Tire size vs Gear ratio

[prchrman]

Can anyone tell me if this brainstorm has any validity. If you want to up your bottom end performance and do not want to change rear ends and be concerned with warranty issues (as I am). If my calculations are right going to a 245 X 35 X 18' will give you in reality a 4.10 ratio when compared to the stock 245 X 55 X 18'? I believe someone out there does know the answer to this.

[TAF]

anyone else?

[LincMercLover]

Umm... I think that's the opposite. I think you'd need a 245/80/18 (which is a non existant tire) to achieve a 4.10 ratio... I could be wrong though...

[RF Overlord]

Willie:

Making the outside diameter of the tire smaller is the same thing as changing to a higher numerical gear ratio...but it may upset the ABS, which changing the gears would not...

I don't know which tire size would result in the desired 3.55-to-4.10 change; show us your calculations...? 3.55 to 4.10 is about a 13.5% increase...

[prchrman]

245 X 55 = wall height of [(134.75 mm X 2) + 18" ie. 457.2mm] X 3.14 = cir***. of 2281.8 mm
245 X35 = wall height of [(85.75 mm X 2) + 457.2] X 3.14 = cir*** of 1974 mm
1974mm is 86.5% of 2281, hence 3.55 is 86.5% of 4.10.
So RFO you are correct in the 13.5%. I do not know if this calculates to real world power equality where the rubber meets the road. This is the real question. By the way 245 X 40 equated to about 3.95, which might be nice too.

[TAF]

not + not X (Ellie May's measurements in a bathing suit) + (number of X's on Granny's "medicine") = ...Oh forget it...

[LincMercLover]

Disregaurd my previous post. I'm:rasta: for some reason because I did the diameter ratios backwards... You got it dead on there though. I got 4.103. However, a 35 profile tire would be EXTREAMLY skinny, but if you're just using it for race, go with a slick in that size. Hoosier has a nice tire for that!

[prchrman]

Really skinny, not to good on wheels, maybe, but oh what handling!Falken has a 245 X35ZR for 181.00 on Tires.com.
The dealer can reprogram for this size to eliminate any issue with the ABS. This is just a thought and wonder if anyone has tried this. Please reply if so and what are the results.
By the way todd it's knot X knot. I speak fluent woodhick.

[nomad]

It would probably achieve what you're looking to do, but man, talk about low ground clearance and a real Flintstone rock-ride!:help:

[SergntMac]

Originally posted by Willie Anglin
Can anyone tell me if this brainstorm has any validity. If you want to up your bottom end performance and do not want to change rear ends and be concerned with warranty issues (as I am). If my calculations are right going to a 245 X 35 X 18' will give you in reality a 4.10 ratio when compared to the stock 245 X 55 X 18'? I believe someone out there does know the answer to this.

If this a test, I'd rather try to pass the Bar exam first. Me and numbers are not good friends, however, I would like to toss my .02c in, perhaps from another angle?

Warranty issues aside, Willie, you want to replicate a permanent performance feature with a consumable product. I don't know the driving qualities of the tires suggested, but someone mentioned they are 180. per. One set of tires buys the 4:10 gears and two sets of tires later, your in the red. The dollars don't make sense, financially speaking.

IMHO...head to head identical MMs, one with the the 4:10, the other with your tire specs, and I would bet my money on the 4:10 car. You still have to get both cars launched from a standing start, therefore, both sets of tires will have to deal with the same sudden power. Both will suffer from identical launch dynamics, the "ballooning" and sidewall deviation of any tire, from adhesion under sudden power. (think what slicks look like in a fuel rail's burnoutl?) In effect, you change the tire's size and footprint, but change them equally in this scenario. I would not expect your tire advantage to overcome the presence of 4:10s, until you start out with tires that are size calculated to meet your specs under hard power...I think.

I'm sure someone can explain this better than I have, so, have at it, I'm outta nickles.

[LincMercLover]

Sarge makes a valid point. The tires do not change the power to the ground factor, but merely the roll ratio. Once the car is moving, then yes, the tires will be identical to that of a 4:10. It's the intital force that it's lacking. With 3:55's out back, you're still only getting 3.55 revs to the ground, as opposed to 4.10 revs, which means less power from the get go.

