Shift problems

[bigbear89406]

I have sum FR500 chrome rims that are 18" when i put these on my 94 they make my trans shift funny but when i put the 16's back on it clears up has anyone had this problem or know whats going on??

 

[toyman]

I have sum FR500 chrome rims that are 18" when i put these on my 94 they make my trans shift funny but when i put the 16's back on it clears up has anyone had this problem or know whats going on??

What do you mean by shifts funny? What are the tire sizes for the 16" and 18" rims? If you mean that the shift points have slightly changed that would indicate that the overall wheel and tire diameter are different between the 16" and 18" wheels. You would also see an error in the speedometer reading.

 

[bigbear89406]

The size of the tires are 245/40/18 but like when i get on it and it kicks down it will take the RPM up to 4000-5000 and hang,i have to let off the gas before it shifts but seems like when i have the 16's on i dont have any problems i have 3:55 gears in the RE.

 

[toyman]

Is it behaving the same whether it's downshifting 2nd down to 1st and 3rd down to 2nd gear? Have you changed the speedometer driven gear for the 3:55 gear change? Is there a shift kit that's been installed? The tire overall diameters appear to be the same so it can't be anything directly related to the different sized wheels. I suspect the issue probably occurs closer to the 5000 rpm range (not 4k) and it may be that you are hitting the rev limit (about 5.2k-5.4k for an AODE) before the shift point. IIRC the Ford PCM shift point is based on throttle position vs. speed. If the speed calculation is incorrect I can see where the rev limit may be reached before the expected shift point causing what you describe as hanging. If what I'm saying is correct I'm still puzzled that the change in wheel size seems to be part of the equation when the tire height is the same.

 

[bigbear89406]

No shift kit is installed and it shifts normal when i accelerate through 123 gears but when i step on the gas and it kicks down if i hold the pedal half way it will shift soso but if i floor it it will rev to 5000 rpm and hang then i half to let off gas for it to shift seems like when i have the 18's on it does all this but it could be coincidence.

 

[Chythar]

I was just about to write up a long post until I realized Richard had posted the exact same thing. But I can add a bit about the tires. The stock GT tires are 205/65/16, and you have 245/40/18. According to the tire size calculator, the tires you have on your 18's are a bit taller than stock so they will throw the speedometer off just a little bit. According to the tire size calculator at RimsNTires.com, your speedometer will be 1/2 mile too low at 60mph on the speedometer. This is a small amount, but if the speedometer isn't corrected for the 3.55 gears either you may be off just enough to run into what Richard is talking about.


Rim & Tire calculator:
Custom rims, wheel tire packages for your ride - RIMSnTIRES.com

 

[screamin5oh]

the width has nothing to do with it its the height of the tire that will throw off the speedo,

 

[Chythar]

the width has nothing to do with it its the height of the tire that will throw off the speedo,

Correct, dunno why I typed width. I corrected my post; nice catch.