So, my '85 GT has the 2.8 and the 4-speed manual. At 60mph I'm spinning about 3,000RPM in 4th. This seems a little high. It has 215/60R14s all around. According to CowsPatoot's speed calculator http://www.gafiero.org/speedcalc.shtml, I should be doing 73. Any ideas? I'm thinking about getting bigger wheels anyway. Would I have to change the VSS in order to keep the speedometer accurate? Could the problem be an inaccurate speedo in the first place? Help?
The transmissions are not labeled. You should have the M17.
Check the RPO's on the driver side inner front fender. That sticker with the build codes should give you one for the speedo gear. The particular one that you are looking for is going to be a D4N,D4O,D4P,or D4Q. The N is Red W/ 29 teeth, O is Purple W/ 30, P is Blue W/ 30, and Q is White With 31. But remember that this is calculated from the final drive, transmission and tire size. Your tires are the right size for your car, but other cars used the same transmission with a different VSS gear, so you should check to see if the gear in your transmission is the same as the one listed on the sticker.
It also could be that your speedometer is not correct.
I posted this a couple weeks ago in another thread:
"According to an R&T road test back in Nov 84 with an '85 GT, 4 speed transmission, and 215/60/14 tires, which are pretty close to the same overall diameter as yours, the car should travel 23.3 MPH for every 1000 RPM in 4th gear. So at 2500 RPM, you should be running 58 MPH."
It would seem that you probably have the wrong colored gear on your VSS. According to the chart below, it should be a purple 30 tooth gear.
Newer TomTom and likely others GPS are accurate enough to tell your vehicle speed. Just borrow one. My TomTom is ~1-3MPH match to speedo on most caps.
Likely causes: speedo w/ problems. Get ECM scanner to see if ECM says same as Speedo. VSS gears wrong. PO swapped 4 speed w/ low finals. Example M19 w/ 4.10 final.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
The only accurate speedo,tach tire size I had dealt with is to use a old hot rod formula that I have had for years. The speed calculator is not close to what the formula produces.I thought I was way off using the gafiero calculator. Dug around and found my formula and it is right on within 1%.
The only accurate speedo,tach tire size I had dealt with is to use a old hot rod formula that I have had for years. The speed calculator is not close to what the formula produces.I thought I was way off using the gafiero calculator. Dug around and found my formula and it is right on within 1%.
Ok there is 4 variables. RPM=Mph x 340 x Gear Ratio Divided by Tire Diameter Gear Ratio= Rpm x Tire Diameter x 2.96 divided by 1000 x Mph Tire Dia= Mph x (Ratio x 1000) x .34 Divided by rpm Speed=Rpm x tire diameter x 2.96 divided by 1000 x Gear ratio So if you want to know the rpms youre running,or gear ratio,tire dia,or speed place your numbers in the equation. So on rpms at 60 mph put in 60 x 340 x gear ratio Divided by your tire diameter.
[This message has been edited by rcm (edited 11-23-2013).]
The formula used for 4th gear at http://www.gafiero.org/speedcalc.shtml is speed = Math.round((TireSize*RPM*31.415)/(1056*TranUser1[5]*TranUser1[1]))/10; TireSize = Math.round(((TireTreadWidth*TireProfile*2)/25.4)+(TireRimSize*100))/100; The point of dividing at the end was to get the Math.round function in Javascript to round to the correct decimal....it was calculated into the formula. In the speed formula, 31.415 is pi*10....in the tiresize formula, TireProfile is technically a percentage but is used as a whole number to compensate for dividing by 100.
Now...to translate that formula....TransUser1[5] is the 4th gear ratio of the trans.....TransUser1[1] is the final gear ratio.
So....with your 4speed in 4th gear at 3000rpm with 215/60/14 tires we have: TireSize = Math.round(((215*60*2)/25.4)+(14*100))/100 TireSize = Math.round(1015.748+1400)/100 TireSize = 2416/100 TireSize = 24.16 (as you can see, that math matches the site's response)
Speed = Math.round((TireSize*RPM*31.415)/(1056*TranUser1[5]*TranUser1[1]))/10 Speed = Math.round((24.16*3000*31.415)/(1056*0.81*3.65))/10 Speed = Math.round(2276959.2/3122.064)/10 Speed = Math.round(729.3121....)/10 Speed = 729/10 Speed = 72.9 (again...my math matches the site....don't worry, I am not trying to prove the validity of the site to you, but QC on work I did years ago is always a good thing)
So....now lets go with the formula above: Speed=Rpm x tire diameter x 2.96 divided by 1000 x Gear ratio Speed = (3000*24.16*2.96)/(1000*0.81*3.65) Speed = 214540.8/2956.5 Speed = 72.566
It's close, but far enough that one of the formulas isn't quite right..... In my formula... 2*pi*R gets you the radius....2*R=diameter......so TireSize (diameter)*3.1415 (pi) divided by gear ratio gets you inches per minute. Multiply by 60 to get inches per hour.....divide by 63360 gets you miles per hour. 63360/60=1056. Basic conversions simplified by algebra....if you want the complete algebraic proof, you will have to wait till I get some sleep to figure out how to format it for this site.
