i've decided to either go with 18x9's or 18x10's. my question for all you guys who have already done this is: i know you get more traction with a wider tire, but if i go with a wider tire/wheel will this speed me up or slow me down any? thanks guys
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2007 GT Premium 5 spd w/ IUP Diablo Predator BamaChips 93 race tune JLT II CAI SLP LoudMouths
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Joined: 10/9/2007 From: Harrisburg PA Status: offline
My experience is the change is negligible. My 'winter' tires are 235/55/17 on stock rims (not the bullits, not sure what the style is called). My 'summer' tires are 255/45/18 on 18x9 DD Bullits all around.
I have switched back and forth twice (had each set on twice) and can't say the car felt very different. I definitely would not say the car is less powerful with the bigger tires or they in anyway slow me down. Also I am quite happy with the handling both ways, although the bigger tires have more of a tendancy to be tail happy (but thats more a result of the tire, not the size). But this VERY much depends on the tire/rim combo you switch to. In my case, the bigger tires and rims only weigh about 2lbs more per corner. You can get 18" rims that are far lighter, so you could actually drop a few pounds by moving to a bigger tire/rim combo depending on what you buy.
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07 Vista Blue GT 5spd, BBK CAI, Brenspeed, Raptor, 18" DD Bullits, MGW Shifter, 4.10s, Eibach Pro, D-Specs, Violators, Cervini's Grill, Spohn adj PHB & UCA
My takeoffs are faster nor have I noticed any difference at speed. You'd think rolling resistance would increase but I haven't seen it in my fuel mileage.
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Rick '07 GT Prem, Alloy, MT, 3.55's, Steeda CAI/Xcal2 w/Brenspeed tunes, 14" shorty, 275/40/18 Goodyear Eagle F1 Allseasons on SVT 2007 Cobras, FRPP Stinger
but if i go with a wider tire/wheel will this speed me up or slow me down any? thanks guys
If the new wheel/tire package is heavier than the existing one, you'll slow down ever so slightly. Enough to be measured with timing equipment that's good to 0.001 second such as you might find at the dragstrip or at most autocrosses, sure (if you're that consistent anyway). But not enough to tell using just your butt-o-meter.
Some numbers to chew on using Argonaut's 2 lb/corner difference taken half as added wheel weight and half as added tire weight. These are the amounts of time quicker and shorter distance required for a 25 - 45 mph sprint in each gear (MT, no shifting). The distance numbers also represent the difference in a side-by-side race where the wheels/tires are different in weight but not diameter and everything else about the two cars and the driver capabilities/reaction times is absolutely identical. I doubt that anybody is good enough to notice accelerations that differ by 1/5 of 1%. The differences in the distances, you'd notice, but only by comparison. You'd never see them if you first drove one car and then the other.
I would agree that wider tires = more rotating mass, but the traction different between the two should make up for that. When I put on my Shelby rims, I didn’t notice a difference in weight. I sure there is but it can be more then a few pounds. The car grips much better now and I can’t wait to get it to the track.
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2005 Mustang Gt 5spd with 3.55 Brenspeed tuned BBK CAI, JBA LT headers, BMR LCA&Panhardbar, Steeda UDP, and Pype Mid-Muffler 289RWHP and 307RWTQ
.................more rotating mass too. Takes more energy to turn that big sucka.
Got the rotating mass effects part of it entered right, but I didn't also add the 8 lbs to the total weight. The spreadsheet works well enough, but isn't 100% user-friendly quite yet (even for its developer. )
Posts: 656
Joined: 10/9/2007 From: Harrisburg PA Status: offline
Hey Norm - thats good stuff. Can you do that calculation for 20" rims where the rim and tire weighs...lets say 10 additional lbs per corner (is that a good number?). I'm curious how big an affect installing 20s has.
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07 Vista Blue GT 5spd, BBK CAI, Brenspeed, Raptor, 18" DD Bullits, MGW Shifter, 4.10s, Eibach Pro, D-Specs, Violators, Cervini's Grill, Spohn adj PHB & UCA
Tire weights I can find, but wheel weights are anybody's guess depending on the design. The sheet is on the other computer, so it'll have to be tomorrow.
It's almost a wash at 5 lbs per wheel and identical tire weights, believe it or not. In the case of the 20's, you gain just enough on having a shorter rolling radius to pretty much make up for 5#/wheel. Keep in mind that rolling radius affects engine rpm, which affects where on the torque curve you're at. And when the weight differences are so small, the effect of overall gearing and torque curve shape become relatively more important.
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Here are the summarized differences between Car 1 and Car 2
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I did find a couple of very minor things in the sheet that weren't quite right. They affected the 1# per wheel / 1# per tire scenario by a maximum of about 0.001 seconds and 2".
Here's the actual input area. The torque and HP curves are fit to the HP and torque figures at their rpms plus an estimate of a low rpm data point. It's crude, but if you're comparing different gearing you do need to have some idea how the torque varies with rpm. Whether it's close much past the maximum HP point hasn't been all that important, as I normally set the redline within 500 rpm of where peak HP is developed.
thanks guys. i don't think i can go wrong going from a 17 x 9 with 235/50's to an 18 x 9 with 255/40's b/c i want the chrome deep dishes and they aren't offered in a 17. but i can't decide if i want to go 18x10's in the rear so i can go wider tires or just stick with 18x9's all the way around. it's bad when you lose sleep over this stuff
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2007 GT Premium 5 spd w/ IUP Diablo Predator BamaChips 93 race tune JLT II CAI SLP LoudMouths
thanks guys. i don't think i can go wrong going from a 17 x 9 with 235/50's to an 18 x 9 with 255/40's b/c i want the chrome deep dishes and they aren't offered in a 17.
Don't you mean 255/45-18? A 255/40 is about an inch smaller in diameter than a 255/45 in the same wheel diameter, so unless you're reworking everything to suit 26" diameter tires . . .
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but i can't decide if i want to go 18x10's in the rear so i can go wider tires or just stick with 18x9's all the way around. it's bad when you lose sleep over this stuff
How much wider of a rear tire are you thinking about? Might be that even 10" rims aren't really wide enough either.
Do you need/want to be able to rotate the tires?
Is having a wider rear tire worth the slight increase in understeer? If not, are you willing to retune the suspension?
Norm
< Message edited by Norm Peterson -- 4/7/2008 4:33:37 AM >
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RE: wider tires speed you up or slow you down? - 
