I am considering heavily a 315/35/20 on the rear (with height-matching 275/40/20 front) which is an increase from my stock (non brembo) 27.25 up to a 28.68 inch tire.

I have 3.31 gears which are already fairly tall. I have considered 3.37's but I am not sure its worth the ~$600 after install.

My question is- does anyone know a way to figure out what the final drive difference is between the stock tires and these 315's? Not just a final drive number but to put it into perspective as a rear gear ratio---

So stock is 3.31- these tires would be like running a "3.2 rear gear"

I really want 315's in the back as I love the look but I am worried about gearing this car to the moon which will of course lessen performance. It is a 3.2 MPH difference at 60 so I am thinking it will not be a drastic change but I want to know for sure before I pull the trigger.

 

No comment on the math, im on my phone so i cant figure it out, but make sure the talle tires wont rub.

 

http://tire-size-conversion.com/gear-ratio-calculator/

 

If they fit without rubbing then your only issue would be the spedomeeter being of by ~5.3%. So when your speedo shows 70pmh you're really going 73.5mph.

 

This, and there's one on discounttire.com

 

Thanks so much for that calculator- didnt know that kind existed.

According to that calculator I would need a 3.48 rear gear to negate the taller rear tires, so it would be the equivalent of running a "3.14" gear if I kept the stock 3.31's with these tires. That seems like a pretty sizable difference! If I were to install 4.10 gears it would actually be a 3.88 gear. If I installed 3.73's it would be as if they were a "3.53".

I am told there is no rubbing in the rear with a 1.5 drop and a slight rubbing in the front with a 275/40/20 with the same 1.5 drop but only on big dips.

The speedometer being off does not bother me at all but feeling like that drastic of a gear change is worrying me. The mustang fund is going to be tapped after the tires and I am not sure having 315's is worth having that tall of gearing considering I already wish it was shorter! I may have to stick to the 19 inch rims instead and run a square 275/40 setup as those are 27.66 tall tires, so .4 inches difference instead of a 1.4 inch difference.

 

Another consideration is the weight of the wheels/tires, especially with one set being taller. More weight will be further from the centerline of the axle, which hinders performance. Even if the two setups weighed exactly the same, the taller tires would sap a little more performance then the shorter ones as the weight is further out.

I'm not an engineer (yet), so I don't really know the specifics, or whether or not the difference will be significant in your case...but it's something to consider.

 

Ya those are both a heavy consideration. I know rotating mass is the LAST place you want to add weight to a car.

I think this is another reason im leaning away toward huge meats in the rear...the 315/35/20 tire weighs 39 pounds (varies slightly between manufacturer) and the wheel weighs 27 pounds (AMR) for a total of 66 pounds.

The stock wheel/tire from what I can find is 55 pounds.

The 275/40/19 tire on SVE drifts comes in at 59 pounds.

Going to 275/40/19 would be an increase in total rotating mass of ~ 16 pounds. Going to 315/35/20 in the rear and 275/40/20 fronts is an increase of ~ 36 pounds! I am guessing that is noticable in both acceleration and deceleration.

Thanks everybody for the help! I think I am going to go with the SVE Drifts in a 19 inch 275 square setup- there are just too many downsides to having those huge 315's out back.

 

What is your concern with the resulting gear ratio with the larger tire? There will certainly be a difference in the acceleration performance, but is there anything else you're worried about besides that?

P.S. I'm planning on running a square 275/40-19 setup for track days starting this summer. I have the 3.73s in my Track Pack, and I find the gearing a bit too short for the track. This setup will give me a few extra mph in each gear.

 

My concern is the slower accleration felt due to a taller effective gearing due to taller tires. That, combined with the extra 11 pounds per wheel/tire i feel will really zap the acceleration of my stock 5.0 with 3.31 gears (which already feels slow compared to a 3.73 car).

 

In my experience you will notice the weight more than the acceleration. If your car had 235/50/18 on it (Stock plain Jane GT), rather than going with a 315/30/20, go with 315/35/20. The difference between the 235/50/18 and the 315/35/20 is only a few points different in axel ratio, looks just fine, preserves some of the ride quality and adds a greater margin of safety from wheel damage. In order to reduce weight, if you can swing it, go with a good lightweight forged wheel to cut a lot weight off each corner.

John

 

I was looking at 315/35/20. The tires are 9 pounds more each than the stockers and the rims 2 pounds more. I am on a tight budget for rims so i was between sve drifts for 19's or AMR for for 20's.

There are no other good sizes in 295/305/315 that are not hugely expensive so 315/35/20 was the only option in my price range.