J-Scan tire change broke speedometer and threw lots of codes

[MacGuyver]

The important figure is the unloaded diameter of the tire. It might be more accurate to measure your spare if the tires really are 35 inches in diameter. The real measurement that counts in calulating speed is the circumference of the tire rather than the diameter. You'ld be travelling about 3% faster than your speedometer tells you I think. Probably not enough to worry about, except where your mileage is concerned. It would be better than you thought.

If you have an automatic you really want to be as accurate as possible. The 'running' diameter of the tires is them on the ground. Probably measure all four add that up and then divide by four for the best result. If you are off it will mess up your shift points.

If you have a manual it is not as important. However, set it correctly so you know the mileage on the vehicle. My TJ had the wrong gear (TJs use a gear in the transfer case to set the speedometer) for who knows how long. I also have no idea how long the 33" tires have been on it so I don't know what the actual mileage is on that vehicle. It could be well over 10,000 miles off. They originally came with 27 -28 inch tires.

Set your speedometer!!

 

[Duster]

If you have an automatic you really want to be as accurate as possible. The 'running' diameter of the tires is them on the ground. Probably measure all four add that up and then divide by four for the best result. If you are off it will mess up your shift points.

If you have a manual it is not as important. However, set it correctly so you know the mileage on the vehicle. My TJ had the wrong gear (TJs use a gear in the transfer case to set the speedometer) for who knows how long. I also have no idea how long the 33" tires have been on it so I don't know what the actual mileage is on that vehicle. It could be well over 10,000 miles off. They originally came with 27 -28 inch tires.

Set your speedometer!!

The tire industry for some reason sometimes refers to circumference as "Roll out" - https://ntwonline.com/tire-shop-talk/revolutions-per-mile-chart/. The chart makes the same point I was making. The circumference of a tire is to a degree affected by the load and inflation pressure, but that's critical estimate. Many sites for tire sales will give you all pertinent data for calibrating. You can check the speedometer on your TJ by finding some location that gives you a measured mile. Measure the time it takes to travel that distance a steady speed - back in the day we used a stop watch - and then do the arithmetic. On many roads there are mile markers that give you tenth-mile intervals. If you aren't being stopped for speeding repeatedly, you are probably good.

 

[Duster]

I would be willing to bet that the spare tire would be about 35 inches. It's not being compressed by the weight of the vehicle. The previous owner had it set to 35 and the speedo and shift points were out of wack. You need to measure the tires on the ground and not the spare.

Yep. Mostly you can look up the diameter, and the rest of the technical details.

 

[BLACKJKU]

When I reset mine I just used a GPS to match up the speed.

 

[mrjp]

When I reset mine I just used a GPS to match up the speed.

X2 that's what I did. The tire size that I had to input does not match my tire so just match it up to the GPS speed.

 

[Duster]

When I reset mine I just used a GPS to match up the speed.

Back "in the day" working on a '68 Mustang, we used a stopwatch and mile posts on US 50. None of us had thought about things like tire size changing the speed at the same RPM until a CHP patrolman explained. We didn't even get a ticket because he had a Mustang at home.

 

[Anybodyhome]

Back "in the day" working on a '68 Mustang, we used a stopwatch and mile posts on US 50. None of us had thought about things like tire size changing the speed at the same RPM until a CHP patrolman explained. We didn't even get a ticket because he had a Mustang at home.

Wow, yeah, I had a 69 Camaro that we used to do the same thing with on 163, which later became US 15 north of San Diego headed toward Escondido... when there was nothing out there.

 

[SteveF]

Back "in the day" working on a '68 Mustang, we used a stopwatch and mile posts on US 50. None of us had thought about things like tire size changing the speed at the same RPM until a CHP patrolman explained. We didn't even get a ticket because he had a Mustang at home.

Back then your transmission wasn't computer controlled

 

[Duster]

Back then your transmission wasn't computer controlled

That was in the early '70s. You probably couldn't fit a computer in a car. That Mustang had no electronics at all except in the radio, which never did work. The engine was strictly vacuum feed back controls with the sole exception of the thermostat, and maybe the tachometer. I wish I still had it. The engine was the 390 cu.i. big block. You had to go easy on the accelerator or you could pop the front end off the ground.

 

[Duster]

Wow, yeah, I had a 69 Camaro that we used to do the same thing with on 163, which later became US 15 north of San Diego headed toward Escondido... when there was nothing out there.

I worked for fellow up until 2018 who has a classic Camaro he religiously starts up and drives around the block every weekend he's home. I don't recall the year, but the body style is probably late '60s or early '70s. "Cherry".

 

[SteveF]

That was in the early '70s. You probably couldn't fit a computer in a car. That Mustang had no electronics at all except in the radio, which never did work. The engine was strictly vacuum feed back controls with the sole exception of the thermostat, and maybe the tachometer. I wish I still had it. The engine was the 390 cu.i. big block. You had to go easy on the accelerator or you could pop the front end off the ground.

I had a 1973 Chevy Nova with a 307c.i. Also a 69 Cougar XR7

 

[Anybodyhome]

I worked for fellow up until 2018 who has a classic Camaro he religiously starts up and drives around the block every weekend he's home. I don't recall the year, but the body style is probably late '60s or early '70s. "Cherry".

Mine was a 69. I traded my 72 Malibu SS straight up for it in 1975. The Camaro had the original 327 badging on it, but the yellow traction bars visible on the rear end was the giveaway that something more sinister was involved. Lifted the hood, and there it was- a 396 big block.

I'm still looking for either to add back to my garage (I'm gonna be 67 next week and I still want the cars I had when I was 19....)

Not my cars, but representative of the models I owned.

The silver & black was the color scheme:

The Camaro was the olive green, except mine had the slightly darker vinyl top:

 

[Duster]

I had a 1973 Chevy Nova with a 307c.i. Also a 69 Cougar XR7

The fellow I bought the Mustang from rebuilt another, to which he added the Cougar rear panel and brake lights, and the dashboard. They have the same frame, so swapping body elements is fairly easy. It was a beautiful job for about two months, then it encountered an idiot driving at night, in the fog, with no lights.

 

[BA-Ohio]

New here, first of all, thanks everyone in this community for all the help and advice,

I finally decided to use JScan to change my wheel size from 31.25” to 35”’s, which was a simple task. Unfortunately, after this 1 single change I am now getting these trouble codes:

C1078 (tire revolutions range performance)
C100A (left front wheel speed sensor circuit)
B2305 (wiper park switch input stuck high)
U1417 (implausible left wheel distance signal)
U1418
U1412
U1409

And now my CEL is on, my speedometer doesn’t work and my mileage isn’t increasing. I’m pretty desperate for any help or suggestions, I think I’m going to try and reset the ECU, but I have no idea how to fix this. I’m hoping Jscan didn’t fry any of my modules,

Has anyone had any similar issues? Any suggestions?

You’ve probably resolved your issue. But I wanted to document that this just happened to me also. Veepeak OBD2 BLE+ with the ODB JSCAN app.

In one place, I was instructed to keep the Jeep running while the change happened. Wrong!!! Speedometer stopped working and error codes popped up. I pressed the “?” In the JScan app and it says your Jeep must be ON but not running. So, I redid the short process with the key on and the codes disappeared, my speedometer started working again, and most importantly, my tire sizes are now registering the correct speed.