Smaller diameter tires

B

baias6230

Member
Car
2009 FX35
Name
Octavian
Guys i have a couple of questions regarding smaller diameters tires than stock ones. I would like to switch to this combination 275/40R20 and 315/35R20 (planning on buying BC Coilovers soon and this tires are the ones that i want).
Now, i understand the effect of smaller diameter tires on the speedometer and odometer (30.5 vs 28.7 diameter difference equals an almost 6% difference at 60 mph) but how about the effect on the transmission shifting points and also the redline of the motor. From my understanding you will make more rpm (i don't know how much...) at the same speed with smaller tires and that the engine will actually come quicker to redline so you have to shift earlier...?:thinking: I don't know if i get this part right...
I have seen some members that are running similiar setup (275/40R20). Anyone, even if they have a 1st gen, please chime in and explain to me a bit how this works because i have travelled the whole car forums but i did not get any clear answer.

Thanks in advance.
 
Baias, your top speed will be less, but you will have quicker acceleration from a standing stop.

It will rev up faster and your shift points will be at the same revs but your speed at each shift point will be less. Just make sure your front and rear diameters are the very same.

I have done this on rwd cars many times.
 
Thank you, Wayne. The tires have the same rolling diameter. I know all about matching tires and the effect of the smaller diameter tires on acceleration and top speed but the one thing i don't get is the effect on the transmission. That is why i am asking. So you are saying that with smaller diameter tires you don't have to shift earlier than redline? Even when driving in D? I really want to know all what it is to know before i make this mod.
 
All my experience is with my older rwd cars that I put smaller diameter tires on. On the ones with automatics, the cars shifted by the revs of the engine automatically, and those shifts were now at lower road speed. Your redline is still the same, it's just your road speed at your redline is now lowered.

Explaining another way : With the the smaller diameter tire you will get quicker acceleration, your redline is the same but your road speed is lowered although you should get to your road speed quicker.
 
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Ok, i understand now.Thank you, Wayne! Do you know how much the top speed will decrease? You could presume that your top speed will lower at the same percentage as the tire diamater decrease (6% in my case)? it is just pure curiosity because how often do you drive it at top speed...? I have done it only once and never passed 140 mph.
 
Everything is basically reduced 6%. Your engine/transmission don't know that the actual speed of the car is lower. They simply go by revolutions. So your car will still shift at redline, but your actual speed at redline in each gear is just lower by 6%. I'm running 275/40/20 and the car shifts entirely normally. Car shifts at redline if floored and at normal shift points otherwise. Your speedometer will show the same speed vs. rpm in each gear, but it's not accurate. Top speed would be reduced by 6% as well.
 
Thank you, shadow. Now has anyone any experience with Falken FK453 tires? Those are the tires i will buy (275/40R20 and 315/35R20). I've surfed the web and they've got some pretty good reviews. They are the cheapest good brand tires i can buy below 1000$. Just wanted some input from some fellow members.
 
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I purchased used wheels wrapped in Falkens. I test fitted and tried running around with it and took them off immediately. I find them too stiff making the ride harsh of an already firm ride quality of the FX. Open your wallet a little bit and stay with the median price range performance tires . I tried Continental extreme contact DWS all season for the first time and love it. It's quiet and most importantly very composed at high speeds around corners and very good wet traction.

I got a good deal from tire buyer.com with free shipping and a manufacturers mail in rebate.


Sent from my iPhone
 
Are you talking about FK453 tires? I have Yokohama Advan ST with reinforced sidewalls and i'm droped on H&R so stiffnes is not a problem. From the shops that are selling tires (including on-line) Falken is median price range performance tire. From the premium brands i would buy Michelins but the total price for 4 tires in that size is double the ammount of the Falkens... And i want to stay away from low-end tire makers that are offering this size like Zeta, Nankang, Federal, etc.
 
Yes, it was the FK453. Not sure for sure how many miles it had from the previous owner but it still has about about 50% tire thread. I can't speak for the Yoko Advan as I haven't tried it. I like the Yokohama Paradas too, though it wears faster. Probably my driving habits. :disapointed:
 
I don't know about the FK453, but the Advan's are pretty stiff. I ran them on my 22"s and they rode a little harsher then the Pirelli's I had before.
 
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