Hello all. I had a question on tire sizes. I have a '07 G35 S Sedan. I switched over to Dunlop Direzzas. I had ordered 225/45/18 fr and 245/45/18 r from a local tire shop. I had read on this forum that people have been doing that without any problems. Well, apparently the manager heard me wrong over the phone. He ended up ordering 245/40/18 rears. I noticed it when they were installing it. He told me that the 245/40 made more sense since the VDC would be thrown off with 245/45. The car would think the front tires are spinning. He even called Crest Infiniti to verify. He told me 225/45 and 245/40 setup makes all four tires pretty much even in height so the speedometer wouldn't be thrown off. Is this true?

I have tried all the tire calculators and of course it says my actual speed will be less then speedometer speed which makes more sense. But I want to know whether the difference in speed is aggregate? eg- if one of the front tire is off by 1 mi/hr, and one of the rear is off by 2 mi/hr, will my total reading be off by 6 mi/hr [ (2 fronts * 1) + (2 rears * 2) ]? Just curious on all your thoughts. Thanks again.

-David

 

No, you don't add up the differences. Your car takes its speed reading off of the output from the transmission. Since you have a 2wd car, the rear wheels are the ones you should calculate for your speedometer reading. Therefore, by your research, your speedometer would show a reading that is 2mph faster than your actual speed.
By the way, can you take some pictures for us to see how the 245/40 and 225/45 tires look on the stock wheels? Thanks

 

Sorry. Just got back into town. I will take pictures tomorrow and post them up for you.

 

he'll tell you anything to get u to keep the mistake he made........I never ever believe anyone who sells me anything ever, just the way the world is.. do your own homework...and make sure you get what you ordered and paid for.

 

Here are pics of 225/45/18 fr & 245/40/18 rears. Sorry it took so long.

 

go back to the shop and get the 245/45's asap... i wouldnt believe anyone that works at any of those shops, they'll BS you when they make a mistake just so they dont have to do what it takes to make it right.
-GP-

 

Yeah those rears look too small...even if they aren't...they look it...

 

It appears to me the tire dealer is right. The diameter for the front tires is 25.97", the diameter for the rear tires he sold you is 25.71" and the diameter for the rear tires you ordered is 26.68". That would be over 3% difference between front and rear diameter and it would likely cause problems.

 

i've noticed my GPS speed is about 2mph slower than my speedometer speed, so my speedometer shows i am going 70mph and the gps shows i am going 68, and thats on the stock tires on the 07 sport

 

Here is a good source to clearly see his mistake http://www.miata.net/garage/tirecalc.html
Take them back and have him exchange it for what you bought. With the info at the tire size website you can take it to the end with this guy.

 

In my coupe i wanted a bit more of a stretched look in the rear so i went ahead and ordered 225-50-18 for the rears. If you run your calcs it will balance out by the .7% too slow, which is not a big deal for nice look