Just bought a car and speedo is way off.

mat82284 · Feb 5, 2007

merc123 said:
He could use that diagnostic mode on the car and then drive to see what it's really turning. If his speedo says he's running 60 but the displays says he's running 55 then you know it's off by 5.

hey thanks ill do that today to check it out. I didnt think the diagnostic mode told the actual speed.

stangman · Feb 5, 2007

merc123 said:
He could use that diagnostic mode on the car and then drive to see what it's really turning. If his speedo says he's running 60 but the displays says he's running 55 then you know it's off by 5.

so if the car has nothing done to correct a gear, how is the diagnostic mode going to tell the correct speed?

merc123 · Feb 5, 2007

stangman said:
so if the car has nothing done to correct a gear, how is the diagnostic mode going to tell the correct speed?

It reads transmission speed? I dunno but I know it works. When I had Ford install my 3.73's I didn't install my speedcal until I got home. I did diagnostic mode it showed me doing something like 65 when I was only doing 40 or so.

mat82284 · Feb 5, 2007

merc123 said:
It reads transmission speed? I dunno but I know it works. When I had Ford install my 3.73's I didn't install my speedcal until I got home. I did diagnostic mode it showed me doing something like 65 when I was only doing 40 or so.

Well mine was different just tested it and its only 2mph off. When im at 70 at 2k my speed in diag is between 57-58mph. When i lifted the car I turned the wheel once and the drive shaft turned 3 3/4 times. So now im confused. If this is my true speed i wonder if a speedcal is already installed. but it still doesn't explain when my speed says im going 95 and everyone else is going about the same speed. I know here in CA people don't speed like that. especially knowning i was next to a old lady.

trinity_gt · Feb 5, 2007

stangman said:
but the one absolute that he'd need there, is to know for a fact that he was going 80. & that would be a problem for anyone in that situation

True. I appreciated the fact that his 80MPH figure was a guesstimate.

He said he was showing an indicated speed between 90 and 95, so I used a value near the split of 92.

If traffic had really been moving at 70MPH instead of 80, he'd have a ratio of 1.31, giving rear gears of ~4.28:1 (i.e. 4.30s or maybe 4.10...) Still possible, sure. 75MPH would give 4.01...3.90s? Maybe...

If traffic had been going at 90 instead of 80, he'd have a ratio of 1.02 or a rear gear of 3.33s. Not really close to 3.55s, he'd settle back to 3.27s.

It's still a valid way to calculate. If need be, instead of jacking a car up, crawling under it and measuring driveshaft rotations or better yet, removing the rear cover, spilling oil everywhere to look for markings or counting teeth, this is by far the simplest, even if you need to verify the numbers by having a friend pace you in his car going a known speed. Even accounting for speedo errors, you'd still get the right number without getting under the car or being on the hook for gear oil.

fast97gt · Feb 6, 2007

heres a web site that i used to determine the mph per rpm and its pretty accurate, its www.1bad69.com/calc.htm try it you will like it

tmoneyballin · Feb 6, 2007

A disclaimer I would throw out to these math formulas is whether your car has stock compression. I have a N/A built motor and with 4:10 gears my car accelerated like a 3:55 because I had my motor built with the intention of supercharging - it does make a difference because my friends slower cobra with stock compression accelerates just like my car but he has 4:10's and I have 4:30's now. My point - his 4:10's compared to my setup with the same 4:10 gear was not close as far as determining gear ratio based on highway speeds. My gear pulled "like" a 3:55 at speeds like 70-80 mph - I could pull quicker at those speeds than his ratio. On the other hand, speeds like 60mph would leave me dead in the water - I could drop down to 2nd and hit 3rd immediately, but any decent car would already begin to pull away if they had better gearing. Another point I am making is if you have a stock 32 valve engine with 4:10 gears you cannot drop it into 2nd going 60mph.
I know the compression factor probably does not apply in this thread's initial inquiry - but it may be relevant for those who have had any engine work done to their car that they may or may not be aware of.

