I have an 85 TPI 5.0 liter,with 700R4,3.27 rear end and 275/40x18" rear tires(overall dia 677mm or 26.65 inches)
Can you calculate speed at 4000rpm,4500rpm and 5000rpm in 4thOD

thanks

John

 

Hi John.
Intersting question! Now, considering I am a maths teacher, and have a degree in special maths (!), I should be able to work this one out! BUT, I have been known to make mistakes-so someone correct me on this if they want.

There are a lot of assumptions to be made here first.
i.e 4th OD is 1:1
No flat spot on tyres when rolling
Going in a straight line. i.e Diff in direct drive-no slipping.
Taking full value of pi.

Formula is
0.024387 multiplied by rpm. (Calculations available on request!).

So, at 4000 rpm, speed is 97.5 mph
At 4500 rpm, speed is 109.7 mph
At 5000 rpm, speed is 121.9 mph.

This is purely theoretical and can cause real life errors. As an example, if the flat spot on the tyre is taken into account, the speed will decrease slightly.

Hope this will suffice.
Does anyone else want to contribute or comment? Does it sound close in real life? I haven't checked this in practise!! has anyone else?

Andy.

 

It would help your Math skills if you would DIY. Just check Tech Central on this Sites's Home page and look up Equations for your problem solving.

 

Andy,

Thanks for the try but 4OD is not 1.1 thats why its OD.Not sure what it is though.I'll check the other "helpfull" suggestion
PM

 

And don't do tire math to get diameter. Measure the true static radius of your car from the ground to the center of the rim. It will be less than the computed value. And a stiff sidewall street tire isn't going to grow that much at speed, so that number is good enough.

 

mph = (rpm * r)/(168 * gear)

in a 700 3rd is 1:1 , 4th is 0.7:1

 

Ed.
Yep, another assumption there. Tire growth negates the slight radius reduction caused by the flat spot on the road.

Your formula does not specify units. r is in inches? Or some other unit? I assume "gear" is the final drive? What about the actual gear you are in in the transmission? Can you clarify?

PM
The formula now changes. Assuming the OD ratio that Ed says is correct, the speed is now 0.03484 multiplied by the rpm.
So, 4000 rpm is now 139 mph
4500 rpm is now 156 mph
5000 rpm is now 174 mph!

IMO, this seems unrealistic. Anyone like to comment?

Andy.

 

The car won't go anywhere near that speed.

Probably somewhere in the 135-140 range is as fast as it will go; air resistance rises very rapidly as speed increases. It takes an enormous amount of power to push one of these cars much faster than that.

 

correct, you wont reach that speed without serious power.
here is my true topspeed according to gearing : in 6th gear with my 2.77 rear at 5000rpm i "can" go 276 mph!
but i dont have nearly the right power to do that!!!

here is a good site:
http://www.f-body.org/gears/

 

I understand people's reluctance to use a static radisu for their tire for speed calculations. What fun is a lower number?

Unless you reasearch the actual growth factor for your tire, you are just guessing that it cancels out the 'flat-spot' at speed. I understand why you'd want to do that, but thats also why anybody with a brain doesn't listen to people spouting 'i went 155 mph in my stock LG4' I was just trying to give advice that gives much more realistic numbers.

BTW, my 'flat spot' numbers match exactly what the factory speedo calibration gives, and my speedo is 100% matching my VSS signal as verified on a scan tool. Thats why i use the low numbers. I don't need to lie for an ego boost.

 

There is more to it than just resistance. For instance, you cant floor it in 4th gear without modefication. So of course you wont be able to make your top speed in 4th gear stock,or without specific modefication.

with 2.77 rear I was able to get my IROC up to 165, but I was close to the upshift point.

 

Ed.
As I stated earlier. This is a purely theoretical calculation-as performed in many classrooms/lecture theatres around the world. If you want air resistance to come into play, then a first order differential equation needs to be produced (actually needs 2nd order, but who's counting-this is good enough for this forum). Air resistance is roughly proportional to the speed squared. This is a reasonably straightforward maths principle, but not entirely necessary in this case. Powermite wanted a speed calculation produced-I don't think he wanted an exact figure. Curiousity and entertainment value perhaps?

You said "I was only GUESSING that tire growth cancels out the flat spot". I was actually ASSUMING it, to make the calculations easier. There is a big difference here. My math qualifications go way beyond that. I do this for a living. I think you may be underestimating me here.
Lets not get all serious now!! I don't think Powermite wants an ego boost! And I certainly don't. My racing/bragging days are over-been there, done that, looked stupid, moved on. All part of the growing up ritual. This post is certainly good for entertainment value though!!!

Researching tire growth at certain speeds is beyond my leisure time! Perhaps in my retirement!

Hey Powermite, I have a much more interesting way to sort this one out. Let's try it for real! A straight open road, a sunny day, no cops, the burble of a V8,.....................this is what dreams are made of!! (Would the M1 do?) I could get from here to the Easter show in Portsmouth in,.. how long? Oh no, not calculations again!!

Andy.

 

4000rpm->222km/h
4500rpm->249km/h
5000rpm->277km/h
These speeds are acurate,IF your tire diameter is 212cm(67,7cmx3,14) and your torqueconverter is lock at wot.
Ps mine -85 Z-28 takes 4800rpm at 4th(3,23 axleratio,th-700,255/40/17tire 305 tpi)

Jari