Why am i....?? gear related

[jmir018]

why am i crossing the 1/4 mile almost at 5.5k rpm in FOURTH GEAR!!! at only 97mph?
my car used to be an aod. i installed 4.10s which i loved but i was crossing the traps at 5.5k rpm in 3rd gear.
Since i planned on getting more parts and going faster, I did a whole 5 speed convertion swap and installed a tremec 3550. i figured that would help me since it would add 4th gear to the equation. boy was i wrong. now i cross the traps at 5k rpm in 4th gear. wtf?
I remember having 4.10s in my 01 cobra and i'd barely be in fourth for a couple of seconds at the end of the 1/4mile at over 105mph? :whoknows:
Can anyone explain to me please why this is taking place?
I will be changing to 373s since i will be trapping more than 105mph now... is that going to be enough gear to make sure that i won't redline by the end of the 1/4 mile?
Thanks,
Jersey

 

[bimmertech]

third gear in an aod is 1:1. fourth gear in most manual transmissions is 1:1. thus not changing your rpms.

 

[larrendeuce]

The AOD in 3rd and the 3550 in 4th should be the same 1 to 1 ratio. Same with the Cobra.

Wow I was too slow to post.

If your running a different tire diameter it would throw it off. Also your speedo and tach could be inaccurate. When you swapped out the AOD for the 3550 did you get the proper speedo gear?

 

[jmir018]

i'm using the correct speedogear, but that only affects the speedometer, not the tach. the tach would be accurate regardless...
so, i'm simply left with getting a higher gear ratio once i modify my car, huh?

 

[BaXTeR3221]

Tire size might have something to do with it. Otherwise, that doesn't make much sence to me either.

 

[z9_87]

Yeah something doesn't sound right... Stock tack???

Look for a rpm's @ mph w/ X gear ratio calculator on the net. That should tell you how off you are.

With my crapy time on the left i barely needed 3rd (1:1)

 

[hllon4whls]

What the others have said is true. 3rd gear in most auto's is 1:1 and so is 4th in most manual.

The little bit of slip by the converter is apparent and lowers the trap rpm by approx 500. We also have to take into consideration the trap MPH. Possibly the stick is trapping a little slower.

 

[jmir018]

i guess, i'm not being clear, or i'm simply not understanding some of the replies.
My car (stock rims) with the aod and gears went 14.7 @94 or so crossing the finish line at around 5k rpm. after the swap, i've ony driven it once. 14.5 @97 or so @ 5.5 or so rpm by the end of the quarter mile.
I understand the whole 1.1 ratio and stuff.. that's not my question, my question is why am i already over 5k rpm in 4th gear at only 97mph?
I barely had to change to 4th gear on my former cobra at around 105-106mph with the same gear ratio.

 

[z9_87]

Here we go

GEAR RATIO = RPM * Tire Diameter / MPH * 336



RPM = MPH * Gear Ratio * 336 / Tire Diameter



MPH = RPM * Tire Diameter / Gear Ratio * 336



TIRE DIAMETER = MPH * Gear Ratio * 336 / RPM

RPM= 87.42 * 3.23 *336 = 94875.1776 / 21" (guess) = 4517rpms when I crossed the line in 3rd (1:1)... Fill it in and see what RPM you should have. We'll then go from there.

 

[super302]

your 01 cobra reved to 7k rpms, that is why it barely needed 4th gear

 

[jmir018]

your 01 cobra reved to 7k rpms, that is why it barely needed 4th gear

you are a genious..
I forgot about that..
i guess it's normal to be redlining on 4 gear with 410s..

 

[jmir018]

Here we go



RPM= 87.42 * 3.23 *336 = 94875.1776 / 21" (guess) = 4517rpms when I crossed the line in 3rd (1:1)... Fill it in and see what RPM you should have. We'll then go from there.

Hey, thanks for that equation.
here's what i did.

96*4.10*336/26 =5086.52
I thought i was close to 5.5k rpm.. maybe because my tires were 26 inches tall but barely had 20psi of air, so they were technicaly smaller than that.

Hypothetical situation,

110* 4.10*336/26= 5828 i could still go up to 110mph before redlining 4th gear.

Still doesn't quite work with my friends vert. he's redlining at 6k rpm in 3rd gear (aod) at only 101, 102 mph.

 

[larrendeuce]

Just thought I'd post about your transmission choice. I'm looking into getting a new transmission so I looked into a few things. Most 5 speeds use a 1.00 4th gear, most 5 speeds also split the 2nd gear on automatics giving a different 2nd and 3rd. A road race TKO has a .80 fifth gear so you have more pull when trying to top out. The cobra spec T-56 has the same ratio in 5th along with a .62(.63?) 6th gear. Then you have better acceleration to top speed along with the lower OD for fuel economy.

I tried a gear calculator on www.allfordmustangs.com and 4.10's top out in 3rd or 4th at 117-118.

 

[jmir018]

so.. is my 4th speed a 1.00 ratio?
that's what i'm trying to figure out. i should be hitting 6k rpm at higher than 101-102.. which is what seems to be happening.. i wonder what's cuasing this.

 

[maverick0716]

Yes, your 4th gear is 1:1 ratio. If you had a 26" tall tire and 4.10 gears you should only be trapping at 5121 rpm....I think your tach's off.

If you were trapping at 105 then you'd be at 5500 rpm.

 

[z9_87]

Like maverick said, your tach is off. If it's a factory tach and you're only off 500 rpms in the 5000 range, I think you're in good shape.
And yes your 4th gear is 1:1

Oh and advantage of my lower gears is I can to 120 in 3rd (only advantage)