Big tires = engine damage?

[Yeret]

Hey folks, seems it's been a damned queen's age since I last posted here. Not a whole lot has really happened in regards to the ole Ram, which isn't necessarily a bad thing, LOL.

Anywho, while poking around Hughes Engines to do some research on Victor cylinder heads (for an entirely different engine/application of course), I came across this article stating that big tires on a truck that hasn't been reprogrammed to factor in the larger tires will result in the computer screwing with the engine parameters with damaging resolve. You can read here...

http://www.hughesengines.com/Index/bigtires.php

Is there really truth to this? For one, I didn't think that the computer used the vehicle's speed as a factor for adjusting operating parameters. And two, I know in our trucks that the computer uses the speed sensor in the rear axle to calculate speed. Now, this sensor sends pulses to the computer as a result of the toothed tone ring and the Hall effect. Since the tone ring is affixed to the axle shaft rather than the wheel, would changing the tire size really effect it's rotational speed? Same goes for the ring and pinion as well. I was always under the impression that swapping the tire size wouldn't affect the output speeds of the transmission, axle gears and the related sensors but the tires themselves would change speed depending on whether they're taller or shorter. I mean, 70 MPH is 70 MPH irregardless of tire size, right? If the tires are taller, they're gonna rotate slower but the vehicle is still moving at whatever speed the speedo shows, right?

Or am I just totally wrong on all this? Quite frankly, as much conflicting stuff as I've read on changing the tire size from OEM, I've never been absolutely certain about the effects on output speeds on the drivetrain. Educate me if I'm wrong, dudes!

 

[BWL]

I'd be more concerned about the transmission as it'll throw your shift points out of whack adding more strain on the transmission. I believe the speed sensor counts axle rotations so your truck will be going faster than it reads on the speedo since you'll travel further per rotation.

 

[Ramjet]

Seems to me if the computer can judge when its time to change your oil by monitoring how you drive, then larger tires would probably throw things off.

 

[Csanders1992]

I think it’ll actually take less output speed to go a specific speed with taller tires. But it takes more effort to do so, which I feel the transmission takes care of


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[muddy12]

How’s this for an explanation?
The speed displayed on the dash is determined by the RPM of the axle shafts(speed sensor in rear dif) multiplied by the circumference of the tires. If we assume a 33” stock tire, then that means that the truck is going to move roughly 104” for every rotation of the axle shaft and tire.
When you increase the tire size, because the circumference is greater, you change the amount of distance moves per rotation of the axle shaft and tire.
So if you step up to a 35” tire, the circumference increases to roughly 110”. This means that the truck is now going to move an extra 6” with every rotation of the tire.
However, this only changes how far the truck moves. Tire size change will not change the rotational speed of the axle shaft. One rotation is still one rotation, so as far as the computer is concerned, you’re only moving 104” per revolution, rather than the 110”.

As an example, I am running 35” tires, with no speedometer correction. Because of the increase in size, my speedometer is off by 5%.
When the speedometer shows 30MPH, my actual speed is 31.5MPH; And when it shows 60MPH, actual speed is 63MPH(actual speed checked with gps).




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[Yeret]

Thanks for the explanation, I think I got it now. Basically, bigger tires mean the vehicle is moving faster than the speedo is showing (unless the speedo has been recalibrated) because bigger tires cover more distance per rotation than smaller ones. So, if I understand it right, if you took a small tire and a big tire, spun them both to 1,000 RPM and then released them, the bigger tire will move faster and outrun the smaller tire despite both spinning at the same RPM.

That certainly clears things up. When I got my tune, Ryan adjusted the speedometer as well but I've since changed to a larger tire so it may be time to recalibrate it.

 

[dapepper9]

Older trucks used a gear and sensor in the side of the trans. Hotrod has an article where supposedly the tire and gear change from stock led to the pcm running the truck lean and melting the valve guides. Hughes built the engine.

 

[KansasArt]

How’s this for an explanation?
The speed displayed on the dash is determined by the RPM of the axle shafts(speed sensor in rear dif) multiplied by the circumference of the tires. If we assume a 33” stock tire, then that means that the truck is going to move roughly 104” for every rotation of the axle shaft and tire.
When you increase the tire size, because the circumference is greater, you change the amount of distance moves per rotation of the axle shaft and tire.
So if you step up to a 35” tire, the circumference increases to roughly 110”. This means that the truck is now going to move an extra 6” with every rotation of the tire.
However, this only changes how far the truck moves. Tire size change will not change the rotational speed of the axle shaft. One rotation is still one rotation, so as far as the computer is concerned, you’re only moving 104” per revolution, rather than the 110”.

As an example, I am running 35” tires, with no speedometer correction. Because of the increase in size, my speedometer is off by 5%.
When the speedometer shows 30MPH, my actual speed is 31.5MPH; And when it shows 60MPH, actual speed is 63MPH(actual speed checked with gps).




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So if this is true...and I have no reason to think it isn't correct. I have no idea, just throwing a 'what if' into the mix.... If the calculations are based off of tire diameter, would under inflation cause a issue? What about tire wear? Or are these variables too insignificant to matter?


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[muddy12]

Only time you “might” see an issue with tire wear (or under inflation)would be running a bald/flat tire and new/inflated tires together. Most ABS and traction/stability systems have speed sensors at each wheel. A significant difference it tire circumference, could cause an issue with these systems.
I’m sure there is some “acceptable difference” built into the programming, but I have no idea how much it is.


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[dapepper9]

Here's the article for anybody interested
http://www.hotrod.com/articles/hot-rod-rescue-dodge-ram-combustion-chamber-meltdown-fix/

 

[Stegs]

big tires = bigger strain on drivetrain

to combat this, simply re-gear your truck to relieve the strain on the vehicle

Somewhere on the web there is a chart that shows you your existing gear ratio, tire height and what ratio to go (to try and keep it as if it were stock)

as for me, I went with 4.56 gears and 34" tires on 17" rims. No strain with 4.56 gears!


Its also fun then to throw small tires back on, leave 11's ever where you go

 

[DodgeDude99]

https://www.florida4x4.com/tech/gearcalc.php

 

[Yeret]

Thanks for all the input, guys. I remember way back when I had my engine torn apart, the exhaust valves were looking pretty toasty. The engine also burned a ton of oil, likely mostly through the blown plenum gasket but could have been sourced from other places as well. Factory tire size on mine, according to the door tag is 245/75/16 and when I first got the truck (with 156,000 on the clock), it had 265/75/16.

When I filled out a form for a tune from Ryan, he wanted to know tire size. One of the first things that I noticed when I flashed the computer was that the torque converter locked up MUCH later than before when shifting into overdrive. Prior to the tune, the converter would lock as low as 1,500 RPM causing the engine to drop to 1,100 RPM at 40 MPH which was a real bog to say the least. Spark knock was a real issue as well. Post tune, the torque converter will not lock into overdrive until I'm doing at least 60 MPH, in which the engine is running around 2,000 RPM. Needless to say, this adjustment alone really helped overall engine performance, even during light throttle conditions.

Now, going by the article that Pepper posted, excessive cylinder quench, which seems to be a significant issue in post-emission Mopar V8s, is certainly going to cause issues and if I ever pull the engine for more than just a re-ring, this is something that I'm going to have properly addressed. I'm thinking decking the block to zero and dropping .040" Cometic gaskets on top of it.