cc 82Z-28

iTrader: (7)

Well, I did a search, that didn't help so I thought I would throw out my problem and question. I changed over to the 700R4 a number of years ago and the speedo reads high starting at about 30mph and as my sped increases the gap widens, for instance, when I am doing 70mph by my GPS, my speedo reads 90mph and I have tried different gears several years ago and it helped but did not fix it, anyone have a fix? Before the 700R4 trans I had a turbo 350 the speedo was very close but hasn't been since the change. The rear gear has been 3.73 with both transmissions. Thanks for any help with this.

Atilla the Fun

If it's off by, say, 5 MPH at 30 MPH, then it will be off by 2.5 MPH at 15 MPH and by 10 MPH at 60 MPH. Calibration error is always a percentage. If it was off by 5MPH at all speeds, that would be a gauge error.
Had you bothered to read my sticky, you'd know you need a 15-tooth drive gear. Driven gear depends on rear tire size, the math is simple and the formula is given so you can figure what driven gear you need. Then part numbers are in my other sticky.

cc 82Z-28

iTrader: (7)

Thanks, I overlooked the sticky you did and have read them both and will now do the math to see what I need.

cc 82Z-28

iTrader: (7)

Ok, did the math and according to the math, I am running the correct 43 tooth gear with my 15 tooth drive gear so I don't what to try next. I guess for the time being I just keep using my GPS since it is correct but would sure like to figure this out. Thanks.

rusty vango

remove it and send it out to be repaired and calibrated. its the only way to be absolutely sure it works as intended

jmd

iTrader: (4)

Yep. Most speedometer gear selection online sites give different gears than what GM used and is accurate. Yours is a prime example.

A 700-R4 / 3.73 third gen used 15 / 45 maybe 44 if the tires were shorter.

I've run the 15/45 with 235/60 and 245/50R16 tires. My odometer matches mile markers and my speedo matches GPS and roadside radar every time. There's zero reason I would ever try and run a 43 and believe it would be accurate.

"The Math" that an OEM spends billions on in R&D is right. What you found, isn't.

Anthony Alfred

How do you remove the kph needle on the gauges that have mph on one side and kph on the other?

cc 82Z-28

iTrader: (7)

Thanks for the replies, I had the speedo removed checked and calibrated a while back but as for the using a 45 tooth I may as well give it a try since what I am using doesn't work. My tires are 255/45/17's and are 26.1" tall. Thanks for the help..

cosmick

I used that formula for my car, and I'm exactly accurate now, at 60 MPH, according to those digital-display radar trailers the police leave out.

Atilla the Fun

LIAR! Not only does that formula come direct from GM, but here's scientific proof from scientific facts. By the way, facts are things that are independently verifiable. Fact 1: Speedometers are calibrated for 1000 revolutions per mile. Fact 2: There are 5280 feet per mile. Fact 3: There are 12 inches per foot. Fact 4: The distance a rolling tire travels per 360 degrees of rotation does not change based on vertical loading. Fact 5: PI is 3.14159xxx, which is commonly rounded to 3.1416, which is accurate enough for this. Fact 6: Performance car tires have their diameter advertised, but not their circumference. Fact 7: 5280' x 12" per foot, divided by PI = 20168.06723, exactly as given in the formula. Fact 8: 20168 divided by any tire's new, unloaded diameter will give the factual revolutions per mile, no matter how much vertical load is added to that tire. Fact 9: When you go on www.tirerack.com, you will find wrong numbers for revolutions per mile. Fact 10: Those numbers are obtained by measuring the tire's minimum vertical radius at maximum rated load at maximum rated pressure, then multiplying that by 2, then dividing that into 20168.06723. Fact 11: I have tried calibrating several speedometers based on those numbers, the results were always grossly erroneous. Fact 12: As tires wear out, the vehicle will be going slightly slower actual speed for the same indicated speed. Fact 13: This is why we calibrate for new tires, so nobody gets speeding tickets that are not deserved.
You think my formula is wrong? Follow it anyway, then go do 15 MPH over the posted limit in front of a police officer or highway patrol officer, and see what speed you get cited for. Fact 14: I will not pay your ticket.
Now, apologize and stand corrected like a real adult, or STFU and GTFO.

jmd

iTrader: (4)

Are you telling us the site he used to come up with 15:43 was wrong or he entered the wrong information?

Or would you like to present a using his tire size to show that your formula comes up with the correct 15:45 ratio?

sofakingdom

iTrader: (1)

This is usually the problem.

For some reason not apparent to me (but I'm kinda dummm, so don't let that bother you too much) people always want to use the tire size molded into their tire to "calculate" their speedo. And EVERY SINGLE PERSON that ever does this, comes up with GARBAGE; and then blames the calculator.

Humans. As long as I am stuck on this planet with them, I will probably never understand why they can't think their way out of a wet sheet of toilet paper.