Where did the other formula come from? edit to add: 3.14/1056= 0.002973.....2.96/1000= 0.00296. I suspect the other formula was modified to make the math easier but keep the result as close as possible.
The math is right....but the math is also theory. The formulas assume a perfect world. They assume no slipping in the clutch, they assume accurate measurement of the RPM and Speed, and they assume all tires are made the same. We don't live in a perfect world. I have compared countless cars to this chart. In every case where the car didn't match the chart, it was the car that was wrong, not the math. Either they had different gearing than we thought....or the speedo was off.....or the tires weren't really the size calculated (not all 215/60/14 tires are the same size)...or (most often) the tach has been off (reading high every time).
In your case, speed is easy to check with GPS. If the Speedo is correct....I will put money on the tach reading high. For the record....almost every v6 tach I have dealt with was reading about 500 rpm high....you say you are reading 3000 at 60mph....my chart says you should be reading 2468 at 60. What does your car idle at? Does it sound like it is idling that high?
[This message has been edited by CowsPatoot (edited 11-24-2013).]
I agree that math is math and tires are not created equal.But old school formula before the day of electronics this formula has served over 40 cars gear ratio to speed to rpm.But as tires are not always correct diameter I would measure the tire which still is not 100%,but still can get you close.My idle is 1k and the gafiero calculator shows 2750 at 70 mph.The formula I use shows 3433 which the tach shows 3400.This is with a m17 with 3.65 ratio and 205-70-14 tires.
Could be that the previous owner swapped out the speedo and/or the whole instrument pod. There are several different instrument clusters depending on factors such as model year and engine. If you posted a photo of the cluster, I or someone else might be able to tell you whether the pod is correct for your car.
According to my phone (another variable, but it correlated with my speedo), the speedo seems accurate. Travelling at 25 mph in second gear, the tach showed 3,000 rpm. Both the speedo and the phone showed 25 mph. On a cold start, the tach held steady at 2,000 rpm (it tends to idle high when cold), and when parked after the test it still showed 2K, but didn't sound excessively high, but then suddenly it idled down to 1,000 and sounded quite low. I'm really puzzled now, but am leaning toward the tach being off.
This is the only formula you need, to calculate what speed you should have:
code:
gear ratio = ratio of current gear x final drive ratio
rpm x tire diameter mph = ------------------------------- gear ratio x 336
Use the full exact numbers for input that you know, and don't round until the final output. For every input that is rounded, you increase the margin of error and reduce the accuracy of speed. Math is math, but it's easy to get the wrong output by using the wrong inputs.
With this formula, and assuming properly inflated 215/60-14 tires, stock M17 trans, and stock gauge panels, you should be going approximately 70 MPH at 3000 RPM in 4th gear. If the 84 performance 4 speed was swapped in, and uses the correct speedo gear, you should be going about 62 MPH at 3000 in 4th.
You can verify what you have in a number of ways. The quickest way is probably to look at the trans case to see if it is an 84 or 85+ case: //www.fiero.nl/forum/Forum2/HTML/063783.html The 84 case is smooth and has cast-in FWD shift cable brackets. The 85 has no FWD cable brackets. You can use GPS to tell you that you are going ~60 MPH in 4th gear, but if the trans was swapped, the speedo and tach may be correct. You should verify your MPH vs RPM in multiple gears. Check both at 25 in second, and 60 in 4th, for example.
Unless your tires are extremely flat or extremely over-inflated, using the standard diameter measurement for your tire size is going to be fine. Differences to tire size as a result of manufacturing processes and normal tire wear, are negligible. The diameter can change by 0.001 inch easily enough by simply driving down the road, by the tread wearing down, and the tires heating up as you drive, causing the pressure to increase slightly. They are rubber after all, and the diameter will change with changes in load, road conditions, wear, etc… in normal usage. However, you will rarely see a change that would be great enough to affect your speed reading by any noticeable amount, assuming proper inflation of the tires.
