Those speed/engine rpm calculators...BS...

mrvette

Phantom of the Opera
Joined
Mar 24, 2008
Messages
15,207
Location
NE Florida
There is something drastically RONG with those speed/rpm calculators on the net....do NOT trust them.....

One of my oldest friends from '66 yet, came down by himself just to get hell out of Wash DC area, he and g/f were here last month.....so we went on a trip south to Coco Florida, right off I 95 and close to COCO Beach....just south of Titusville NASA.....GREAT restaurant there BTW....DIXIE CROSSROADS....best damn seefood you will EVER SEE.....

ANYway, we driving and Larry keeps saying my speedo is too high...we start in with mile markers....and calculators....I keep swearing and showing him the calculations with my specific tire size, supposed tranny o/driver ratio, what I damn well KNOW is the rear gear ratio....and the tach is dead on at 1500 rpm ....300 rpm high at 4500 and 6 grand..... I have to assume 200 high at 2500....did not check it there....

so what I THOUGHT was the speedo tracking the tach and both of them agreeing as to speed.....is in FACT rong...I dunno why, just IS.....

I went to get 2 new hood pins, having stupidly LOST one recently, stainless from West Marine next door.....order them In a few daze ago....picked them up this morning....looked at a GARMIN NUVI 200w.....GPS....sales guy called GARMIN and asked about the top speed it's capable or reading....up to 1000 mph, supposedly...aircraft guys use them......120 bux + tax....

OK, make me a offer I can't refuse, down goes the plastic fantastic, and out I go for some driving....

what speedo said was 100 mph was in fact only 88, so I changed the speedo gear....what it says NOW is weird....

the speedo is dead nutz ON, at speeds up to 60mph....after that, it gets screwy 70 is +2 mph.....90 is +4 mph...and 100 mph is +6 mph.....all on the over indicated side.....I didn't have safe room today to run any faster on the upside...

I really don't CARE about being rong on the top speed runs I thought I had done, but what bugs ME is since I proved the tach accurate enough....when the calculators on the net....2-3 different sites...say the same shit, and they prove dead RONG.....

WTF is doing ON here??? rear tires slipping at speeds??? BULLSHIT, not possible....:banghead::banghead:

so anyone with a true intelligent answer???

AND why is my speedo so non linear/inaccurate at higher speeds....5% over at 100mph...and dead nutz on at 50.....

:crap::bomb:
 
A few thoughts:
Compare the advertized tire diameter with what you actually measure.

If you do the math (rpm, gear ratio and actual tire circumference) then that's giving you the real speed and that should be close to what the Navi says.

5% off at 100mph isn't really all that bad for a 37 year old mechanically driven speedometer.

Any gauge, speedo, tach, pressure, temp gauge is only accurate at a specific range. You want the gauge to be accurate where you need it to be accurate or better, in the most likely operating condition - speedo for example should be accurate up to 70mph (speed limit).... why would anyone spend money to make a speedometer for a street legal vehicle accurate at any speed up to 150mph ??
A $400 VDO speedometer might be accurate at speeds up to 180mph - dunno....

When they came up with the 85mph speedometer it sure solved this issue :nuts:
 
Don't forget that the tires grow a little. Not much on street tires, but I bet there's little growth.
 
Don't forget that the tires grow a little. Not much on street tires, but I bet there's little growth.

IF the tires really grew much, the speed of the car would run faster than the speedo....it's slower....100 is really 95 etc....

but then it don't agree with the tach....and I can't see how my tach cal can be off ANY.....

whole instrumentation situation pisses me off no end....
 
My Speedo is 2 mph higher at all speeds up to 80mph when comparing to TomTom. Did't check any higher than that. Good enough for me and my 39yo ride.
 
Do you have an auomatic trans, maybe you are getting converter slip?

200 4r auto, locked tighter than whale pussy, and that's waterproof.....


BTW....Welcome to the motley crew of the Flying Dutchmen....


:D:yahoo:
 
My Speedo is 2 mph higher at all speeds up to 80mph when comparing to TomTom. Did't check any higher than that. Good enough for me and my 39yo ride.

