3.73s too much with 18s (25” tire)?

[biggsy]

I would like to move to 18s (25” tire) next year but after doing some reading and going over my last time out at Summit, I think the gears are too short. I know at some point in this journey I may go down to 3.31 to help it out but while I still try to get seat time, I just want to know if I’ll be fine with shorter tires. Home track are between Summit and Dominion. I plan to venture out to NJMP, VIR, Pitt Race a couple times each next year. I appreciate any insight.

 

[blacksheep-1]

You'll be fine, but 3.73s are well within the operating range of most tracks, some even 3.9 or 4.10
It's also not just the wheel size, you can get 18s with a sidewall that are taller than some 19s.
Companies like Hoosier will have that info on their website

Gear Ratio Calculator

Easy to use gear ratio calculator. Use our gear ratio calculator to find the correct gear ratio to run when changing tire sizes to keep your stock gearing.

tiresize.com

 

[biggsy]

You'll be fine, but 3.73s are well within the operating range of most tracks, some even 3.9 or 4.10
It's also not just the wheel size, you can get 18s with a sidewall that are taller than some 19s.
Companies like Hoosier will have that info on their website

Gear Ratio Calculator

Easy to use gear ratio calculator. Use our gear ratio calculator to find the correct gear ratio to run when changing tire sizes to keep your stock gearing.

tiresize.com

Thank you for that! Just didn’t want to run out of 5th all the time. I understand how tire sidewall impacts overall diameter. I was looking at 295/30/18 in the new Continentals contact force

 

[JDee]

I think the only remaining mod I'm doing to my car is to go to 3.55s. With 3.73s I am banging into the 7250 redline that is in the Ford Racing PP2 tune on the back straight all the time and I don't think that's good for the motor. It might cost me a bit of time, though I doubt it will be much, but it may turn out to be a wash as well because there's at least 1 other place on the track that I have to upshift a gear for a couple of seconds to keep revs below redline and a lower numeric gear would avoid that. I think it's going to be better for the engine at the very least. That's with 305/30/19s. There seems to be 1 or 2 spots like this on every track I run.

Once again, it ain't racing, it's lapping, so the point is maximum fun for minimum bucks.

 

[RodS197]

I am driving a 2014 Mustang GT and have the 5th gear 1:1 as well and 3.73 gears. My car originally came with a 19" wheel, so my running with 18" has produced the same results you are anticipating.

My home track is Utah Motorsports Campus. I have been thinking about going to 3.55 in the off season as well, so I took the telemetry from the AIM Solo 2 DL that I run and calculated the RPM values at my top speeds for the straight and lowest speeds for the corners.

You would have to do similar for your track, but for me it will make it so I can use 3rd out of some of the tight corners and get some legs from it, and stretch 4th on some straights that I now have to use 5th and I should be able to do the long straight in 5th (we call 6th the parachute) and reach about 142 at about 6600 RPM, just above my self imposed 6500 max. I will have to shift more than I do now, but that isn't a bad thing as I need to be a bit more aggressive, but I just don't like running above 6500 all the time. I am going to go with the 3.55. By may calculations the 3.31 was just too tall to work on my track.

I have attached some screen shots. If you think it would be helpful I could get you some spreadsheets that might help you decide.

 

[RodS197]

One other thing, I would suggest you put the tires on first and drive the car before making other changes. That way you would have a baseline with which to go on.

 

[biggsy]

I think the only remaining mod I'm doing to my car is to go to 3.55s. With 3.73s I am banging into the 7250 redline that is in the Ford Racing PP2 tune on the back straight all the time and I don't think that's good for the motor. It might cost me a bit of time, though I doubt it will be much, but it may turn out to be a wash as well because there's at least 1 other place on the track that I have to upshift a gear for a couple of seconds to keep revs below redline and a lower numeric gear would avoid that. I think it's going to be better for the engine at the very least. That's with 305/30/19s. There seems to be 1 or 2 spots like this on every track I run.

Once again, it ain't racing, it's lapping, so the point is maximum fun for minimum bucks.

I agree with the minimum bucks lol. I have an 18 manifold and I read that the redline increases as you upgrade manifolds but I dont know the science/math behind it. I have a problem being too high in the power band so if I can help it then that would be great because shorter tires are not going to make it an easier.

I have attached some screen shots. If you think it would be helpful I could get you some spreadsheets that might help you decide.

