Rear Gearing question - Ford Mustang Forum
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post #1 of 18 (permalink) Old 3 Weeks Ago Thread Starter
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Rear Gearing question

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.

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post #2 of 18 (permalink) Old 3 Weeks Ago
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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


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post #3 of 18 (permalink) Old 3 Weeks Ago
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2018 manual trans rear axle gear ratios are as follows.

3.31 on standard models, 3.55 with the Performance Package, 3.73 with the Torsen Performance Package.

Could it be that you didn't actually start out with 3.73:1 gears?


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post #4 of 18 (permalink) Old 3 Weeks Ago Thread Starter
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Quote:
Originally Posted by Dana W View Post
2018 manual trans rear axle gear ratios are as follows.

3.31 on standard models, 3.55 with the Performance Package, 3.73 with the Torsen Performance Package.

Could it be that you didn't actually start out with 3.73:1 gears?
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.
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post #5 of 18 (permalink) Old 3 Weeks Ago Thread Starter
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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..
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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.

using this on-line calculator
https://tiresize.com/calculator/

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.
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post #7 of 18 (permalink) Old 3 Weeks Ago Thread Starter
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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.
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post #8 of 18 (permalink) Old 3 Weeks Ago Thread Starter
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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
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post #9 of 18 (permalink) Old 3 Weeks Ago
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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.

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post #10 of 18 (permalink) Old 2 Weeks Ago Thread Starter
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Quote:
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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.
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Quote:
Originally Posted by Rpinaiii View Post
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 would say go burn them off, but that will be tough without breaking something. haha What brand tire you get? I think you will get used to them. I honestly can't believe you can feel a difference. I have never felt a difference other than having more traction, but I have never gone that large or that much difference from one set to another like that. Hmm I am trying to think of a solution for you. Years ago I would suggest a intake and AM BAMA tune, but not after doing that as they never got my tune right so I will never try another mail order tune. With that said though if you have any local perfomance shops I would check in their and get them to dial you in a nice custom dyno tune and that will add some power in to help make up what you are feeling is a loss. They will also be able to help you with the gears and questions you have there.
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My personal opinion, for what it's worth: there is no such thing as "too big" when talking about tires on these cars; assuming you are talking about width; and assuming the tires fit the wheels correctly and fit inside the fenders without contacting anything.

These are fairly heavy, and fairly high torque/horsepower cars; and they like a lot of tire. It takes a lot of tire to get close to the "tire to weight" ratio of a Miata, LOL! The road track guys do everything they can to stuff the widest tires under there that will fit. 305 or 315 is what they shoot for, on all 4 corners.

Even the diameter of the 305/45-17's isn't very far off from the "standard" 27.2" (I am not positive, but would not be at all surprised, if that is still the standard coming out of the factory) The only issue I would have with that tire choice is to make sure the wheels are correctly sized to match the tires -- pretty sure you need an 11" wide wheel for those; with the correct offset so there is no rubbing anywhere through full suspension travel.

Quote:
Originally Posted by Rpinaiii View Post
. . . the offset on my wheels made them stick out like and inch . . . .
Other than the wheel sizing, potential rubbing or the very small change in final drive ratio, I really don't see any "problem" with those tires. For the road track I would go with 18" or 19" wheels and lower aspect ratio, but I assume you are going for more of a drag car.

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post #13 of 18 (permalink) Old 2 Weeks Ago
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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
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post #14 of 18 (permalink) Old 2 Weeks Ago
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It’s only .8 inches taller. Not even 1” you really should not feel a difference. Just a tune will easily make up for it
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post #15 of 18 (permalink) Old 2 Weeks Ago Thread Starter
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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.

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