Other than not being able to tell how fast you're going, what are the disadvantages of running oversized tires that are 3-4% bigger than your stock tires? Bigger meaning bigger overall diameter.

For example, stock size 225/50/17..... running 245/45/18 is 3.3% off (slower)


~limsandy

 

Speed being off can send false info to the tranny causing it to shift at the wrong times. That could cause tranny problems.(or at least thats what the guy who fixed mine said.)

 

I thought tranny shift is dependant on rpm and how deep you step on the gas pedal? But what you said does makes sense too.

I don't really think the speed issue would be that big of a problem. At 60 mph, I'm actually running 62 mph. And the lower the number, the more accurate it becomes.

Does anyone know how our speedo gets the number from? I believe it's from the driver's side front wheel, but please correct me if I'm wrong.


~limsandy

 

Main disadvantage is the slight variance in calibration of your speedometer and odometer. The Tach will read correctly, however, although a bit lower at a given forward speed in a given gear (which will not be readily evident unless you calculate your new speed as "speedo reading X 1.033". Actually, the height of the 225/50/17 would be 25.858 inch versus the 245/45/18 at 26.681 inch...a diff of .823 inch or 3.183% of the original 25.858. The difference is not that much and within spec for a passenger car. After all, the oriiginal tires do wear out and when they get worn down its like having a tire that is at least 10mm shorter (5mm wear top and bottom) and sometimes 20mm shorter (10mm wear top and bottom) when they really get worn down which is nearly an inch in height! (25.4mm)
The business of it throwing off the calibration of your transmission is just bull. The transmission shifts at different RPM based on load (throttle application), right? It will also shift differently if you have 5 people aboard and 400 lbs of luggage in the trunk. A 3% tire diameter increase is designed into the car. In fact, engineers usually design in for tires that are two sizes larger, and in the case of SUV's and Trucks, up to five sizes larger. Not to worry. I just put some 265/75/15 on my Pathfinder that was originally equipped 235/70/15. That is going from 27.95 to 30.65 diameter...roughly 10%. Speedo reads lower, as does odometer (have to add 10% miles to calculate M.P.G.). The Pathy now turns 2050 RPM at 60 mph, 2390 at 70, and 2733 at 80. The ABS works fine, the car rides better than ever, tracks better than ever, more stable, better traction, and it gets better highway mileage. It doesn't have as much oomph around town, but now the low range 4 wheel drive is very usable on steep roads in the typical NE Snowstorm.
One word of caution: you are going up two tire widths from 225 to 245. Be careful about any further increases without consideration to wheel well, rim offset, and suspension travel . You are probably OK to the 245 with properly fitted 18's but I can't say for sure and I would be very cautious beyond two size increases, even with the lowered profile. My experience with cars has been that the third size often produces some rubbing, but each case is different.

 

As said above, I'd be more worried about potential rubbing issues unless you know exactly what you are doing (fitment-wise) and have some reliable anecdotal evidence to suggest that it wouldn't rub. In any case, this is a trickier issue because you are changing the tire size in 2 dimensions instead of the usual 1 dimension (width only).

 

bump anyone?


~limsandy