Thank you very much! I will do the same with the data I collected form my last time out. Your sheet was really helpful

 

[Beermoney]

I run 315/30/18s (25 inch tire) on a 2013 Boss which should be the same gear ratio as your car. I think the updated gear ratios for the MT-82 came in 2018.

On occasion it is inconvenient but overall it isn't an issue. Usually can feather the throttle at the top of the redline to avoid the shift if your super close to a braking zone.

 

[PaddyPrix]

I run 315/30/18s (25 inch tire) on a 2013 Boss which should be the same gear ratio as your car. I think the updated gear ratios for the MT-82 came in 2018.

Heh, yeah, they're very different on the MT82-D4, and reading the first line of the post my mind immediately jumped to, what in the hell track are you on that you're topping 5th, when I'm taking 4th to 160mph . The T56 Magnum XL and MT-82D4 are (generally) very similarly wide geared double overdrives, just like how the MT82 and TR-3160 are but single.

 

[JDee]

Usually can feather the throttle at the top of the redline to avoid the shift if your super close to a braking zone.

I've been told by an old school racer that this isn't a good thing to do, but I have to do it to keep out of the rev limiter. The theory is that when the engine is under load at high revs it's happier than when it's lightly loaded. This is due to clearances in the engine, when heavily loaded they are consistent but when the engine's revving high but lightly loaded things can kind of "float" for lack of a better term and that floating is not good.

I'm not sure if this is legit, but it seems to make sense to me. Maybe not so true with modern engines, I don't know. But that's the biggest reason why I am looking at 3.55s.

 

[docs302]

Since you mentioned NJMP I will give you something for comparison. I have a 13 Boss with the stock MT-82. I have experimented with many of the 18" tire sizes. At all the tracks (including both at NJMP) with the stock 3:73 I was always getting into 5th way too early. The 305/650 an 660 were far too small and it was only by dropping to a 3:31 that I was able to keep 5th to a minimum. The 305/680 slick and 315/30 DOT are the ideal sizes for my combination but even with the two short tracks at NJMP I hit my rev limiter (7500) and need to grab 5th for a couple of seconds. If I extended my rev limit to 7800 I could stay in 4th until the brake zone. I'm usually at well under 140 mph in 4th when I hit the limiter so at long tracks (WGI, VIR) I will be in 5th longer and that's a crappy gear.

 

[2012-Boss]

The T56 Magnum XL and MT-82D4 are (generally) very similarly wide geared double overdrives, just like how the MT82 and TR-3160 are but single.

The close ration T56 Magnum XL is not geared anything like the MT-82D4. The only thing they have in common is a double overdrive. The close ratio Magnum XL gear spacing is actually closer in most gears than the early 1:1 5th gear MT-82.

The ratios in the MT-82D4 are horrible for a performance car. First through fourth are way too widely spaced - your never seem to be in the right gear around a road course. One of the best modifications we have done to the 2018 and newer cars is to put an older close ratio MT-82 into the car.

As far as the OPs question, drive the car after you make the tire change. Having said that, most of the people I know who swapped the 3.73s to 3.55s for road course work were happy with the results.

 

[biggsy]

Thank you all for the information and feedback. I am still focused on seat time right now so not focused on time. I was hoping going with 295/30/18 or 315/30/18 wouldn't make it unbearable. I guess I will have to be more attentive to the tach if this is the route I go.

 

[Bill Pemberton]

I guess my reaction would be why are so many hesitant to be in 5th gear at a few of the high speed tracks? I run a 315/30/18 , but almost every track I run I love getting out of the corners harder and I honestly seldom run out of 4th gear. When one does need to go to 5th occasionally you also have to balance what the best gearing is for the rest of the track. Often we get caught up in the top speed in one straight where the rest of a track finds lower gearing needed elsewhere. I understand the need to go to 5th at various courses, yet I would wager the majority of the tracks only need 4th - heck I don't run out of 4th at Topeka at 130 with 315s.

The new Continental/Hoosiers ( Force ) is a great tire designed for Endurance racing or the happy HPDE driver , so I concur that would be a very solid choice.

 

[Beermoney]

I've been told by an old school racer that this isn't a good thing to do, but I have to do it to keep out of the rev limiter. The theory is that when the engine is under load at high revs it's happier than when it's lightly loaded. This is due to clearances in the engine, when heavily loaded they are consistent but when the engine's revving high but lightly loaded things can kind of "float" for lack of a better term and that floating is not good.

