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tires size vrs computer problems


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becarefull guys changing tire size on newer 4x4 . and if you have the 4x4 auto setting it will really mess it up , since it activates by tire rotation of rear tires. if that size is changed then the slip in time will lag and youll have a ruff lock in every time. also anti lock brakes will be in active (light will probably come on). computer will be confused because engine RPM and wheel speed will be out of sink.

 

 

 

 

dont want to bust and bubbles but thats the facts

 

 

 

Pastor Scott

 

former off road equipment mechanic and 4x4 speacialist

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I have added slightly bigger tires to every 4x4 and suv I have ever owned starting back with a 95 Grand Cherokee up to the 02 Silverado I have (planning to on 04 soon) with no problems. You can reprogram the ecm on the newer trucks but it is not necessary. My auto 4x4 works fine and so does my ABS......so I am not sure where you got your information. Possibly from the same person who expects us to ride our bikes to work.....

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Problems can arise from running oversize tires. Other then speedo error the second most common problem is shift points. Some ECMs will command an upshift or downshift based on vehicle speed as determined by diameter x tire compared to rpm y using gear/rear end ratios.

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becarefull  guys changing tire size on newer 4x4 . and if you have the 4x4 auto setting it will really mess it up , since it activates by tire rotation of rear tires. if that size is changed then the slip in time will lag and youll have a ruff lock in every time. also anti lock brakes will be in active  (light will probably come on). computer will be confused because engine RPM and wheel speed will be out of sink.

 

 

 

 

dont want to bust and bubbles but thats the facts

 

 

 

Pastor Scott

 

former off road equipment mechanic and 4x4 speacialist

 

 

 

 

I have heard this before but am skeptical. I have changed tire size (and know many who have changed size) on many OBS and NBS trucks with none of the above problems.

 

How can the engine rpm and wheel speed be different just from tires. If you have a 4:1 rear end than the tires will spin 1 time for every 4 engine revs. Changing tire size can't change this. So the computor will always see the same wheel rev at the same rpm. The actual vehicle speed will be different but all this will effect is your milage calculations and speedo calibration.

 

ABS senses the difference in the rotation between the four tires as long as they are all the same size it will not know what size they are.

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becarefull  guys changing tire size on newer 4x4 . and if you have the 4x4 auto setting it will really mess it up , since it activates by tire rotation of rear tires. if that size is changed then the slip in time will lag and youll have a ruff lock in every time. also anti lock brakes will be in active  (light will probably come on). computer will be confused because engine RPM and wheel speed will be out of sink.

 

 

 

 

dont want to bust and bubbles but thats the facts

 

 

 

Pastor Scott

 

former off road equipment mechanic and 4x4 speacialist

 

 

 

 

I have heard this before but am skeptical. I have changed tire size (and know many who have changed size) on many OBS and NBS trucks with none of the above problems.

 

How can the engine rpm and wheel speed be different just from tires. If you have a 4:1 rear end than the tires will spin 1 time for every 4 engine revs. Changing tire size can't change this. So the computor will always see the same wheel rev at the same rpm. The actual vehicle speed will be different but all this will effect is your milage calculations and speedo calibration.

 

ABS senses the difference in the rotation between the four tires as long as they are all the same size it will not know what size they are.

 

 

 

 

 

When you put larger than stock tires on a truck with a 4.10 axle ratio, you effectively raise your gear ratio. 285 tire w/ 4.10 ratio is closer to 3.73 w/ stock tire size.

Its not engine revs/tire rotation. Its driveshaft rotation/tire complete revolution. A larger tire takes longer to complete one revolution than a smaller tire. That in turns drop engine RPMs at any given speed.

 

What the original post is saying is correct about going to larger tire size. If you think adding larger tire size has no effect, then mount up some larger tires, go for a drive and take your handheld GPS w/ speedo function w/ you. You will see a discrepency between actual speed(GPS) and what your speedo is showing.

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Trans shift points are determined primarily by output shaft speed and throttle input. You are accustomed to a certain SOTP acceleration feel. To feel that same SOTP feel when you install larger tires, you have to mash the accelerator harder because you have effectively changed gear ratios so the engine is working harder to provide that same thrust with the 'new' ratio.

