles on that little thing hidden someplace in a trunk they call a spare.

ABS is useless on glare ice roads as is 4WD, just ask those hundreds of SUV drivers parked in the ditch about this. On some GM ABS systems will set a difference in wheel sensor code if you have a blow out, and only way to reset it is with an ABS scanner.

I learned on my 82 Pace Arrow, the speedometer was very accurate on the interstate, but was curious about the engine speed, found by turning the driveshaft, do have a 4:11 rear end ratio and learned that at 60 mph, the 454 is cranking at 2,673 rpm. Now wondering where the peak torque occurs in this engine.

Amazes me that I can buy a twelve buck speedometer for my bicycle and adjust it for a wide range of tire sizes to within a millimeter, pay a fortune for a car, and can't do a damned thing with the speedometer calibration. Old Tom McCahill always used a fifth wheel for his famous 0-60 times in Mechanics Illustrated. Could never trust that gauge that came with the car.
 
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#12
Re: Scare tactics

Bottom line. No one wants you to do anything to your vehicle. Soon even waxing it will be frowned upon.

Where I live, bigger tires on trucks is an everyday thing. My 06 Dodge has 35s on it. In 06 Dodge decided you can't change the pinion factor for anything that is not offered stock. The Power wagon comes with 33s, so that is the biggest option. So there is three things wrong with my truck now. The speedo is off 5 MPH at 55. None of the odometers are accurate, and neither is the MPG read out. ABS works perfectly, even on glare ice, transmission shifts perfectly and the Cummins will burn those big tires off the truck if I wanted. When I am doing 60 MPH the truck just thinks we are cruising along happily at 55 mph. I checked my road speed with my GPS to determine speedo error.

There is nothing on the truck that can determine what the wheels are made of. And you can put bigger wheels on, install tires with the same as OEM outside diam and the truck will never know the difference. Except you will get better mileage and a little better performance. Because aluminum is lighter then rubber.

Anyone who has ever played around with stock car racing knows getting a matched set of tires is not going to happen.

There are limiting factors to tire size, size of axles, size of brakes, clearance issues, local laws and so on. Put a set of 44s on a Ford ranger and you now become a liability on the road. Put a set of 33s on 1 ton and life is wonderful.
 
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MikeP

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#13
Okay, so my thinking is pretty much correct. I figured as much, based on the pure physics of the whole thing. And true, you need to keep the tires the same on all 4 corners, that was never an issue. It was the tire guy that said I couldn't take my Eighty-eight with steel 15" wheels and swap them with Eighty-eight 16" alum. wheels who, quite frankly, pissed me off. Especially since the final outside diameter of the tires were identical.
I can agree, however, that drastic changes can make things act strangely, but I expect that has more to do with the load the engine feels. I once put 4 donut spares on my Bonneville, then took it on a ride down my lonely country road. It upshifted real quick, was practically in 4th gear 30 feet out of my driveway, speedo thought I was going 45 MPH already.. But, talk about changing the gear ratio!! Wow! Just touch the gas with those little guys, burn them off the front...
Mike