Ok i have 450+hp 351w in my 68 coupe and it doesnt go any faster than 145mph. I imagine most cars with this power reaches at least 175-190mph? Why is that?

 

you have a lower gear ratio in your differential than the others. Change your pumpkin over to a 2.73 and itll do the same with no motor changes. But then, you'll lose acceleration at the lower speeds.

it's a tradeoff

 

because your car doesnt wanna kill you probably...

aerodynamics, gearing/trans, weight and open roads probably limit everything the most

 

Aerodynamics are a huge factor as well. Older Mustangs, and Mustangs in general, aren't terribly aerodynamic. You need more power in a less aerodynamic car to go the same speed. And yes, gearing will also effect top speed as well. Ideal gearing would have you at peak torque in the engine's rpm range at top speed.

 

1. No they don't. Most cars with 400 HP from the factory do not reach supercar speeds unless you bought a supercar.

2. Top speed of a car has very little to do with the horse power level of the motor. Top speed is a function of gearing, RPM range for the motor, and finally enough HP to over come drag.

3. You're speedo is not calibrated correctly either and a speed of 145 is suspect.

4. You were on the back straight of what track when you came to the stupendous conclusion that your 40 yr old mustang coupe tops out at 145 MPH (I bet you still have primarily a stock style suspension too and done very little to actually improve the aerodynamics since that fits the profile...sorry I always jump to conclusions).

You have a 351 in a 68 coupe that you have just been looking for heads for, so I have to assume it is not a crate 351 but rather a replant from another motor? What cam do you have in the motor? Stock 351w will redline (run out of air by about 6k). 6k max RPM, c4 tranny with a final drive of 1:1 and a 245 45 17 tire which is usually about 25.7 inches tall.

Assuming you have a
2.79 rear gear - TS is like 165 <= this would be a silly rear gear to have in a performance car
3.25 rear gear - TS is like 141
3.55 rear gear - TS is like 129
3.70 rear gear - TS is like 124
4.11 rear gear - TS is like 112 <= this would be a silly rear gear to have in a car with a motor that has a 6k red line and no OD

But here's the cincher most on line calculators will not tell you. These are meaningless values based 100% on the math of the simple calculation for the ratios. They do not take into account: converter drag, driveline drag, tire friction, wind drag.

on edit: this is one of the better calc available because it does try to take some factors into account by fudging a little bit and I have found to be 'more' accurate than others when comparing it to the speeds at given RPMs for my own mustang

http://www.car-videos.net/tools/spee...B1=Recalculate

Here is a link with it setup with c4 ratios, 6k redline, plug in a rear gear of your choice.

 

Ouch[:@][:@]

 

I'm amazed why anyone would want to do 145 MPH in a classic mustang? Much less the 175 or 190 like you are wanting. It sounds like you just asking to be killed.

 

dam I get scared when I hit 85 in my stang.........things get scarry when you dont have air bags and cages.

 

Anything over 90 is scary. At a certain point, Mustangs tend to develop enough lift to pick the front end right up off the ground. Before that, the front end tends to get very light, giving you minimal steering. A good gust of wind at this speed can pick the car right up off the road and flip it. I've talked to someone who lost a brother because of this.

Without lowering the car, adding a hood with vent louvers(Shelby-style), some very wide and sticky tires,and front and rear spoilers, and some really beefy brakes,I wouldn't feel even remotely confidant driving the car over 90 for any extended period of time. It's absolutely unsafe.

The only car I've ever been in that's been even remotely safe at 100+ speeds ("safe" being relative at that point)was my roommate's late model F-body (blasphemy, I know, sorry). With 295/45ZR17Goodyear F1 GS-D3's,road race shocks,and a body designed specifically for aerodynamics, it had the traction and stability to keep on the road at those speeds. Your classic Mustang does not. Don't risk your life trying to go as fast as you can with 40yr old technology. If you want top speed, buy a Corvette.

Besides,in reality, unless you're racing it on a track with a long straight, there's no need for that kind of top speed. Throw some gears at it and it will get up to speed faster; that's what's more useful on the street, and much less dangerous.

 

here's me

225/60/15 tire
3.40 gear
1:1 C4 trans
7000 redline
---------------
151.5 mph

i've had my II up to 6500 RPM and held it there for a couple minutes - it was a beautiful thing. thats 140.7 according to the calculator which sounds about right, cause when i let off the gas it took awhile before the needle moved from the MPH which is past 120 on my speedometer