I'm not sure if this is legit, but it seems to make sense to me. Maybe not so true with modern engines, I don't know. But that's the biggest reason why I am looking at 3.55s.

It makes a ton of sense as far as valve to piston clearance and things like that come into play. I'm not sure if stock engines have tight enough tolerances to make this happen but at 7k+ I'm sure it's possible.

 

[2012-Boss]

I guess my reaction would be why are so many hesitant to be in 5th gear at a few of the high speed tracks? I run a 315/30/18 , but almost every track I run I love getting out of the corners harder and I honestly seldom run out of 4th gear. When one does need to go to 5th occasionally you also have to balance what the best gearing is for the rest of the track. Often we get caught up in the top speed in one straight where the rest of a track finds lower gearing needed elsewhere. I understand the need to go to 5th at various courses, yet I would wager the majority of the tracks only need 4th - heck I don't run out of 4th at Topeka at 130 with 315s.

The new Continental/Hoosiers ( Force ) is a great tire designed for Endurance racing or the happy HPDE driver , so I concur that would be a very solid choice.

You need 5th gear for a lot of the East coast tracks and some of the Midwest tracks I have run (VIR, Watkins Glen, Summit Point, Mid-Ohio, Road America) with the MT-82 and 3:73s in cars from 2011 - 2017.

Your 4th gear is 1:1, which is why you seldom run out of 4th gear with the Tremec in the 302S. The OPs 4th is a 1:3 (i.e. your third gear).

 

[Bill Pemberton]

Thanks 2012 Boss, Senior moment on my part and you learn or are reminded of things often as you age ---kind of like learning new items , ha I almost listed the tracks I knew one would run out of 4th gear , since I have run VIR, Sebring, Road Atlanta, Road America, Mid Ohio, Utah Motorsports Campus , etc.. Currently there are so many newer tracks without a really long straight, yet I was thinking Boss S and not the M-80 Bossesand appreciate the correction. I still stand by wondering why folks worry about running into 5th for a bit, and as BS-1 noted the 25.5 diameter tires do work well on the Mustang , but your input may help the driver decide based on the majority of tracks he/she run. The Boss S also had the rev limiter set at 7800 and that helps stretch it's legs a bit more than the 7500 cut off on a regular Boss 302. Thanks for your input , I had completely forgotten the 4th gear ratio differences.

 

[consultjr]

I would stick with 3:73's with 25.5 inch tires. The only place you may run into an issue is on the outer loop with braking at the 2 marker (based upon your 6500 pm limit). I would feel comfortable with your engine at 6750 rpm's. Even if you get close and breathe slightly off the throttle because you will be a lot faster coming out of Blackrock (turn #5), out of Right hook and Knockout (#6), Attitudes (#8) and Windup (#12?). I feel your lap times will be a lot faster with the 3:73's. Remember on the Coyote engines, they are a lot more efficient in the higher RPM range (i.e keeping RPM above max torque at approximately 5000 rpm's). That being said the 3:55's are not the end of the world but it depends on your goals, tracks you plan on running and what you feel comfortable with.
Note: this response was in reference to RodS197 post. Also, if you are going to run Toyo RR's, I believe they make a 315x35x18 which would help to achieve the approximate 5% difference without having to change diff.

 

[RodS197]

I would stick with 3:73's with 25.5 inch tires. The only place you may run into an issue is on the outer loop with braking at the 2 marker (based upon your 6500 pm limit). I would feel comfortable with your engine at 6750 rpm's. Even if you get close and breathe slightly off the throttle because you will be a lot faster coming out of Blackrock (turn #5), out of Right hook and Knockout (#6), Attitudes (#8) and Windup (#12?). I feel your lap times will be a lot faster with the 3:73's. Remember on the Coyote engines, they are a lot more efficient in the higher RPM range (i.e keeping RPM above max torque at approximately 5000 rpm's). That being said the 3:55's are not the end of the world but it depends on your goals, tracks you plan on running and what you feel comfortable with.
Note: this response was in reference to RodS197 post. Also, if you are going to run Toyo RR's, I believe they make a 315x35x18 which would help to achieve the approximate 5% difference without having to change diff.

Great points JR. I will need to look at some more data and see where I am on those critical corners...

Rod