look at my sig for details
tire sizes are 225/55/16

 

GTUs rearend p'raps?

 

Youre off by a lot....4.30's wont put you anywhere near that. Especially considering his taller than stock tires(225 55 16) If his speedo is right, he has 5.12 gears.

 

dude if i have 5.12's O_O

 

4000rpm in 5th at 80mph sounds right.

FYI different tire size is meaningless. you could have 4" tall tires and at 4000rpm your speedo would still say 80mph. but you wouldn't be going that fast.

 

 

so you're saying that the speedometer doesn't measure off the transmission? What kind of magic does the car use to measure speed, then? It just guesses?

Tires certainly will give a misreading.

 

Yes, the wrong sized tire will make you rspeedometer inaccurate, but no matter what tires you have on your car, your speedometer will always read the same # at the same RPM in the same gear. (That's why the wrong sized tires CAN make your speedometer inaccurate.)

In other words, the speedometer reading would be the same if the car was sitting on blocks, with no tires on it, spinning at 4000rpm. It has nothing to do with the size of the tires, or if the car even has tires. The reading is based on how fast the output shaft of the transmission is spinning.

 

I don't know if I believe that.

Having different sized tires is also like changing your rear end gear. Hence, there will be a difference in rpm to indicated mph.

Of course, maybe I'm wrong...I'm trying to actually put it all on paper now to either prove or disprove myself

 

Different size tires will make a difference, they transmission will turn at a difference rate depending on size. If the speedo cable is measuring transmission rotation, different size tires will throw the speedometer off.

 

hmmm In my sport I do 80 at about 3100 rpms ???

that sounds off somehow. Perhaps you didn't get the GTU's gear for the speedo cable?

 

"Different size tires will make a difference, they transmission will turn at a difference rate depending on size. If the speedo cable is measuring transmission rotation, different size tires will throw the speedometer off."


Wrong! The transmission and speedometer will always read the same no matter what tires. I think there is confusion here. Even though the speedo and tach read the same, if you have smaller tires than OEM, you will be going FASTER than what the speedometer is reading. If you have bigger tires and rims than OEM, you will be going SLOWER than what the speedo is reading.

For example, if the tires are bigger, the car must travel further to cover one complete rotation of the wheel. The tranny does not know or care about this because the speed calibration is coming from the output shaft of the trans, NOT the drive wheels. Therefore, the car will read 80MPH, but because the wheels have to travel more distance per rotation, your acutal speed is slower. This is why people re-gear their differentials for greater acceleration, especially if they are running 17" or 18" rims, they need to offset the difference in outer wheel diameter.

The only effect that bigger or smaller wheels have on a transmission is acceleration time. You will accelerate faster with 205/45/15 than you will with 205/50/16s, but the speedo will still read the same.

Kurt G.

 

I get 80 at 3K on my 87 GXL tach.

 

I think you have it backwards on the speed difference but please corect me if I am wrong.

If you have a tire with the circumfrence of 100" at 4k rpms it will spin at x speed say 4k rpm with a 1-1 rear end. therefore you are turning the wheel and mothing 100" per rotation times 4k which would make it 400,000" per minute.

Now if you have a larger tire with a circumfrence of say 125" spinning at 4k rpms with a 1-1 rear end you are turning the wheel and moving 125" per rotation times 4000 which would equal 500,000" per minute. How would that make larger tires go slower?

Santiago

 

N/a's have taller 5th gears than TII's. A larger than stock tire will make you actually be going faster than the speedo reads, while a smaller than stock tire will actually make you be going slower than the speedo reads. The speedo is taken off the output shaft of the tranny, so a change in diff gear or tire size wil throw off the actual speed, but without changing the speedo drive, the reading on the speedo will not change.

Like I said before, I did the math, and according to his speedo, he has 5.12 gears. Obviously, for the speedo to be right with those gears, the speedo gear would have to be changed, but that would have been done with the 5.12 gears.

 

do you run out of 1st gear instantly?

 

Not sure how you define that as wrong but the speedometer would definitely be showing the wrong mph, that's my point. Speedo would not read the same because your actual MPH is not being indicated on the speedo.

 

sorry for the double post

 

Wow definitely can see the physics majors in this thread.

Those who said that the output shaft speed doesn't depend on tire size are correct.

The speedometer is taking the reading from the shaft but the inaccuracy is coming from where the tires grip the road. smaller tire = spinning more times per rotation of the shaft, and vice versa.

For the tire size to be affecting the speedometer it would have to mean that the size would be slowing down or speeding up the output shaft, which makes no sense. there is no noticible difference in drag between tire sizes. edit - the only thing i could see is if maybe the rim itself was lighter or heavier it might affect the output shaft but that wouldn't affect your speed being accurate or not.

 

I think we can agree that its normal to be at about 3000-3200 rpms at 80 mph, considering its a stock na tranny, and 4:11's. Could it be something as simple as his clutch is slipping, cuaseing it to do that, taller tires would also put a little more strain on it, trying to pull harder in that tall *** 5th. Just an observation... Ill let you physics dudes figure it out.

 

Sorry for the confusion guys, what I meant to say and what I wrote were two different things, you're right that I got it backwards. I meant to say smaller tires read that you are going faster, but are actually going slower. Bigger tires read that you are going slower, but are actually going faster.

Kurt G

 

you're right, i think now that i think about it. longer legs = equal amount of energy = faster

 

His clutch definately isnt slipping. Taller tires would put more strain on the engine, but that wouldnt affect the speedo reading. It would only require more power to go the same speed.

FWIW, n/a's have a .711 5th gear, while TII's(and the 89-90 GTUs) have a .756 5th gear. TII's should be higher in rpm in 5th than n/a's, but not nearly that much. And its not just his 5th gear thats off, its every gear. How many TII owners can barely break 60 mph at 7400 rpm in 2nd? How about topping out 3rd at 90?

 

my tii is in stride at 90.

do you think the previous owner replaced the tranny with something else?

 

I think the owner replaced the diff gears. The tranny is very unlikely.