The CORRECT way to get the radius of your tires, is to simply MEASURE IT. Which is just STOOOOPID EEEEEEZIE; all ya gotta do, is park the car on a more or less level surface, and use a tape measure to see how far off the ground the CENTER of the wheel is. Then plug THAT into the "calculator" instead of that stewpid crap printed on the tire.

It's a good thing that there's no "math" involved with a speedometer; all it is, is SIMPLE ARITHEMETIC. A 5th grade kid should be able to do it and get it right WITHOUT a "calculator", IFF (geek shorthand for "if and only if") the numbers s/he starts out with ARE RIGHT. Which reading them off the side of the tire IS NOT.

Once that's done, the formula WILL work right. It has no choice. Even if you live in Tennessee where it was illegal for a time to use any number other than 3 as pi (because it says in the Bible that a wagon wheel goes about 3 times as far each time it turns, as it is across; and that of course is what pi is). But if you plug in the wrong numbers, the formula will give the same wrong answer EVERY TIME.

Next thing to be concerned about is, correctly counting the # of teeth on the gears. Even though one doesn't even have to have reached the 5th grade to get that right, adults still screw it up all the time. That's where I'd look next if I was trying to fix such a problem.

cc 82Z-28

iTrader: (7)

When I put the trans in back in 2005 I went to the web site that was in the sticky and when I did the numbers it comes up as 43 and it didn't work right so I bought the 45 to try since what have I to lose but 11.00 and just for the record the tire height is 26.1", that's what tire rack says for the size tire I am using and when I measured it back in 2005 it was 26.1 and today it is still 26.1". I can't say what else is causing it at this point at 35mph my speedo say 41 at 45 it reads right around 52 and it progressively climbs to where at 70mph the speedo says 90mph. I have had the head checked and cable is lubricated, it doesn't bounce around, it just doesn't read right. I see on the web site that they have a speedo recalibration unit that you put in between the trans speedo gear and the speedo cable and it corrects it. There are different units for a given % of difference reading to actual speed, may try that next is the new gear doesn't work. I know at times I may not be the brightest bulb in the bunch but I do try to understand things that people have suggested and do try and research it out. Oh, one suggestion was to check it against one of the police radar setups they use, I did and it was exactly what my GPS said as I was going by it so I am confident that my GPS is ok. I do agree that some people can really screw up simple things but I really try and have the facts about any problem that I pose questions with on this site, thanks for all the input.

jmd

iTrader: (4)

43 teeth requires fewer spins to turn the cable a given rpm than 45. So it's not only a question of whether that's the right tooth count for you based on factory use (it is.) It's the right direction to take based on everything you've described and tried so far (mph needs slowed down @ the speedo based on verified road speed.)

45 is the highest tooth count available in the 700-R4 series of gear combinations. However, there's no reason for your combination of parts to require anything higher.

Sorry about the perceived cheap shot. Just deflecting a glistening lump of indignance.

cc 82Z-28

iTrader: (7)

None taken, it frustrates me that I can't get a handle on this, as it turns out when I did this conversion to the 700R4 back in 2005, my first gear was the 45 tooth gear when I had a different set of 15" wheels and tires on and then when I upgraded to the 17's that I currently run is when I bought the 43 tooth gear that is now in there according to the web sites formula. As I remember the 45 tooth was way off and the 43 tooth was better but never right. Looks like I may have to try the ratio adapter that transmission center sells. I guess what I will tomorrow is go out and using my GPS start at 20 mph and see what the speedo mph is and write it down and work my way up through 70 mph and then figure out which ratio adapter to buy. They offer 37.5%, 50.%, 26.7%, 17.6%, 10.%, and 2.56%.
So, should I try a lower number gear such as 40, 41, or 42? Am I going the wrong direction with the higher number when I should be going lower? Like I said, I appreciate all the input and sometimes this 58 year brain a little help figuring things out and that is why I come to this site, great people who are for the most part very knowledgeable about third gens.

jmd

iTrader: (4)

You need more teeth on the driven gear (45) rather than lower (41.)

Think of it this way: for the same number of spins of the output shaft, a 43 tooth gear will spin 1000 times whereas a 45 will turn 955 times. That will slow down the speedo, which is what you need.

No need to jump to a ratio adapter unless you still have an inaccuracy after the 45 swap.

cc 82Z-28

iTrader: (7)

I have the 45 tooth gear coming and I will give it a try before going to the ratio adapter. At this point that's all I can do, but thanks for working with me on this it is much appreciated.

cc 82Z-28

iTrader: (7)

Took the car out today to get some data for different speeds the speedo shows versus the GPS. Starting at 10mph, GPS 10mph, 20mph, GPS 21mph, 25mph, GPS 22mph, 35mph, GPS 31mph, 45mph, GPS 39mph, 55mph, GPS 48mph, 60mph, GPS 51mph, and 70mph, GPS 59mph. Right now it shows an increase in the difference as the speeds increase. I have the new 45 tooth gear and I will be putting it in soon to see what happens. I really hoe that it will fix the problem, I will keep you posted as to what the results are. Thanks for the help..