Changed back to OEM tires 255/50/ ZR 16 from 245/50 VR16 and Changed the PCV valve on the car. Car sounds noticeably different and runs noticeably different. Smoother but less torque. I used to be able to put car in 3 and get to 90+ before shifting it up into 4 or OD. np no redline even. Now i really have to push the pedal down to get it to do it and its not as snappy or quick to get there. Also i seem to not be able to break the tires loose at all. Did I really change the car that much? I'm not really happy with it's performance now. I did make sure I hooked all the vacuum lines and hoses back up right. the one on the back of the TB, the stovepipe, the one on the passenger side im not sure what it's called and the smog hose. runs good other than the loss of power.

 

Generally speaking there is about a half inch difference in diameter (26.1" vs. 25.7") between the two tire sizes you mentioned. The shorter tire (245) is going to make the car accelerate faster and "feel" like there is more power. It is also very likely that the 90mph you were reading with the shorter tire was not really 90mph as the shorter tire will affect indicated mph. The only way to be sure, would have been to check it with a GPS.

Any time you switch to new tires, assuming they are both of similar compound (treadwear rating?) you're going to experience greater traction. The older a tire is the more heat cycles it has experienced and the harder the rubber becomes. I changed from 8 year old Goodyear Gatorbacks on my '85 that felt like I was on ice at times and would spin effortlessly to new Goodyear GS D3's and the difference was almost night and day. Much more effort was required to break them loose.

You haven't mentioned what tires you had and switched to, the mileage on your car or even what car you have (I checked your profile to see you have an '84 C4). You'll need to provide more information to get a more meaningful response. There may be something else going on with your car, but based on what you've told us so far, I'm not surprised you noticed a difference.

Good luck... GUSTO

 

Old tires. Goodyear Eagle ST. narrower than the new ones not taller or shorter and checked with gps my speedo reads true. New tires are BF Goodrich Sport Comp 2's the only company that still makes the OEM tire size for this car a 1984 Base coupe. I'm more inclined to think it has to do with the vacuum and something that changed under the hood and not so much the tires. But what you say makes sense about the tires so I'm dealing with a larger change of what is perceived due to changing multiple things at once. I still think it should accelerate faster at speed. I will keep digging.

 

Chris, your profile says '84 coupe. Is it an '84, or an '85? How many miles are on the car? How long have you owned it?

How is your gas mileage? Mid 20's on the highway is not uncommon for a mid-80's TPI, not sure about the Crossfire however. If the mileage hasn't changed that's a good sign.

A vacuum leak would normally make it run rough and possibly set the Check Engine Light off. Has your CEL come on? Have you tried to check for codes? If any codes show up that would give you an idea of where to start looking.

If all of that is normal, the next thing I would try is to get it on a dyno and see what that tells you. There should be one in Iowa City or Cedar Rapids if there isn't one closer.

Good luck... GUSTO

 

jeesh my fat fingers...... 84 yes.

 

Nothing appears to have changed but the way it accelerates on tope end. No CEL no codes im aware of. The biggest problem is I have nothing to compare it to. I don't know what normal is for this car so my car is the norm for me and it's no longer normal LOL.

cruising in the 60- 70 mph it says its getting mid to low 20's but i have a parrasitic energy drain i haven't been able to find so the trip comp and fuel mileage computer keep resetting it's self every day. I have to DC the battery when I park it. so I can only go off of instant. I have so many electrical problems with this car ie in the headlights that I don't even want to dive into finding the drain yet.

 

Well, your old tires were even smaller in diameter because of being worn, so going to new tires in the stock diameter (& being heavier) I'm sure you'll feel a change.

 

There must be something else going on to change the performance.

The tire change is equivalent to changing the axle ratio 1.5564202% or equivalent of changing 2.59 axle ratio to a 2.63. Or a 3.07 to a 3.11 ratio. I find it hard to believe you will feel that little of a change. Certainly not as much as you described. I have changed from the 285/40/17 to 275/40/17 and vice versa. They are the same 25.7" and 26.1" as your sets. When doing this change they are so similar that I have never noticed a performance change like you have had.

The PCV valve is trivial. It should not cause the problem either. However, if you created a vacuum leak it could show some symptoms. Usually you would see it at idle, but you say it idles good and smooth.

In summary, I doubt that it is due to the taller tires or the PCV. I would look for something else that might have changed.

Good luck.