Roddimus Prime

<~~P5 races on 15" steelies with drag radials and baby spares in the rear!

arapau

Prime- I think you've proved that heavier rims are slower to accelerate. You are still accelerating when your trap speed is measured and heavier rims will slow this process down. If your tires are 205/40/17, than your diameter is a little less than the stock 195/50/16 and should help acceleration. Your numbers actually show how much 12 lbs of rotating weight can affect the performance of these cars. Is this weight difference just the rims or the rim/tire combo?
I test drove a new Saturn Vue with optional 18" rims and the salesman warned me that the acceleration would not feel like the Vue with the standard 17's. Overall diameter was same for both. And the Vue has a 250 hp Honda V6.

GNO

In the eariler statement, I was discussing acceleration. Shorter tire --> more acceleration. 1) less inertia 2) numerically higher gearing.

In the second statement, we are trying to achieve numerically lower gearing. In this case we are not attempting to effect accelleration, we are studing a steady speed -- crusing. Icreasing the tire diameter will lower cruising RPM. As 03ESPro stated earlier To sample this, use two cylindrical objects with different diameters. Mark a point on each object. Start with the marking touching a flat surface and roll each object one complete revolution, until the marking is touching the surface again. You'll see the larger diameter object rolled futher.

Now when you increase the tire diameter, one rotation of the tire yeilds a greater distance. When going in a straight line and the tires aren't slipping, the speed of the vehilce is equal to the circumferance of the tire X the RPM of the tire. Since the circumferance of the tire is increasing, in order to MAINTAIN the same speed, the RPM must decrease proportionately. If you decrease the speed of the tire and maintain the same gear, the speed of the engine will decrease.

VagaBond-X

hehe well i got too bored to read the LOGN LONG posts near the end...

but where is the speedo gear located.... that will make a difference to what speed u see and what speed u are actually traveling....

I knwo GNO mentioned with larger tires u will be seeing 70MPH but u will actually be travellign faster.... i think that will amke a big difference int his whole discussion about speed, vs tire size...

from my understanding... (well i dunno where the speedo gear is located, so i could be totally wrong) but no matter what tire size you will always see the same RPM at the same speed....

like say you put on larger tires (taller) alright..... you speed upto 60MPH, u will still see the same RPM.... but you will actually be travelling at liek 65MPH....

now it will drop the RPM if you are ACTUALLY travelling at 60MPH, but u will see like maybe 55MPH....

i think it basically depends on where that speedo gear is located...

and as stated earlier.... the tire size will change the overall gearing, but just what u see might be different...dpendent on where the speedo gear is

03ESPro

Sorry Vaga, but you're really complicating this far beyond what it is. We're talking about actually reducing RPM's at actual speeds, although if tire size is changed significantly, the speedo will need to be adjusted. Probably needs to be done in the ECM, most new cars can't be adjusted with a speedo gear change like the old carbed cars.

Roddimus, one that might be confusing you about GNO's post is that you have to remember that gearing a car higher means actually going to a lower numbered gear (ie: a 4.22 to a 3.XX). This will reduce your cruising RPM's at the same speed, and will have the same effect as adding larger tires.

2.0GT

Drive slower to lower your RPM's

shane racing has a final drive gear of 4.30. It doesn't make sense to me that a smaller gear (smaller diameter or fewer teeth?) would reduce speed.

03ESPro

If the Protege has a 4.22 final ratio as Roddimus stated, then the 4.30 gearset offered would increase RPM at the same speed (mph), but would offer better off the line performance (ie: 0-60 time, trap speed), and possibly better performance all around, with the exception of gas mileage and maybe top speed.

Although, now that I think about it, unless your top speed is limited by redlining in your final gear, it may increase you top speed by keeping your engine closer to its peak power RPM when its needed most to overcome the huge amount of wind resistance at 120+ mph.

VagaBond-X

sorry about my complications hehe!!!

but hmmm i have a speed governor at 118MPH

so hmmmm maybe i should goto those lower gearings so that i'm at a good part of my powerband when hitting the 118MPH speed governor....

might give me some good accelleration lol

say l8z to my feul economy tho.... and me ever having a quite engine at highway speeds.... lol i probably get people passing me saying "change into 5th gear"... my reply... "I AM IN 5TH GEAR"

lol

03ESPro

After doing some refresher reading on Edwin's FAQ, I've seen that the actual final drive ratio is 4.105:1. No big deal.
What I did find interesting though, is that the Mazda3 with the 2.0 shares the same automatic tranny as our 2.0, although the final drive is 3.683:1 instead of our 3.904:1. This would reduce your engine speed by 200 RPM at 75 mph, and probably help mileage and cruising comfort considerably (less engine noise).
I haven't driven a 3 on the highway yet, just a little around town, and don't remember what the mileage is listed at on the sticker though. The other thing, is the 3 runs a 25.1" tire, while we run a 23.7", and going back to our discussion earlier, this would also lower the RPM at cruising by it being a larger diameter.
I'm not familiar with FWD trannies and final drives, but I assume they're basically the same as a RWD tranny and rear end, just jammed together. So... would the 3's final drive gear possibly be an easy swap for our Protege's?

GNO

Unlike RWD, you can't just change the gears in the rear end. Gotta go internal. . . how easy is it to pull the tranny?

03ESPro

Yeah, I figured it would require pulling the tranny which isn't quite as easy as a rear gear swap, but if it doesn't involve tearing into the tranny itself it could very well be a DIY job for those somewhat advanced enthusiasts.

GNO

You suck! I'm going to have chapter 5 of the shop manual memorized by sunrise if you keep this up.

03ESPro

I'd help you out if I had a shop manual..... but I don't... who needs sleep anyway?

GNO

Should be able to do it with just the ring gear on the diff. Might have to swap the secondary and output gear as well. Change your ratio while installing the LSD. The PCM will have to be adjusted or the VSS signals spoofed, otherwise, you're in for a whole heap of electronic related issues.

Roddimus Prime

GNO, where can I pick one of those shop manuals and what brand are you reading? Hayes?