 

DEWFPO

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Guest 007FL
becarefull  guys changing tire size on newer 4x4 . and if you have the 4x4 auto setting it will really mess it up , since it activates by tire rotation of rear tires. if that size is changed then the slip in time will lag and youll have a ruff lock in every time. also anti lock brakes will be in active  (light will probably come on). computer will be confused because engine RPM and wheel speed will be out of sink.

 

 

 

 

dont want to bust and bubbles but thats the facts

 

 

 

Pastor Scott

 

former off road equipment mechanic and 4x4 speacialist

 

 

 

 

I'm sorry pastor scott but if you are a former mechanic and 4x4 specialist you should already know some of this basic information.

 

ABS senses differences in wheel speeds. As long as the tires are all the same size it will make no difference whether they are much larger or much smaller (other than much smaller tires may skid easier activating the ABS).

 

If yur auto 4x4 is engaging harder it may well be due to the extra strain you are putting on it as a result of the effective final gear ratio change due to the larger tires. Ease off the pedal some and you may find it smoother like before.

 

Mattt

The larger tires do effect your actual speed and result in errors on your speedo and odometer. These are easily corrected using any of the common handheld programmers.

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becarefull  guys changing tire size on newer 4x4 . and if you have the 4x4 auto setting it will really mess it up , since it activates by tire rotation of rear tires. if that size is changed then the slip in time will lag and youll have a ruff lock in every time. also anti lock brakes will be in active  (light will probably come on). computer will be confused because engine RPM and wheel speed will be out of sink.

 

 

 

 

dont want to bust and bubbles but thats the facts

 

 

 

Pastor Scott

 

former off road equipment mechanic and 4x4 speacialist

 

 

 

 

 

I have heard this before but am skeptical. I have changed tire size (and know many who have changed size) on many OBS and NBS trucks with none of the above problems.

 

How can the engine rpm and wheel speed be different just from tires. If you have a 4:1 rear end than the tires will spin 1 time for every 4 engine revs. Changing tire size can't change this. So the computor will always see the same wheel rev at the same rpm. The actual vehicle speed will be different but all this will effect is your milage calculations and speedo calibration.

 

ABS senses the difference in the rotation between the four tires as long as they are all the same size it will not know what size they are.

 

 

 

 

 

When you put larger than stock tires on a truck with a 4.10 axle ratio, you effectively raise your gear ratio. 285 tire w/ 4.10 ratio is closer to 3.73 w/ stock tire size.

Its not engine revs/tire rotation. Its driveshaft rotation/tire complete revolution. A larger tire takes longer to complete one revolution than a smaller tire. That in turns drop engine RPMs at any given speed.

 

What the original post is saying is correct

 

 

If you read the original post it states "computer will be confused because engine RPM and wheel speed will be out of sink". As I stated this cannot be correct because any size tire will turn the same number of revolutions at a certain rpm. So a 35" tire will spin the same number of times at 3000rpm as a 30" tire. It does not take a longer time for a larger tire to complete the revolution they both complete the revolution in the same amount of time. Effective ratio is just that the effect you feel not the actuall mechanical change. Yes you will feel less acceleration with bigger tires because it takes more torque to turn the bigger tire. There is no dispute actuall vehicle speed will be off but this doesn't "mess up" the computor because it doesn't measure actual vehicle speed.

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You are correct about a 30” tire and a 35” tire taking the same TIME to spin 1 revolution, but I think he is talking about distance. A 35” tire does cover more distance per RPM than a 30” tire does, thats what throws the speedo off. Changing tires actually does change the overall mechanical drive ratio. The torque multiplier for the rear gears remains the same at 3.73:1, but there is also a torque multiplier associated with changing the tire size. Without getting too much into the physics of it all…. It isn’t that it takes more torque to turn larger tires it’s the fact that larger (taller) tires have a larger moment arm than smaller (shorter) tires. This means larger tires exert less force to the ground to move the truck than smaller tires do. That is why you need to change the rear gears when putting on bigger tires to get the same acceleration(force to the ground). Changing the gears to 4:10 increases the torque multiplier which offsets the decrease torque multiplier of the larger tires.

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ABS senses differences in wheel speeds. As long as the tires are all the same size it will make no difference whether they are much larger or much smaller (other than much smaller tires may skid easier activating the ABS).

 

 

 

 

 

sorry but you guys are wrong on this one,

 

if you go 10 percent bigger or smaller on your tire size without reprogramming it can cause a problem.

