Ok, need to hear from some gurus. Fairly simple question, and to make sure my thoughts are correct. I have a 2018 GT. Manual, so it has a 3.73 gear. I added some 305/45/17 Thomspon SS rear tires, the car feels a bit slower with moving acceleration, of course thinking the larger tires which are about 2 to 3 inches larger than stock caused this, I did a rear gear calculation and my final rear gear is now like 3.44 or so. Would adding more power offset the larger tires. I am going to add bolt on mods eventually, I am just curious if more horsepower is enough to get it to accelerate harder again. I don't want to change rear gears. This gearing thing is very confusing lol. People run these tires with automatic GTs with lower rear gear ratio so just trying to figure out if I am thinking correctly. Of course more horsepower is always good.
Unless you are talking blower type mods 9bolt on) ...there is no bolt on that will add any hp/tq…..to offset you are going to need about 100 hp and 75+ lbs of tq
Damn good point. I thought I was at 3.73 gears. Isnt the torsen standard? I need to check that. I have the base GT manual, non PP. But if I started at 3.55 then the larger tires really brought down the acceleration. I'll check this on the weekend. If I don't have 3.73 then getting that would fix the problem quick . I'll update you all.
Here is a pic of the tag. I am pretty sure I was wrong, it's 3.55 not 3.73, but gonna call dealer tomorrow and verify. If that's the case, the car really slowed down with the larger rear tires..
The first thing I would do, is downshift -- unless you are already in first or above about 5,000 RPM, you have a lower gear which will more than make up for the loss of torque due to the bigger tires. But yes I do understand you don't want to be screaming around at high RPM in a low gear all the time . . . but when you want maximum acceleration, that's what you need to do.
I find the diameter of those tires to be 27.8" or 726 Revs/mile . . . I am not sure of your original tire size, but in the past the standard was always 27.2" or 740 revs/mile . . . if so, that is only a 2% difference which really isn't a big deal, I am surprised that it is noticeable. The difference between 3.55 and 3.73 gears is 5%, so that would more than make up the difference.
Be forewarned, 3.73's tend to whine, and I don't know if that has been corrected recently . . . try some searches to learn more details about that.
Hi all, thanks for the great responses. I called the dealer and verified my mustang GT is 3.55. For some odd reason, I thought I had 3.73 SMH. I have to go ahead and check some more info on the 3.73 before i change anything. Gonna check the difference in tire size from original, I did go after market twice lol, i have two different set of wheels and tires. Plus, adding some small HP gains would help. I just need to really get on it in higher RPMs to see the difference and if it pulls better that way.
I am still checking on this, getting different researching results for rear gear on the internet. Called a different dealer and they said 3.55 as well. lol
Depending on variance you may need to calibrate the tires for the speedo to read right... Also I am surprised nobody has mentioned what I am about to, but why such big tires for a stock GT? I mean good grief those are huge tires for a stock 460HP car? I mean 275's or 285's would be great, but man 305's are HUGE! Heck my GT 500 has 285's and when I do tires I might go to 295's or 305's, but I am supercharged and putting down around the 550hp mark. I am not hatting the choice I have always like big rear tires on my Mustangs, but on my GT's I usually went 275-285 and never had any issues or complaints. Just my opinion for what it is worth.
I think you will for sure notice a difference in 3.55 to 3.73, but depending on what you use the car for 4.10 you will see a HIUGE difference. If using the car daily I would recommend going with anything greater than 3.73 though.
Yes you are correct. I went too big on the tires. Didn't realize it until it was too late, my first mustang so learning from all this. I've seen others have them and thought it would work out. Not Only that but the offset on my wheels made them stick out like and inch as well lol. My other sets are just fine. I am not gonna supercharge or anything gonna stay NA, I think the gear change would help with all this. But no rush and gonna think through this. I appreciate th straight forward response that is why I ask in the forums.
I’m with JBert the wider the better and you’re only at 27.8 so it’s not a huge tire diameter I run the same size diameter. If you go from the 355 to 373 you will almost fill no difference. If you just have the car tuned it will make enough power to make up for the difference. these cars Naturally aspirated with headers can put 500 horse power to the ground on E85 that will definitely make up for your small bump in tire diameter. If you plan to put big power in the car then definitely do not change to 3.73 you will never be able to keep traction. Most people actually go to the 3.15 when they start making good power to help with traction And they still have problems. If you want to go down in size I have 305/35/19s I would be interested in trade wheels and tires
I hear y'all. They are Mickey Thompson SS. I am staying the course. I'll do some bad a$$ pulls tomorrow and see. I think the difference I felt was just regular driving. Doing a tune adding CAI, e85, etc should help. Waiting for Ford to release the power pack for 18-19. See what they have to offer. I've done suspension. Shoot I even changed the front brakes to Baer so 17s would fit. The wheels look awesome all around. I think 3.55 is fine. I didn't realize the SS tires were less than 1 inch larger. They looked huge next to my other sets. I'll keep you all updated.
Update, It was the tires rubbing. I have BMR performance lowering springs had a 1/8 spring spacers in rear, I knew tires rubbed on fender but it was more than I thought. I added 1/4 spring spacers which raised the rear back to stock height, and no rubbing. That was affecting the acceleration. Took out the mustang and got on it hard. It was way better. The tire size is fine, not a large difference at all. Once I get a tune etc, it's going to be even better.
Wow, that's some serious rubbing! I assume you already checked the tires, to make sure the sidewalls are not nearly cut through . . .
I guess the underlying cause is that the 17" wheels are not sized correctly for those tires. If the width and offset were correct, I would not expect any rubbing unless there was a lot of lowering involved.
@JBert yeah. The offset is aggressive about +22. So the tires stick out about an inch. I noticed the loss of acceleration when someone else was with me in the car adding more weight. And I saw the rubber on the fender. I knew they rubbed by not as much as they were.. I checked the tires and they are fine. The wheels look nice and I got such a great deal I couldn't pass it up. Here is a pic and a close up, this is before adding the 1/4 rear spacer which brought up the rear by 0.50 inches..you can see the ring on the tire where it was rubbing...
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