Tire Upgrades: Revs per Mile and Engine Computers

[Moose-Hunter]

How many revs per mile are needed to throw off a engine computer?

With so many things (read: everything) on new vehicles being dependant on computer feedback, what does it take to confuse them?

My stock tires are 31.5" dia (640 revs. per mile). The new tires are 33.2" dia (627 revs. per mile). Is this difference (13 revs per mile) enough to really mess with the computer on my new 2011 Z-71 Silverado?

[Macgyver55]

The main thing I would watch for is incorrect shift points, especially under load and while pulling. You definitely don't want the transmission shifting up and down excessively. Taller tires turn slower (at a given speed) so it will lower the engine RPMs at the same time adding more rolling resistance. (Taller tires are usually wider too)

Unless you have the tire size corrected in the computer, it will make a difference, but I dont think it will allow that much variation from stock. Entering the largest available size would help speedometer correction and performance as much as possible.

The tires wont "mess with the computer" but the computer may "mess with the vehicle" to compensate for them.

[MNice]

When I put bigger tires on my 01, they were able to change the computer so my speedo & odo are accurate. Don't know if thats an option on new, but I would guess it still is.

[311Hemi]

I know Dodge will allow a good amount of variation from stock, they were able to program mine for the 35" I had on there after I showed them how.

I think this also plays a part with the ABS system, so I would get it changed.

[Powerstroke]

I'm not sure about the newest vehicles, but just a couple years old and all they do is changed the rev's per mile in the programming and it will read and react as it should.

As far as "messing up the computer" that minute size change will hardly be noticeable. The greatest change you will notice is the added weight in a the heavier tire and if its a wider tire there will be a bit more steering feedback. I've had larger tires on every vehicle I've owned in the last 10 years (4 trucks and an SUV) and none of them have shown any adverse effects to the larger tires. ABS, 4wd, traction control etc have always worked just fine.

The only noticeable change is in the speedometer and also the odometer. Your speedometer will read 5% slower. In other words, at 30mph you will read 28mph and at 60mph your speedo will read 57. At 70 is will read 66mph. So, thats not even enough to get you a ticket if you're trying to follow the speed limit.

If you're thinking about going to 35's, then I would be start thinking about getting the speedo recalibrated.

[delcecchi]

But if you are doing "73" in a 65 zone, thinking you can skate at eight over..... you are really like 12 over and smokey be watching.

[Moose-Hunter]

From what I've been told by more than oner Chevy service department, there is no option in computer for recalibration to allow for larger than stock tires. BUT... All is not lost. After a bit of research, I found one of the optional tire sizes offered by Chevrolet, is VERY close to what I'm looking at. In fact, it's just a shade larger! This means that there's a very, VERY good chance none of the trucks systems will be adversely effected. Better yet... Because my tire size is within specs already dictated my the Chev Warranty team... My 5yr/100K warranty is remains intact!

All that said, I grabbed my GPS and hit the road for some on-road testing of the stock tire set up. From what it looks like is that with the addition of the new rubber, The difference will actually "reduce" my speedo error and I'll be within 1-2 mph of a speedo marked 30, 55, and 70 mph.

My topper will be installed in a couple weeks. So I have between now and then to choose my wheels and tires. I know... Hardly enough time to weigh all my options but I think I can muddle through some how.

[Macgyver55]

none of them have shown any adverse effects to the larger tires. ABS, 4wd, traction control etc have always worked just fine.

None of these features would be affected by tire size no matter how much they vary from stock, as long as all 4 tires are the same size. ABS and traction control are controlled by individual wheel speed, and then the computer actuates ABS or traction control as needed when wheel speeds don/t match. The 4wd system only requires that the front and rear wheels turn the same speed as to not damage the transfer case, so the actual tire size doesn't really matter (as long as they all match).

[JOHNBIGDOG]

I find that odd that Chevy doesn't have an option to recalibrate the pcm for different tire sizes. I know for a fact a Ford can, and in certain situations has to be told what size of tires are on the vehicle(pcm replacement), within a certain amount of variance. odd...