 

the front wheels are going off the hubs, but the rear is measured off the trans tail shaft and the computer uses the gear ratio along with tire size to compute the speed, if the tire size is defferent from what the computer is programed for the two readings will differ. if it is more than 10 percent difference between front and rear the computer will turn off the abs, flip on the light and record a 238 code for wheel speed mismatch. this will also mess up the auto 4wd since the computer sees the front and rear moving at different speeds and always sees slippage.

 

also will throw off speedo, odometer and shift points

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ABS senses differences in wheel speeds. As long as the tires are all the same size it will make no difference whether they are much larger or much smaller (other than much smaller tires may skid easier activating the ABS).

 

 

 

 

 

sorry but you guys are wrong on this one,

 

if you go 10 percent bigger or smaller on your tire size without reprogramming it can cause a problem.

 

the front wheels are going off the hubs, but the rear is measured off the trans tail shaft and the computer uses the gear ratio along with tire size to compute the speed, if the tire size is defferent from what the computer is programed for the two readings will differ. if it is more than 10 percent difference between front and rear the computer will turn off the abs, flip on the light and record a 238 code for wheel speed mismatch. this will also mess up the auto 4wd since the computer sees the front and rear moving at different speeds and always sees slippage.

 

also will throw off speedo, odometer and shift points

 

 

 

 

 

I still don't get it. We are talking 4 tires the same size right? I can see if fronts are dif size from rears then there may be a problem. If all 4 are the same size how can they be turning at different speeds? How can my front tires be going 10 percent faster than my rear? So you take the front sensor from the hubs and the rear from the tail shaft they will still be the same regardless of tire size. The computer dosent know what speed the vehicle is traveling it will read the same speed regardless of tire size. Yes the speedo will be off but how will the computor know it?

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ABS senses differences in wheel speeds. As long as the tires are all the same size it will make no difference whether they are much larger or much smaller (other than much smaller tires may skid easier activating the ABS).

 

 

 

 

 

sorry but you guys are wrong on this one,

 

if you go 10 percent bigger or smaller on your tire size without reprogramming it can cause a problem.

 

the front wheels are going off the hubs, but the rear is measured off the trans tail shaft and the computer uses the gear ratio along with tire size to compute the speed, if the tire size is defferent from what the computer is programed for the two readings will differ. if it is more than 10 percent difference between front and rear the computer will turn off the abs, flip on the light and record a 238 code for wheel speed mismatch. this will also mess up the auto 4wd since the computer sees the front and rear moving at different speeds and always sees slippage.

 

also will throw off speedo, odometer and shift points

 

 

 

 

 

I still don't get it. We are talking 4 tires the same size right? I can see if fronts are dif size from rears then there may be a problem. If all 4 are the same size how can they be turning at different speeds? How can my front tires be going 10 percent faster than my rear? So you take the front sensor from the hubs and the rear from the tail shaft they will still be the same regardless of tire size. The computer dosent know what speed the vehicle is traveling it will read the same speed regardless of tire size. Yes the speedo will be off but how will the computor know it?

 

 

 

 

 

 

lets see if i can clarify

 

4 tires the same size travel at the same speed going down the road

but the computer thinks one set is going faster than the other because

the computer uses a signal off the front hubs calculated by the programed tire size to get speed in mph or kph for front left and right tire

the computer uses a signal off the trans tail shaft calculated with rear end gear ratio and tire size to get speed in mph or kph for the rear tires

if you change the tire size and do not reprogram the computer it throws off the calculations for the front and rear because one uses gear ratio and one does not so the computer thinks one set of tires (front or rear) is traveling at a different speed even though the tires are actually traveling the same speed

the computer just sees what it is programed to see,

if it sees a difference between front and rear it thinks the tires are slipping or skidding depending on what its looking for (braking and looking for lockup or traveling with auto 4wd and looking for tire slip)

the computer is programed to disable the abs if it sees more than a 10 percent difference in ft and rear readings, wich is about 10 percent size change from the stock tire size it is programmed for, it will then set the abs light and throw a code 238 for wheel speed mismatch, will also disable traction control if the truck has it

 

the speedometer and odometer will be off because the computer thinks you are traveling at the speed it calculates from the signal using the programmed tire size, not the tire size you are using

 

also some of the trans shift points will be off because there are shift tables that use mph to determine some shift points and you are actually traveling at a different speed than it thinks you are

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