How much will 20's slow me down?

M

mustangdave

My rearend needs a stud and two nuts.
Founding Member
Feb 26, 2002
2,976
1
56
North Carolina
#1
I f I were to go from 17" wheels, stock tire size to 20's, how much slower would the car be in the 1/4 mile? The weight is one factor but the larger diameter would effectively change the gear ratio for the worse. I have the optional 3:55 gears. Surely someone has made this change and ran at the strip for comparison. I don't know what tire size I would run but I'm sure the overall diameter would increase. Thx in advance.
Also, how would this affect gas mileage?
 

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#2
The weight is a factor, but the tire diameter really isn't. When you go to 20" tires you go to lower profile tires, so the diameter of the tire is the same. Just less sidewall.

Go to tirerack and get the revolutions per miles for the stock tire, then use the wheel guide for 20" wheels. Whatever tire size they suggest, check it's specs, you'll see only a slight variation from stock.
 
#3
I don't have any specific data to prove this. But I just went from 17's to 20's (staggered 255/35/20's Front & 295/30/20's Rears) and I can tell you that "seat of the pants" power there is a slight loss as well as a mpg loss (approx 2 mpg around town). To try to counter-act this problem I'm eventually going to purchase an aluminum driveshaft (to help reduce the unsprung weight and rotating mass). Was it worth it? Yes, cause it looks so much better now! Plus, it gives me a reason to add another mod to the Mustang.:nice:
 
#6
O.k., this is what I found out using the tire calculator: going from stock to a 20" wheel with 275/35/20 tires, the overall diameter would increase by .4 inches, sidewall height would drop from 5.08" to 3.78", and revs. per mile would decrease by 11.1.
Dividing 11.1 by 4 means the tires would turn 2.775 less. That's as far as my old man math skills get me, but it seems that turning roughly 3 less revolutions in the 1/4 shouldn't cost more than a tenth or two.:shrug:
Adding that extra 5 or 10 lbs. at each corner would surely cost also, so as a complete guess, I could see this mod slowing the car down no more than a half second. Good reason for 4:10's or a blower.:D
 
#7
If you plan on racing, keep your stock 17" wheels and put some slicks on them. This will keep you from burning up the tires for the 20s (which are on the costly side to be racing with) AND give you more traction than you had with the 17" street legal tires. Problem solved! And on MPG, going from the 18" polished bullits during the winter to the 20s during the summer every year, MPG change isn't even noticeable for me, the only thing that REALLY hurts gas mileage is your right foot :D
 
#8
A lot more unsprung weight with 20's over stock 17's. It will affect acceleration, braking, and gas mileage. For overall performance I would go with nothing larger than 18's on a mustang, but that's me.

I went from 17's to 20's on my 2007 Silverado with 5.3L V-8 and it negatively affected acceleration, braking and mileage dropped 3 MPG. Enough where I immediately noticed the large shoes.

Just depends what you want.
 
#9
20" Wheels

I went from the stock 17" to 18" because the outer diameter of the wheel and tire is the same as stock (27.5") with 255/45/18 front and 275/40/18 rear...

I did not experience any change in MPG but car had got greater handling and braking due to a bigger wheel/tire combination...

Torque is better because I was smoking the 17" so bad but that could be a result of a wider tire in back...

20" look cool if you want to show your car but as for performance, stick with a setup diameter that is close to OEM stock...

Dave
 
#15
mustangdave said:
O.k., this is what I found out using the tire calculator: going from stock to a 20" wheel with 275/35/20 tires, the overall diameter would increase by .4 inches, sidewall height would drop from 5.08" to 3.78", and revs. per mile would decrease by 11.1.
Dividing 11.1 by 4 means the tires would turn 2.775 less. That's as far as my old man math skills get me, but it seems that turning roughly 3 less revolutions in the 1/4 shouldn't cost more than a tenth or two.:shrug:
Adding that extra 5 or 10 lbs. at each corner would surely cost also, so as a complete guess, I could see this mod slowing the car down no more than a half second. Good reason for 4:10's or a blower.:D
Click to expand...

You didn't factor in how bad the traction would suck. No sidewall, no traction.
 
#16
Some Mustang owners are into looks with their Mustang, some are into performance. Performance wise going from 17's to 20's is going to kill your acceleration. People only look at it from a difference weight-wise. What they don't account for is the fact that it is rotational mass. It's gonna take a ton more effort from your car to get those wheels to spin. The lighter the wheel, the easier it is for the car to turn up. I replaced some 17x9 and 17x10.5" 03' Cobra wheel on my old car with a set of Weld Pro Stars and I was floored at the difference in how much easier the engine would rev up and the difference in quarter miles times. But as previously said, different strokes for different folks.
 
#17
Not a great comparison but maybe give you an idea. With 20's and street tires my car went 13.3@105 last SUMMER (FLORIDA) in march this year with 17's up front and 17's outback on dr's it went 12.70@107 in cool weather. IMO that's not much of a loss. I'd probably trap about that on 17" wheels in the summer while it's 80+ degrees. On the highway, best I got with stock 18's was 31.0 on a long road trip and going to work I could average 29-30 on a regular basis...after the 20's and no changes I averaged 27-28 with a best of 29.5. Everybody has different driving styles and there are many variables but that is my comparison.
 
#18
6 tenths is a huge difference in the world of drag racing. Sure the weather may have accounted for 2-3 tenths difference, but regardless, even a difference of 3 tenths is phenomenal. That's comparable to a large modification to your car. But like I said, different strokes for different folks. Some people like to just cruise, I like to haul ass :)
 
#19
I personally wouldnt go bigger than 19" (lightweight) so that I could atleast still run nitto 555r. In fact I think I will be moving from my 18 deep dish rear with stock bullits up front, to TR motorsport MT1 18x8.5 and 18x9.5... 19lb per rim, plus 32lb mickeys in the back. From what I remember thats 305/45 MT ET streets.

I also agree that half a second is a huge difference. in fact it's about 2-3 car lengths at the 1/4 market
 
#20
Thx. NotQuick; that was exactly what I was wondering. For those hung up on 1/4 times, I would keep the 17's for dedicated drag rubber, but honestly I have very little time to go drag racing. Another reason for 20's is big brake clearance; hard to do with 18's. I'm still debating the change but I know my American Racing wheels (on car when I bought it) won't balance for crap, so if I buy new wheels, I'm looking at Foose Legend due to clearance and chrome all the way to the backside of the wheel.
 
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