Originally posted by PatrickTRoof: According to my phone (another variable, but it correlated with my speedo), the speedo seems accurate. Travelling at 25 mph in second gear, the tach showed 3,000 rpm. Both the speedo and the phone showed 25 mph. On a cold start, the tach held steady at 2,000 rpm (it tends to idle high when cold), and when parked after the test it still showed 2K, but didn't sound excessively high, but then suddenly it idled down to 1,000 and sounded quite low. I'm really puzzled now, but am leaning toward the tach being off.
Tach is almost certainly off. The tach filter is a common failure point on the V6, and will cause the tach to read wrong.
I came up with your tire diameter being 24.157 and at 60 mph your tach is approximate 3083 rpm.
With what gear ratios and/or formula? Are you leaving final drive ratio out of the equation when you make the calculation?
In neither the equations you posted, nor the ones I posted, does ~3000 RPM equal anything close to 60 MPH, with the 215/60-14 tires and M17 gear ratios. 60 MPH in 4th with M17 and 215/60-14 should be around 2500 RPM.
Using only the final drive ratio in the calculations, without multiplying by the 4th gear ratio, will give you ~60 MPH at ~3000 RPM, though. But that's just using the wrong values in the math.
This is the only formula you need, to calculate what speed you should have:
code:
gear ratio = ratio of current gear x final drive ratio
rpm x tire diameter mph = ------------------------------- gear ratio x 336
Algebraically, this is the exact same formula I created except that this one rounds off a bit earlier (that number should be 336.1451535890498 rather than 336....but we are splitting hairs here)
Something else that isn't figured in any of these formulas, and hasn't been mentioned....when a car sits on the ground, the tire isn't round. The distance of the center of the wheel to the ground is less than the radius of the tire. In reality, we should be using "distance from tire center to ground times 2" rather than the tire diameter. .......edit to add.....if you wanna be really picky, you need to use "Distance from tire center to ground on the left tire, and add it to tire center to ground of the right tire" to compensate for differences in tire wear between the two sides. At the end of the day, a 0.1" difference in the diameter will make about a 1mph difference in your speed.
rcm....I appreciate the fact that you have used your formulas for an extended amount of time, but you can't get around the fact that they are approximations. If I use the formula you quoted for Speed (Speed=Rpm x tire diameter x 2.96 divided by 1000 x Gear ratio) and use algebra to solve that for RPM....I get RPM=MPH x 337.838 x Gear Ratio divided by Tire Diameter. I rounded that number off after three decimal places...but the simple fact that it is different than your listed 340 proves that your formulas are approximated. To be fair....mine is approximated too....I rounded pi to 4 decimal points.
Back to the original problem....sounds to me like I nailed the problem, so I will now bow out of this thread until an ECM Scan Tool proves me wrong. I apologize if I sound argumentative on the formulas...regardless of how accurate the formula is, we are still limited by real world issues not matching the theory.
[This message has been edited by CowsPatoot (edited 11-24-2013).]
I agree that math is math and tires are not created equal.But old school formula before the day of electronics this formula has served over 40 cars gear ratio to speed to rpm.But as tires are not always correct diameter I would measure the tire which still is not 100%,but still can get you close.My idle is 1k and the gafiero calculator shows 2750 at 70 mph.The formula I use shows 3433 which the tach shows 3400.This is with a m17 with 3.65 ratio and 205-70-14 tires.
This bothered me a little...I didn't think the formulas were that much different, so I did the math myself....
RPM=Mph x 340 x Gear Ratio Divided by Tire Diameter
RPM = (70 x 340 x (0.81 x 3.65))/25.3 = 2781.2
How did you come up with 3433? I suspect you forgot to add in the 4th gear ratio.
This bothered me a little...I didn't think the formulas were that much different, so I did the math myself....
RPM=Mph x 340 x Gear Ratio Divided by Tire Diameter
RPM = (70 x 340 x (0.81 x 3.65))/25.3 = 2781.2
How did you come up with 3433? I suspect you forgot to add in the 4th gear ratio.
Oh man I don't mean to create any grief just that's my formula I have been using and by no means is it spot on. I am using the 3.65 as gear ratio as you would say a conventional mopar suregrip being a 3.55 for example. Using that ratio and my formula my car shows pretty close.I had posted a thread on the same problem of speedo or tach not reading correctly.Used gafiero calculator and it wasn't close although it was mentioned the tach can read 500 rpms to high but yet my idle is 1k.Unless tach reads incorrect as it reaches higher decimels.
