My speedo isent working on my 87 turbo2......im pretty sure its the cable..just havent gotten around to throwing in a new one.

my question.....

In 5th gear, how fast am i going...

@ 3k
@3.5k
@4k
@4.5k
and if i know theres no cops around...
anything after that.

thanks for any info!!

the fastest i had it was 5th gear at 7k..any have a speed?

mike
87 t2

 

The formula is (RPM*Tire Diameter*Pi)/(1056*Differential Ratio*Trans Ratio)

So for stock RX-7 (RPM*24.7*3.14159) / (1056*4.1*Trans Ratio(3.475, 2.002, 1.366, 1.000, 0.711)

I hope this helps.

 

I think I worked it out before to approximatey 28 mph / 1000 rpm in 5th gear. So you would be going...

@ 3k 84
@3.5k 98
@4k 112
@4.5k 126

@7k that would come out to 196 mph if my estimate's right. hell even if my estimate has some major problems, 7k is still probably above the top speed of the car, so I'm assuming you "misread" the tach or you've done a good bit of mods to the car.

 

Sorry bub, 4500 RPM is 100.8 MPH in 5th. Unless you have the 4.30 gears in which case it is slower than that. 7K in 5th is 176MPH with the N/A trans.

 

hmm.... makes sense, but did I mention that I was running monster truck tires in my calculation?
I had most of that equation derived and converted and all, but i quit screwing around and plugged numbers into another speed calculator I founid and got the estimates I used above. Guess I plugged in a wrng number somewhere along the lines.

 

In TII's each 500 RPM is approximately 10 mph in 5th gear. 89-91's rev at about 3000 at 70, and 87-88's rev at about 3200 at 70.

 

I have 17 inch rims in the rear s im assuming that would factor in.
yeah i was on the LIE and i maintained in 5th gear@ about 6800. did i mention i blew my motor.

anyway...didnt realize i was doing 84 at 3k...doesnt seem that fast.

thanks, Mike

 

rim size doesn't matter as long as the tire diameter is the same as stock.

I spin 110km at 3000, 65mph or so.

 

Much easier to just say circumference instead of pi*d.

Anyhow, for a stock 87 TII tire size of 205/55-16 and a rear end of 4.1, here is what all your speeds come out to:

3K rpm - 71.06 mph
3.5K rpm - 82.9 mph
4K rpm - 94.75 mph
4.5K rpm - 106.6 mph

I'll glady show work if nobody believes me.

 

http://www.miata.net/garage/tirecalc.html will tell you how your tire size affects your speedometer.

 

Hehe, guess so but for the mathematically challenged I thought pi*d would be a little simpler.

Oh, what size tires are you using on the 17s?

TII trans ratios are, 3.843, 2.015, 1.391, 1.000, .762, and the rear gear is 4.10.

What mods have you done?

 

Rebuilt 90 motor
dual friction clutch masters
TID mod--Cone intake
Greddy BOV
G-force ECU
Walbro fuel pump
3 Inch Mindtrain downpipe
HKS 60 mm catback
Removed AC, Pwr Steering, Air pump

Thats all the mods under the hood, i have suspension things but i dontthink that would interfer with the speed.

I guess i should just get the speedo cable fixed and see for myself.

Mike
87 Turbo2

 

erm, is there a way to figure in speed for HP, or what. and is there any way to figure in resistance of road. And where does the 1056 come from? That just a figure to get mp/h out of that number?

 

Node...

If the 1056 is left out your unit is inches/minute, which you convert to miles per hour. 1 mile=63360 inches and 1 hr =60 min. So, 63360/60=1056

As to the Speed from HP, this is not possible using simple mathematics. It requires the use of the torque curve along with about 100 other variables. I know a guy that is working on a spreadsheet to do this calc, but it is not quite finished yet. I hope he markets it after he finishes though. His program takes all the stuff you mention into account. I sent him the numbers for my stock 90 N/a and he calculated a top speed of 124 MPH, which seems to be correct (seeing how I can only get mine to 117). I think I need new plugs and wires .

 

Blake Qualley (he is a member here) did an AWESOME excel spreadsheet that does all this for you. If you have Microsoft Excel, and would like a copy, PM me with your email address. (Blake wrote this for free distribution for non-commercial use.) Or PM Blake. Either way, it's VERY useful for questions like this. You plug in your gear ratios (he has a chart on the second page if you have any questions about what ratios you have) and tire size, and it gives you speed in 500 RPM increments in each gear. Pretty doggone slick!