My last three modern vehicles all ran about 2 mph higher than my GPS shows at about 70 MPH. Vette runs about the same.

I checked my Vette against speed/rpm calculators when changing tire sizes and they seemed to work for me but I did not check various speeds - only 60 to 70 MPH. When I get a chance I'll compare my instrument speed and rpm to the calculators at various speeds as well as to the GPS.

DC
 
200 4r auto, locked tighter than whale pussy, and that's waterproof.....
BTW....Welcome to the motley crew of the Flying Dutchmen....

What did it take to install the 200 4r, send me some info please.....
I have been looking into that conversion...
 
200 4r auto, locked tighter than whale pussy, and that's waterproof.....
BTW....Welcome to the motley crew of the Flying Dutchmen....

What did it take to install the 200 4r, send me some info please.....
I have been looking into that conversion...

Well, first thing to look for is a source for a chebby/universal bolt pattern bellhousing 200 4r tranny...out of a 80's monte/maybe even grand national....
some others.....

it's a direct bolt in for a 350-400 turbo, or even a Muncie....sets on the frame rail directly...toss all the shoe mounts extending forward of the support rail....

you will need a method of attaching the kickdown cable so it's working properly, and that is critical.....

you will need a ten vane front pump, converter, TRANS GO shift kit (NO OTHER) and a soft parts kit to include wide band then maybe some hard parts, but not know that until you tear it down....I got lucky, my core had all good hard parts....been some 4 years now....
you also need the ATSG service guide.....it IS something that if you can read and understand fairly complex crap, have a table to spread out the parts, can read diagrams clearly....take your time, some tools like a good set of snap ring pliars...reversible with interchangeable tips......screwdrivers, etc........

then the electrics for the lock up....simple enough, really, most of the guys here can help with that....

TimAT is one of the tranny guys here, and if questions, can help....I"m NOT a expert, but got lucky once.....

well actually 2-3 times on different trannies....
 
I used the Richmond Gear Calculator when shopping for my transmission and like many others here I also used a GPS to verify the results. My speedometer and tachometer readings are right where the gear calculator said they would be.

http://www.richmondgear.com/101032.html

Site #4 now tried, something is funky in Denmark.....same results....I dunno who to believe....I have to believe Garmin...as to speed....and I electronically verified my tach YET a huge differential with the ratio/tire sites....dunno where to go....
 
All analog gauges of any type will have a sweet spot. Gene, you're a geek, you should know that. Remember tuning the antenna coupling on an A.M. radio?..all about compromise. The speedo and later tach's are magnetically coupled. Shoot for accuracy in the middle of the gauge.
 
Can you post some rpm/indicated speed results? Also what size tires and rear ratio? Maybe someone will see something that makes sense.
:confused:
 
You can use the .xls sheet I have uploaded for RPM and speed calculation, I have checked the math several times and it is ok.

Radial tire grow with speed is really small and I have not cosidered it on my formulas.

Pacha

Ps. I have cheked and the .xls is not available yet
 
ALL the sites tend to agree, with the calculations....so that is not the issue....

thing is, I proved the accuracy of the tach at being some 300 rpm over indicating at 3000, 4500, and 6000 rpm....fine so taking that into account being as it's electronic, and totally repeatable results....I know my Tectronics 453 scope on line synch at 60 hz, and the audio oscillator I used to drive a transistor into saturation/switch mode to key the coil and tach signal with, is totally accurate so at 40 hz that is 5 sparks/second per cyl.....or 600 rpm....

so to go to 6000 rpm it's obviously only 400 hz.....well any point in between is obvious.....so the tach is fairly correct....10% at 3000 rpm 7.5% at 4500 rpm, and 5% at 6000.....not so very awful evil....

so, just how in hell is the speedo so WAY off, I know it's mechanical, but really now....:fishing:
 
45-55 is the speed limit within most city limits right?
Would make some sense that its' most accurate there.
At the upper end I don't think it matters if it's 96 or 100
As far as the calculators being off.....no clue
 
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