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Discussion Starter #1 (Edited)
I apologize if posted before, but I disappear for weeks at a time and have no memory.

Someone correct my understanding:

A higher real axle ratio means more drive shaft RPMs per turn of the axle, which allows the engine to get up into the higher HP ranges faster, which means better acceleration. That same axle ratio, because it results in higher RPM at a given speed, means less gas mileage on the highway and a lower top end speed, since you hit the redline at a lower speed. Is this correct?

Now let's add tire size: A larger tire pushes the car further each time the axle turns, because of the larger circumference. Thus, a bigger tire turning the axle one time puts more pressure on the engine than a smaller tire doing the same thing, since it must move the car a few inches further.

Assuming the above assumptions are correct, and they may be wrong, it seems to me that a larger tire would counteract the benefits of a high rear axle ratio, in that we slap on that 3.73, 4.11 etc. to take pressure off the engine and allow for faster RPM gain, then put on large tires which add pressure to the engine and reverse the benefit of the high rear axle ratio by slowing the climb through the RPM band.

I must be wrong, but I just can't grasp the math. Help!
 

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You kinda got me confused ( not sure what your talking presure on engine) but here's how I'm pretty sure it works. I always called a rear end gear with a higher # a lower gear, so our terms won't compare. We are talking RTs so a 3.06 will turn less RPMs than a 3.92 with the same tranny. This will cause the 3.92 to accellerate faster. A 20" wheel will turn less RPMS than an 18" wheel and the 18 should accelerate faster.
 

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Don't forget about the effect you get with an auto with the torque converter. That's why most auto R/T's will outrun a 6 speed.
 

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That's why I said with the same tranny. The end ratio can change alot of whys by the ratio comming out of the tranny.
 

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True, it's somewhat confusing trying to figure out high/low gear ratios. By high, do you mean a low number or a low ratio? For example, 3.9 is a "high" number, but a "low" ratio. Quicker acceleration, but less top speed (and MPG)> Hope this helps.
 

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Before someone gets on here and gets tech. about the wheels. That can be minipulated too. If you used a low enough profile tire on a 20" wheel so the accual cercumference was the same as the 18" wheels tire they will run the same RPM.
 

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Yeah, but then it would ride like a ricer.
 

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I know and the op was saying it rolls farther so he plans on the tires being the same profile and the 20" being bigger around but some people on here will try to act smart and argue any point. That's why a specified my comment.
'
 

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I apologize if posted before, but I disappear for weeks at a time and have no memory.

Someone correct my understanding:

A higher real axle ratio means more drive shaft RPMs per turn of the axle, which allows the engine to get up into the higher HP ranges faster, which means better acceleration. That same axle ratio, because it results in higher RPM at a given speed, means less gas mileage on the highway and a lower top end speed, since you hit the redline at a lower speed. Is this correct?

Now let's add tire size: A larger tire pushes the car further each time the axle turns, because of the larger circumference. Thus, a bigger tire turning the axle one time puts more pressure on the engine than a smaller tire doing the same thing, since it must move the car a few inches further.

Assuming the above assumptions are correct, and they may be wrong, it seems to me that a larger tire would counteract the benefits of a high rear axle ratio, in that we slap on that 3.73, 4.11 etc. to take pressure off the engine and allow for faster RPM gain, then put on large tires which add pressure to the engine and reverse the benefit of the high rear axle ratio by slowing the climb through the RPM band.

I must be wrong, but I just can't grasp the math. Help!

From what I got out of your post it is correct. Fact I have an 09 SRT8 Auto with a 3.06 rear gear I installed the 3.91 in it. At 80 mph with the 3.06 it turned 2450 rpm's, at 80 mph with the 3.91 it turns 3050 rpm,s. If I went with a taller tire depending on which size it would lower my rpm at 80 mph and also make my car slower out of the hole. If I went with a smaller tire my rpm,s would even go higher than 3050 rpm,s at 80 mph and I would have even more acceleration out of the hole.
I hope this helps some.
 

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our cars make their highest torque at what 4200-4800 rpm. so from idle to that peak, youre not making max torque.

so add a low gear (high numerical) and the engine will rev faster to get to its higher toque numbers faster. add a bigger tire and the revs come down looking at the tire rotation.

add low gears and a small tire and youll be smoking them to eternity. huge tires (diameter) and high gear 2.76's and you have awesome top end but it will take you a day and a half to get there
 

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Discussion Starter #11
Thanks to all. I typed higher ratio when I should have typed lower. Everyone that replied seems to have figured that out.

My understanding is confirmed, but leads to a question. Why does a car maker put a lower rear end in its most powerful version of a musclecar then counteract that by putting larger diameter wheels and tires on that same model? Why do so many folks with eighteen inch wheels want twenties or twenty twos?
 

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Here's a couple of reasons.
Better Handling

  • Larger tires permit a larger area of connection between the tire and the road, improving traction or grip to the road. Better traction prevents slipping and allows for better control of the car's response. This equals better and more consistent handling.

Better Braking

  • Even though brake calipers and rotors do the actual stopping of the car's forward motion, the tires are a part of the braking process as well, because they are the only part of the car that touches the road. More tire-to-road contact allows for a quicker and shorter stop.

Read more: The Advantages of Larger Tires on Cars | eHow.com The Advantages of Larger Tires on Cars | eHow.com
 

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Thanks to all. I typed higher ratio when I should have typed lower. Everyone that replied seems to have figured that out.

My understanding is confirmed, but leads to a question. Why does a car maker put a lower rear end in its most powerful version of a musclecar then counteract that by putting larger diameter wheels and tires on that same model? Why do so many folks with eighteen inch wheels want twenties or twenty twos?

I think its just the times, bigger is better type thing. I went that route with my 4X4 put on 20,s that didn't last long. To much wheel not enough tire, I bought 17's and put on 285/ 70/ 17's and got my look I wanted which was less wheel and more tire or sidewall!!!!!!!
My car came with 20's so I'll leave them on it but my drag wheels will be 17's with a shorter tire for more out of the hole like 275/315 40/ 17 or about 26 to 28 inches tall.
 
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