Rubrignitz

I ordered my GT with 3.55 gears, one of my definite requirements knowing I was going to put 20's on it. I would haveconsidered 3.90/4.10's if they offered them. Everything felt good with the 17's, but now I have 20' rims/35 series tires and it really feels like I need more gears again.Iinstalled 4.56 gears and a Richmond PowerTrax in my truck after hearing how GREAT they were on dodgetalk.com with 20' rims (26' tall tire) and it ROARS on the highway. It's now relegated to justhauling my boat to the lake and of course, moving my friends crap[:@]around every once in awhile, so it's all good. The Richmond PowerTrax is acutually the greatest mod I've ever done to a vehicle. That thing never loses traction.

I want my stang to be a little nicer and somewhat more docile at 70-80 mph. I have a complete JBA (LT's, h-pipe and axle-back)exhaust system, so at 3k rpm she starts to get pretty loud. My truck rushing by scares the crap out of people at 80 mph/3k rpm. It feels great from a start to 60mph, then gets really annoying. I don't want to go through that with my car.

With 20' rims and 35 series tires, would 4.10's be a better choice than 3.90's?Would going from 3.55's to 3.90's really make a cost-justifiable mod?

Jizz

HUH?

Did you change the overall height of your tire? Give me the full facts please...

Old tire ...
New tire....
Old Gear...

PLstang

So if I'm understanding you; you want to stay under that 3000 rpm mark where the car gets loud (mine does that too) and want to know if cruising at 80 with the 20" wheels and 410's will do that?

Rubrignitz

Ok, now you got me outside taking measurements. The old tires with 17' rims and 255/55's were 26'. New tires rims 20' with 265/285 35's are 27' tall. Gearing is 3.55.

Rubrignitz

Yes, pretty much that's what I'd like to know. I really don't want to break 3k rpm at 80mph. I used to have a link somewhere to calculate gearing/tire diameter. If anyone knows where to find that info,...

Jizz

Ok so you need to make up the revolutions...

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
255/55-17 5.5in 14.0in 28.0in 88.1in 719 0.0%
285/35-20 3.9in 13.9in 27.9in 87.5in 724 -0.7%

Higher gearing i.e. 3.73 or 4.10 will make the car rev higher at speed. If you want lower RPM at 80 you need to lower your gears.

Ok ok I see what you are asking now...

MTAS

Yeah you definitely lost a bit of off the line grunt there Put it this way - I have Nitto 555 295/45/18's on the back, with 4.10's. Since those tires are roughly 1" taller than stock, my 4.10's are now an effective 3.90 or so (there's a formula out there on the interweb somewhere I used). When I had the 4.10's installed, I didn't know I'd be putting the 295/45's on 6 months later. Had I known that, I'd have gone with 4.30's from the get go

Rubrignitz

Ok, thanks guys. IF I decide to change out gears in the near future, I'm going with 3.90's. With my current setup that should equate to ~3.73 final and I plan to go with some type of forced induction once my warranty runs out. I don't think I really want to run 4.10's with forced induction...

moosestang

Your stock tires were 235/55/17 for one thing. Your new tire 285/35/20 are 27.9 inches tall. With those tires and 4.10 gears you'll be doing 84mph at 2600rpms in 5th gear.

By the way, this can all be figured out with free online calculators.

http://www.miata.net/garage/tirecalcold.html

http://www.angelfire.com/fl/procrastination/rear.html

Black07gt

so whats a good tire size for 20's? 265/285 or 255/275? or 245/275"?


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