What would the top speed of a car be if it had a t5 with a 3.55 gear and say around 350hp.

 

It depends on several factors. First If your ecm has a speed limiter on it (mine did until I reprogrammed the chip). Second what is your 5 gear ratio? Do you have a RPO Code MK6 or M39?
MK6 .73 5th gear. M39 .63 5th gear.

What Rpm are you willing to go to?

Some of these factors have to defined before an actual answer can be attempted.

 

My car is rid of the computer crap cause I did a v8 swap and I got rid of the computer and most of the electronics. So I am pretty sure that I don't have a limiter. I think I have RPO code M39 and I will be willing to go to 5800 Rpms

 

Theoretically........198 MPH at that RPM in 5th gear. Lol, but there's no way 350 HP could put out that kind of power to get it up to 5800 RPM in that gear. Does anyone know a formula to figure this out? Thanks.

 

He'll need 575-650hp to hit that speed.

 

what kind of horsepower would I have to have to hit 155.

 

I ran the #'s and got 192mph, although 3.55 gears is not a normal GM ratio (8.8" or 9" Ford, yes).

To pull 192mph, you will need 425hp or better, which isn't going to happen. With the big gear spread of the M39, it makes it even harder.

 

121 is what I got. Thats all 4th gear is gonna get ya.

Oh yeah, 7400 rpm's will get you 155 mph in 4th

 

280rwhp gets my brothers 96 T/A 6 speed with 4.10 gears to 155mph at the top of 5th gear I believe.

 

My formula requires tire circumference not diameter. The formula is: RPM times tire circumference divided by rear gear times a constant of 1056 equals MPH, you multiply first then divide. In this case using 4th gear and assuming you have a tire circumference of 80 inches, follows: 5800 X 80 :/: 3.55 X 1056 = 123.77 MPH. The math for overdive is to multiply the rear gear. Example: 5800 X 80 :/: 3.55 X .63 X 1056 = 196.46 Mph. Aero dynamics work against you with increased speed. Aero formulas are complicated because they are cubed. But it is a known fact that to double your speed to 200MPH requires 8 times the HP you use at 100 MPH. so if you need 70 HP to maintain 100MPH you will need 560 HP to reach 200.

 

I just figured 245/50/16 tires for circumference. For your formula, don't you need to take into account the vehicle's mass?

 

I had an 85 Z28 w/355 built to that power level easy with a T5 and 3.23 gears it hit 5,000 rpm in top overdrive gear with maybe another 100-200 rpm to pull. I figured the car was doing a good 155. You would probably need 650+ horse power and a quality set of 2.73 or lower number gears to hit 190. The force of pushing air out of your way increases greatly with speed. The car had headers true dual exhaust and was cpu delete.

 

I would geuss you're pretty close to around 150. I would think the reason Zepher said his bro's T/A will do 155 with only 280rwhp is because of the 4.10's--he's right in his powerband at that speed.

A little off topic but what's the formula for determining tire diameter? Other than going out and measuring it.

-Rippin

 

With 3.42 gears in my brothers 96 T/A, it was hitting 155 at the top of 4th/bottom of 5th.

 

X84 and Rippin----Mass is a concern when accelerating,but once you reach a givven speed your HP requirements stabilze. Think about the Salt Flats rather than drag racing and the formula will make more sense. All that formula tells you is that when you are in high gear and the trans has a 1:1 ratio, your crankshaft is spinning at 5000 RPM the driveshaft will be spinning at 5000 RPM also. The formula uses circumference because that will be the distance the tire move in one rotation. I believe circ=diameter times 3.14. The tire manufacturer web site may give diameter.