[RF Overlord]

Originally posted by Willie Anglin
By the way todd it's knot X knot. I speak fluent woodhick.

Willie: you may speak fluent woodhick, but I speak fluent Jethro Bodine, and I think it's naught X naught... :lol:

[RF Overlord]

LML:

I think I respectfully disagree...since the driveshaft, the differential, the tires, and the ground are all connected (unless you break the tires loose...), then changing either the number of teeth on the pinion, or the ring (or both), or changing the tire O.D., is all the same as far as leverage.

If you think of the tire/ground as a big rack-and-pinion, with the tire as the pinion and the ground as the rack, then reducing the tire diameter is exactly the same as putting a smaller pinion gear in; you get more transmitted power, but less RPM.

Sarge does have a valid point concerning the tire's cost and it being "consumable"...the gears are a one-time cost...and if you drive like LightningVic and transfer all your rubber to your wheel-wells and bumper, you will go through them quickly... :D :burnout:

[LincMercLover]

Think about it. With the 4:10's, you're keeping the stock diameter of the tire (around 28m I believe) and putting an additional 0.55 revolutions from the motor to that "roll." With the tires (24m I think it was), you're getting the same 3.55 revolutions to a shorter "roll." This is allowing more power to the ground in the BEGINING. Once the car is set into motion, there will be hardly any difference. Like Sarge, I'd be putting my money on the geared car...

[Marauderman]

No matter what- It ain't gonna happen ---changing tire size that is --but we have discovered a new thread ----woodchick---to survive with some of these threads --you gotta have humor---Oh Yeah--Todd's was right up there!!! And Sarge---you gotta appreciate his method of dissecting
this whole thing to put in it simple terms----the real gears out weigh--out put- the numbers of tire changes= $$$$...so...back to our new thread --what was it again Todd....

[prchrman]

Thanks for the input-I sure do not know enough to argue with any observations-may wear out 245x55 and try 245x40-not made out of money-got extended warranty-previous AOD owner---can also be nought----woodchick???????

[VaderSS]

The main difference between using gears over tires to achieve a higher leverage, is that a shorter tire will put less rubber on the ground than a taller tire, holding all other parameters the same.

This means that the traction issues that you have with stock size tires and gears, will be compounded when going to a shorter tire, giving the same overall gearing but less rubber, and less traction.

With the same width tire, the shorter tires contact patch will be as wide as the taller tire, but not as long. Example, the contact patch of one tire might be 4L x 5W, while a shorter tire (bit same width) might be 3W x 5L.

Hope this is not too confusing, its hard to explain...

[RF Overlord]

Wayne, thanks...

I agree that going to a smaller diameter tire is not as desirable as doing the gears, my point was simply to answer Willie's question and explain that changing either the gear ratio or the tire diameter will produce the same result purely in terms of launch and acceleration. Changing the gears is in fact the better of the two solutions, for the other reasons that you and the Sarge mentioned...

Peace Out...

[SergntMac]

Some interesting brainstorming going on here, isn't that great? I expect that for a lot of us, we have forgotten how much we do know about stuff like this. I know I've learned an awful lot of new stuff these past few months, and I've remembered as much or more. What more could you ask for, in entertainment from an Internet site.

[RF Overlord]

Originally posted by SergntMac

I know I've learned an awful lot of new stuff these past few months, and I've remembered as much or more.
I agree 100%, Sarge...I wouldn't have had the nerve to install Dennis's chip if it weren't for this site... :bows:


What more could you ask for, in entertainment from an Internet site.
Not only that, but I've made some good friends, even if I haven't actually met any of you...yet (except for Moparzz) :beer:

[smith5365]

Sarge: Absolutely agree. All the ideas I have of what mods to do I've gotten from reading on this site. As you've probably noted, I love to throw out an idea and see how the group here chews it over, and how we finally get a consesus of what make sense.

