Tire size and emission testing

The Great State of Colorado requires that I pass emission testing before I can register the vehicle in my area: http://www.aircarecolorado.com/im240tst.htm

It has been an adventure getting this heep to run, I'm still not satisfied with the current outcome. I do feel confident that it will pass at this time, but I am pondering how much difference taller than stock tires will make on the treadmill.

The vitals: '89 Waggy, 4.0, AW-4, NP242, D-35 with 3.55:1 gears.
It currently has 30x9.50x15s, the door sticker lists 205x75x15s for it's load rating, that is a 3" difference in diameter. I feel there is a significant change to the final drive, the speedo is way off, it takes more throttle in all driving conditions, hills are really noticable.

I have a set of 225x15 rollers, I am wondering if installing them for the treadmill would make enough difference to be worthwhile. The diameter is about 1" over the 205s.

Thoughts anyone?

From my experience as a certified inspection/emissions mechanic I would say that it really is dependent upon the speed at which you are rewuired to run on the rollers. Here, in PA, you are only required to go 15 mph and the difference is negligible. I run the same size tire in a 91XJ with the only difference being my NP231 transfer case and I pass it without any problems. My drivetrain is all relatively new(under 25k) though. If you run at higher speeds it may be beneficial to run the smaller tires as most engines run more efficiently at higher rpms.

I live in the Dallas Fort Worth area and have driven my 93 600-800 mile week for 10+yrs. I run the same size tire BFG AT/TKO 30x9.50x15 and have had zero problems passing.

Thanks for the feedback!
They way they run these cars on the treadmill, I think it's over than 15 mph. I'm not really sure if it is speed or RPM specific. Since the test is on an engine under load, I have to wonder if the increased load at the given rpm/speed will matter. I do know that what ever the criteria is, it takes more throttle to achieve with the taller tire...more in to get the same out. Does that make sense or should I stop trying to think? LOL

Stop trying to think - you'll probably actually notice that with the 30s, your speedometer is actually more accurate anyway. Just give it a good tune-up and make sure the engine (and oil) are nice and thoroughly warm when you go to get tested.

Then, after it passes, make sure you drive past the onramp sensors plenty of times prior to your next e-test coming due. http://www.aircarecolorado.com/rapidscreen/index.html That way, you won't have to worry about the roller test again.

Jim www.yuccaman.com

that screening thing it weird... cool but weird.

It seems like i am always going thru the onramp sensors ,wonder if it would be beneficial to pop it in neutral and idle while coasting thru ???

I'm from MO, we don't have these testers or any emissions testing here.
Can these vans issue a citation to you or any other form of penalty?

When I moved to AZ, I had 33" tires on my YJ with stock gears (3.55). I had a bear of a time getting it through the dyno test because the guy driving it had to have it floored most of the time to keep up with the program. I swapped to 31" tires and breezed right through. Since then I keep a set of 205 tires on stock wheels specifically for running through emissions. Your jeep should be OK with 30" tires, but it does make a difference onder certain circumstances.

It passed...WOO-HOO!

In this case the driveability and tailpipe emissions went hand in hand. It has some noticeable torque with a new cat installed. LOL

1985xjlaredo, they don't issue a summons...yet. The State's system is to have the E-test to get your tags renewed. The renewall notice states wether or not the E-test is required. Theoretically, passing the vans often enough gets you out of the test when your vehicle would have normally been tested.

I've seen the vans on two of the ramps I use in my 7-minute commute. On either ramp, my engine had been running about 90 seconds. Looks like I get hosed when my time is up.