Hey guys..

Was just wondering if there is any deference between the mazda protege 5 trany and the Speed? Besides the LSD??? i was wondering if the gear ratio is the same or different... Cause i wouold lke to drop my revs a little on th HWY...

Tom

 

The tranny gear ratios are identical. The difference is the LSD, the slightly heavier clutch (I doubt it) and the thicker axles.....other than that both cars ahve the 4:22 final drive (I may be off slightly, but it's a short gearing).

You could always drop down to a 16" rim with ultra-low-profile tires to help keep your rpm's a little lower. It'll roll a little faster at less rpm.I think 16" is about the smallest wheel you can fit over the MSP brakes.

 

Unless my mind is shot at the moment, which is possible, I think you have it backwards. A smaller diameter tire is going to raise the rpm's at any given speed, due to the shorter distance traveled by a smaller circumference. A larger tire is going to lower the RPM's, which is why trucks go to a lower (numerically higher) gear to compensate for larger tire diameters, as well as some of the rolling mass.

-Dan

 

The way I understood it is that a smaller wheel will rotate faster(speed) with less power (rpms)..

this is the case when I go drag racing. Switching from my 16" stock rims to a 17" rim dropped my trap speed almost 5mph. Switching back to the 16" wheel picked up speed in the same distance.

 

In one complete revolution, a larger tire will travel further than a smaller tire. This is what makes your speedo out of whack when changing tire sizes if not chosen carefully (width/height ratio). The difference in your trap speed was probably due to an increase in weight with the 17's, as our 16's are fairly light wheels. Even if the weight was close, the fact that the weight is further from the center increases the power needed to get it rolling (and lowering your trap speed), although once rolling it will have more momentum, necessitating larger/stronger brakes with large diameter wheels unless they're superlight.

 

That makes perfect sense. The rest of your post doesn't.. Maybe someone with the actual physics laws behind this can clear this up. I still think I'm right though...either way, the answer to yourmain question is: There is no way to change the rpm's at speed. Either deal with it or have someone custom make you some gears!! I wouldn't worry about it though....after your first 4+hr trip you get used to it!

 

If 1 RPM equalled 10 feet travelled with a small tire, and 15 feet with a larger tire, it stands to reason that you need less than one RPM to travel 10 feet with the larger tire, correct? That would lead you to see that lower RPM's can be achieved at the same speed with a larger tire. (I'm not trying to get into a pissing match)

 

I agree with you that THAT makes sense, but your explanation of why EVERYONE'S (not just mine) racing times are faster on a smaller wheel doesn't make sense. I know for a fact that putting a smaller wheel on my car (regardless of weight) makes my car "faster" (mph-wise) in the same distance when the car is doing the exact same amount of work. Also, there is not but about 3lbs difference between my 16's and 17's...so you think 12lbs is going to increase my trap speed by 5mph?? It just doesn't make sense.

I do agree with you though about needing less rotations to travel the same space, but that has nothing to do with speed which the gear ratio really handles. The ratio is the comparison of engine speed to wheel speed, not distance traveled.

 

I'm not big on drag racing so I haven't had enough experience with trap speeds and tire diameters to say for sure, I was just going with a little logic based on other factors. Although 3 lbs of weight per wheel on our little 4 cyl's would probably be noticed.

The gear ratio is actually a comparison of engine RPM to wheel RPM, which is different than wheel speed. The ratio does not change, so by changing the wheel diameter, you are directly changing the amount the engine has to turn to achieve the same speed.

Like I said, I'm not here to argue, and I'm not the all-knowing god of all things car related, so I'm gonna leave it at this and let a few others chime in if they want with their thoughts as well.

-Dan

 

yeah, I'm waiting to hear from someone else to...I'd like to see how your theory and mine both work, but produce different results.

 

Wheel weight (tire & rim) will have an effect on your acceleration. A heavier combination will slow acceleration down and cause trap speed to be slower. It also effects braking and suspension dynamics. There is an article about this subject in Motor Trend this month. The rpm's at highway speeds will be less if you can increase tire diameter. I am not sure if there is enough room to make a noticeable difference though. This is my biggest gripe about my car. I don't know why 5th gear is not lower numerically to decrease engine revolutions at highway speeds. It's like 4th gear in other cars.

 

so you're saying about 12lbs of wheel weight will cause a 5mph difference in trap speed? nope.

 

Maybe not 5mph, but it will increase your trap speed. Inertia is dependant on mass. Inertia is the resistance an object has to change its state of motion. The more inertia an object has, the harder it is to speed up or slow down. Now since the wheel is rotating, you have to deal with the Moment of Inertia as well. 1lb of rotating mass is like having an additional 6lbs of dead weight in the vehicle. On a rotating object, such as the wheel, having the weight further from the center magnifies this effect. Think of a figure skater spinning. As they begin their arms are extended, when they want to increase their spin rate, they bring their arms in. By switching to a smaller diameter wheel, you're bringing the mass closer to the center.

Roddimus Prime - When you switch from the 17" to 16" wheel is the overall tire height the same? If so, see above. If not and the 16" wheel has a tire that is shorter, you see a higher trap speed because the shorter tires are also effectively lowering your gearing. The engine has to work less to accellerate. The protege's gearing isn't optimized for the 1/4 mile. Using a shorter tire gets you closer. Depending on what your RPM is at the traps, a taller tire may benefit as well.

fasta76 - if you do go with a taller tire to decrease your rpm, keep in mind that your speedometer will have a slower than actual reading (ie. it will read 70mph but the cops will clock you at 77mph).

 

Rod, keep in mind the wheels and tires are UNSPRUNG WEIGHT, reducing the weight of the wheel produces almost exponential results.....dropping 3 lbs off a wheel makes a MUCH bigger difference than dropping 3 lbs off the car, as the the wheel is what the engine is actually turning...funny thing is that is why you see ricers with 17" giovanni's in the rear and 13" steelies in the front of their civics. It really does make the car faster.