[prchrman]

Let's see--less tire contact--spinning--hit power band--holy cow!--hang on!--barge be gone! Great info men

[TAF]

I'm in Charlotte (Matthews) for the next two days. If you want to take one for a spin with ALL the mods...send me a PM.

Todd

[vaderv]

The taller gears will also exert more leverage to the axle which is the one constant in this thread. Taller gear = longer lever. The twisting force at the axle will be much greater.Considering that..this argument is moot.

[SergntMac]

But, it was fun to explore, eh?

[RoyLPita]

Here's a crazy theory:
How about even size tires? Put the 235/50-18 tires that are on the front in the back, too. Advantages to this: chance to rotate for longer tire life, a little better acceleration (may also improve slalom time and speed), and "God forbid" you should have a flat (especially in the back), the spare tire will not wear out the differential clutches prematurely. Drawbacks: you will lose the 1.5" rake that the taller tires give and it will cost you to recalibrate the speedo and ABS at your local dealer. If I had a Marauder, I would do this because I drive 15k+ miles per year and I couldn't tell you how long the Tire Rack is going to keep the tires on special ($85 rear, $95 front +S&P).

RoyLPita
P.S. check the math on the gear ratio, please.

[prchrman]

This info is from a car nut who happens to be a M.E.- smaller dia. tires will put the same torque to the ground as a different gear. He claims it actually will put more to the ground because of the loss power from the numerically higher gear and also because of the loss of weight in the tire. I know everyone is not going to agree with the smaller tire but it will put the same torque to the ground. I am going do this when my factory set wears out and have the Merc dealer change my computer to match the tire size. Will also be able to rotate tires. M.E. will get formula and will post later.

[RF Overlord]

Thank you, Willie...I was beginning to doubt myself (shoulda paid more attention in high school physics class), but it's nice to see confirmation from a Mechanical Engineer...although as I admitted before, replacing the gears is probably still the better of the two options...

[prchrman]

RF - my concern is the warranty issue, I got extended to 72 mons - I'm not sure my dealer will honor if I do gear change - I can change it myself w/no problem if I want and it would be cheaper than tires - and I live in a mountainous reign where handling is a priority.

[SergntMac]

Looking at this gear vs. tire issue from another perspective has been enlightening, it's always nice to see an argument (theory) turned inside out, and finds that it still works, or did, until your last post Willie, when your rationale for such a change came to the surface. When this thread started out, I thought we were just brainstorming options. I didn't think you were looking for a "get-a-round" way to peak performance without risk. Change your tires, Willie, and you'll have the end affect of a taller gear on paper, but you'll come up short in real time performance.

Now, back to your warranty protection for a moment. You think you'll blow your warranty by changing the gears, and I won't disagree that this is a possibility when the repair bill is high enough for LM or the dealer to want to stall paying out big bucks. But, take this advice from someone who used to do this work himself (albeit many years ago) that changing the recommended tire size and recalibrating the ECM and ABS to agree to that change, also voids your warranty.

As someone once said to me here..."LM didn't build your car, you did." and that's what you'll hear, when the bill runs high enough. The MM is a specific design, with exact specifications. Tamper with those specs, and the manufacturer is no longer responsible for maintaining the results of a change in designed specs. My SM told me that if I needed any warranty repairs after my gear change, the car would be restored to it's factory specification of a 3:55 gear. I would have to supply my own 4:10s at that time.

The guys here who have added Class III hitches face these same consequence, whether they ever tow anything or not. The mere presence of the hitch could be a problem. So, be it tires or gear, Willie, YOU have modded your car from designed specs, to the same extent I have in changing gear ratios. Expect the same risk in the mod, and the same answers from LM, when the bill is high enough.

[prchrman]

thanks sarge - I will check with my dealer before changing the tires - never even comprehended tire size would be considered for warranty issues - I do however believe it does equate to real time performance because of past history and the math does not lie

[LincMercLover]

Good point Sarge! I didn't even think about that... If one of his axle seals would happen to give it up, they could blame it on the tire size that caused excessive spin of the axles, prematurely wearing out the seal.