Originally posted by rcm: Oh man I don't mean to create any grief just that's my formula I have been using and by no means is it spot on. I am using the 3.65 as gear ratio as you would say a conventional mopar suregrip being a 3.55 for example. Using that ratio and my formula my car shows pretty close.I had posted a thread on the same problem of speedo or tach not reading correctly.Used gafiero calculator and it wasn't close although it was mentioned the tach can read 500 rpms to high but yet my idle is 1k.Unless tach reads incorrect as it reaches higher decimels.
You are using only the final drive ratio. That would be fine if it was 3rd gear in a TH125c auto Fiero, where 3rd is 1:1, but it doesn't work when the trans gear is not 1:1. You must multiply the final drive (3.65 for M17) with the ratio of the actual gear you're in (0.81 in 4th for M17).
If your idle is 1000 RPM, and you don't have a small vacuum leak somewhere, and the engine is running correctly on par with a new stock 2.8, then your tach must be off as well. Idle should be closer to 700 RPM when warm. 1000 RPM would be reasonable at cold start, but it should drop down a bit after warming up.
Originally posted by CowsPatoot: Algebraically, this is the exact same formula I created except that this one rounds off a bit earlier (that number should be 336.1451535890498 rather than 336....but we are splitting hairs here)
Yes. I wasn't disagreeing with you. But you posted a bunch of JavaScript and a bit of a complex description of the calculator page you made. It's certainly "close enough" for anyone who wants to use it, to give a reasonable approximation of what their speed/RPM should be. It's certainly easier for people to remember the simpler formula I posted (which is exactly the same formula as in Auto Math Handbook.
Also, I did mention the differences due to wear/etc… in my post. Unless your tires are severely under or over inflated, the difference is negligible. Properly inflated to the correct percentage of maximum pressure printed on the sidewall, for the load exerted on the tire, the tire should be very close to round. It will also be slightly more round at speed, especially after driving long enough for the tires to heat up and the pressure to increase slightly, than it will be sitting at rest in your driveway under load. But again, it is negligible, and if your tires are visibly flatter on the bottom from load, you probably need to increase the pressure in them.
Correct I did not input final drive.Per manuals and others on this site the 2.8 should be 900 rpms at warm idle. Ok with my numbers placed in your equation 60 mph with 205-70-14 tires with a m17 for an 86 se 2.8 what would the tach read?Engine runs great..1700 rpms cold and 1000 at idle.
Correct I did not input final drive.Per manuals and others on this site the 2.8 should be 900 rpms at warm idle. Ok with my numbers placed in your equation 60 mph with 205-70-14 tires with a m17 for an 86 se 2.8 what would the tach read?Engine runs great..1700 rpms cold and 1000 at idle.
Tach should be around 2355 RPM for 205/70-14 with M17 in 4th gear.
Wow, I didn't mean to spark such a debate. As I said before, my gps and speedo agreed on my speed, so for now I'm going with a bad tach filter. I'll replace it and take it from there.
Wow, I didn't mean to spark such a debate. As I said before, my gps and speedo agreed on my speed, so for now I'm going with a bad tach filter. I'll replace it and take it from there.
Ok I did a liitle more testing today.According to mile markers and the odometer on my truck against the car its right on.So I did mph per rpm from 50 mph and up in fourth gear. 50=2500 55=2700 60=2850 65=3150 70=3300 So I compared this to a m19 with a 4.10 gear ratio and it looks pretty close at 70 mph=3100 So my tach could be off a little bit but the speedo is reading correctly so the po may have put in a m19.I haven't checked the speedo gear to see what tooth but guess it really dosent matter too much now.
ok did a side by side comparison with wifes 2010 ford escape.She was at 60 mph and the fiero showed 55 mph. So you put that in the equation 55=60 mph the calculator shows 2647 with a 4.10 final ratio and my tach shows 2700.
Update: I disconnected the tach filter and jumped the connector with a wire, which didn't seem to make a difference in the tach reading. I also checked the tach reading with a tach/dwell meter, and it seemed to correlate with the tachometer. The car has a brand new clutch with about thirty miles on it, and it doesn't feel like there is any slippage (at least I hope not!!!). It has been a few years since I had the car on the road, so I don't really remember if it read like this back then. Could the car just be geared that high, or should I keep digging? Could it be in the ECM?
EDIT: I did not check to see if the VSS has the correct gear, because I verified the speed with a GPS.
[This message has been edited by PatrickTRoof (edited 12-23-2013).]