mat82284 · Feb 6, 2007

Ok well my rpms at 2k say im going 70mph. Is this stock or 3.73 or 3.55? Anyone else compaire this?

tmoneyballin · Feb 6, 2007

You can compare all you want, but the only sure way is looking at the actual gear itself for markings or counting the teeth, or comparing tire rotation to drive shaft as outlined above(which I should have tried sooner...). If your analysis is done strictly on comparing speeds you are likely to have other factors throw off your conclusion - like how accurate are the cars you are comparing it to, do you have an aftermarket transmission that can manipulate the rear end gear (make it higher or lower), things like this can change the way the car drives at certain speeds...Personally, I was totally unaware of my ACTUAL gear ratio until I took it apart and looked for myself!

Also what are you looking to accomplish? If you already have a 3:55 or 3:73 gear hypothetically but you are inquiring because you want your car to be "faster" than it already is, you probably need to go straight for at least a 4:10 regardless of what kind of gear you have. Now if you have a 3:27 in there then a 3:73 might be a better step up. I just know cobra engines like 4:10 and up and my old GT had a 3:73, which seemed like a decent match for the 2valve engine.

trinity_gt · Feb 6, 2007

tmoneyballin said:
A disclaimer I would throw out to these math formulas is whether your car has stock compression. ...

Nothing to do with the math. The math depends only on a ratio of indicated and actual speeds. As long as there's a solid mechanical connection between the wheels and the engine (e.g. an automatic with a torque converter clutch locked up, or a standard with the clutch out), it doesn't matter what's under the hood to make the calculation.

If you know your actual speed and your indicated speed and you know the original gears the car should have, you can get a really good approximation of the actual gears using the math, independent of the engine setup.

You're right about trying to deduce gear ratios based on "feel", that is one car pulling much harder than another due to way more engine torque, but the math works fine for steady-state, "instantaneous" speeds where acceleration is not a factor, for any engine.

merc123 · Feb 6, 2007

mat82284 said:
Ok well my rpms at 2k say im going 70mph. Is this stock or 3.73 or 3.55? Anyone else compaire this?

If you're doing 70 @ 2,000 RPM and your diagnostic mode digital read out says 57-58 MPH then you DO NOT have a speed cal.

Additionally, if you're running 70 MPH @ 2,000 RPM then you have 3.27's (if you have a 3650 tranny and stock tires). I have 3.73's and at 70 I turn around 2200-2300.

Here's the numbers:

If you need more: http://www.03sbgt.com/gear_ratios.html

mat82284 · Feb 6, 2007

merc123 said:
If you're doing 70 @ 2,000 RPM and your diagnostic mode digital read out says 57-58 MPH then you DO NOT have a speed cal.

Additionally, if you're running 70 MPH @ 2,000 RPM then you have 3.27's (if you have a 3650 tranny). I have 3.73's and at 70 I turn around 2200-2300.

Here's the numbers:

If you need more: http://www.03sbgt.com/gear_ratios.html

my car is a 2000 i belive it has the t-5?

mrvax · Feb 6, 2007

Once your SpeedCal is in the ballpark:

Find a mile marker on a highway. Note your 10ths on the odometer.Fine tune the SpeedCal so that your odometer reads exactly one mile as per the mile marker.

merc123 · Feb 6, 2007

mat82284 said:
my car is a 2000 i belive it has the t-5?

It's a T-45. I just checked the ratios and it's the same 5th gear ratio (.67) for T-45 and TR-3650's so the RPM won't change on them. Now, if you were in a different gear then it will change SLIGHTLY but not more than +- 5 RPM.

merc123 · Feb 6, 2007

mrvax said:
Once your SpeedCal is in the ballpark:

Find a mile marker on a highway. Note your 10ths on the odometer.Fine tune the SpeedCal so that your odometer reads exactly one mile as per the mile marker.

Never thought about trying that. How do you tune your speedcal though as far as the dip switches go? I mean, how do you know which to turn ON and which to turn OFF?

Just bought a car and speedo is way off.

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