raybroussard

I installed a taller tire (255/55/17)on my car and now my rpms at 70 are around 2350-2400 and my mpg stays over 25 at this speed, With my 4.10 gears and my previous tires which were 255/50/17 my rpms were just a little to high on thehighwayand my car would vibrate and shutter at 75+mph. So after installing these new tires which are a full inch taller, all my pains went away! I can hit 60mph in 2nd gear at only 5900rpms where as before I had to up to 6200 or more...I am really happy about this because I was really starting dislike highway driving and now with this simple tire swap things have changed..I know it seems silly but little things like this drive me nuts!

Happy new years!!!!

AlloyGT07

Effectively what you have done with a taller tire is changed your gear ratio to a higher gear (lower numerically). I'm sure there is a math wiz somewhere thatcould do the calculation, or even an Excel tool that will do it for you. But with a taller tire, you're now running the equivalent of 3.90s, or 3.73s....maybe not as drastic as that, but you get the idea.

06hoss

Yeah, I was thinking the same thing. If thevibration thing wasn't an out of balance tire then you might want to take the car up past 75 mph to see if the vibration is still there. Sometimes that vibration is the pinion angle being off a little after the install of a new driveshaft.

CrazyAl

Exactly.

I am running 295/45s in the rear with 4.30 gears. That is roughly equivalent to 4.10s with the stock size tires.

Ray's setup with one inch taller tiresis like running 3.95 (if such a ratio existed...call it 3.90 to match what is commonly found)

raybroussard

Yep, the car is silky smooth now, I guess since we can't get the ford racing gears in the 3.90 size then you can justgo with the 4.10's and put a 28 inch tire and boom your problem is solved. I can hit 130mph in 4th gear now where as before 4th would run out hair over 120mph. Another thing to mention is my traction is a little better now as well, I can actually use first gear now!

07GT

What size did you install on the front? Is your car lowered?

moosestang

Post some pics!

I agree with whoever said a tire was out of balance. That's why the new tires eliminated your vibration.

hawgman

That is exactly what I did. 4.10 FRPPgears plus Nitto 305/45/18 tires comes out to a 3.87 gear ratio.

The equasion is your stock tire sizediameter divided by your new tirediameter multiplied by your gear ratio.

My stock tires were 235/55/17. My new tires are 305/45/18. I have 4.10 gears. So....

27.18 / 28.81 = 0.943 X 4.10 = 3.868

Johnyrrr

hawgman, i would love to see pics with that tire on the back, i keep seeing everyone with 295's on the rear but i want bigger than that. i had 295's on my 2003 GT and love the wider sive in the back. are there any clearance issues with the 305's think a 315 would fit out back?