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View Full Version : Speedo off... pretty far off too ... but car is stock



93SuperCoupeBoy
06-03-2004, 11:14 PM
Ok...at the track tonight, I trapped around 88mph every run, but my speedo said I was going between 105-110 (just to give it some leeway). Anyways...what would cause that considering I have stock sized tires on it. It is far enough off to seem like gearing?! Just wondering some other opinions to help me solve this mystery. I would give revvs @ certain speeds, but obviously if my speedo is off, revvs won't match the speeds I give!

Nick

BlckOnBlckBrd
06-03-2004, 11:18 PM
Just to add some clarification, at the end of the qtr, he told me he is about 4500rpms, and I have 3.73 gears and I'm at about 4600-4800.

Anyone think his car could have gotten a gear swap without any record of it?

SlinginBeauty
06-04-2004, 01:11 AM
I have this same problem. I have no idea what it could be. It gets progressivly worse as you get higher in speed. If im going 85 on my speedo im doing just over 65 here, at 60 on speedo im going about 45... Dont have any idea what it could be considering the rest of my gadge seems to be working just fine. Didnt notice it as much b/c my tranny swap, so thought it may have been that and different gearing but aods, are aods so...same gearing i would think. If anyone has any ideas what it could be that would be awsome. Pretty sure rearend has stock gears. THere has been a few owners, only one that did any work was the last and he "rebuilt" the lowend, and got a new sc(still stock). SOunds like same thing as you Nick. Hope someone can help us out. Zach

sho taurus
06-04-2004, 06:28 AM
yeah but 93super is other direction.did you just buy the car?sounds like gear 3.55 or 3.73,same speedo gear though i believe.if you don't get that fixed you're odometer will tack on alot more miles then it really has.i friend of mine did that too and for every actual 1 mile it was like a 1 1/2 miles on odometer.he rode around for over a year like that.speedo gear is like $10-$20 and zach haven't a clue of what yours might be.here is calculator http://www.sccoa.com/faq/speedgr.html

John Valerioti
06-04-2004, 08:47 AM
yeah but 93super is other direction.did you just buy the car?sounds like gear 3.55 or 3.73,same speedo gear though i believe.if you don't get that fixed you're odometer will tack on alot more miles then it really has.i friend of mine did that too and for every actual 1 mile it was like a 1 1/2 miles on odometer.he rode around for over a year like that.speedo gear is like $10-$20 and zach haven't a clue of what yours might be.here is calculator http://www.sccoa.com/faq/speedgr.html

My 1990 seems to show a higher reading above 60mph. If I time it on the highway using the second hand on my watch and the mile markers, 60 mph in 60 seconds is accurate....but then about 10 over the faster I go...it may read 90 and I'm actualy doing 75...it has to be the calibration...if you figure yours out...please let me know..thanks...John

Tazer999
06-04-2004, 09:09 AM
FYI: Your trap speed is not the speed your car is going when you cross the finish line

93BLKSC
06-04-2004, 09:41 AM
FYI: Your trap speed is not the speed your car is going when you cross the finish line
It should be fairly close though. Isn't the trap speed a calculation of the time it takes you to travel the last 60 ft. of the strip? Since you're still accelerating, it will be off a little. But how much can you accelerate, in 60 ft, at around 5300 rpm?

My speedo seems to be off too. It shows about 110 when my trap speed is 96-97.

68COUGAR

007_SuperCoupe
06-04-2004, 10:19 AM
There's an easy way to check to see what gears you have in the rearend. That will determine if your speedo is off because of gearing or simply because of tires. Block the front wheels, then raise the rearend of the car. Support it on jackstands so the wheels can spin freely. You can make a mark on one of the rear tires, then climb under the car and mark the driveshaft near the differential. Have another person spin the wheel one full revolution. The number of rotations that the driveshaft rotates will be your gear ratio...i.e. 3 and 1/4 turn will be 3.27 gears...3 and 1/2 turns...3.55's, 3 and 3/4...3.73...you get the picture. That's the easy way to tell what gears you have.

Keep in mind that you speed-o-meter is geared taking into consideration, final drive ratio (tranny and rear gears) and the number of revolutions that it takes your TIRES to travel one mile. That is all factored into it. It is actually very rare that you will have an exact whole number (for the speed-o gear in the tranny) when you factor in those variables. So you round off to the nearest whole # for the speed-o gear. That allows the speed-o-meter to be off by a factor derived by the amount that you had to round off to get the proper # of teeth on the speed-o gear. I've seen it as much as 5 mph either way depending on the size of tires that you have on the car. My experience has been that the stock 225/60/16 tires are generally off by 2-3 mph depending on the treadlife that is left. I currently put on 235/60/16's and my speedo is pretty much dead on. It is less than 2 mph which is good enough for me. Hope this info helps...I can get more technical, but it would get a bit boring...

sho taurus
06-04-2004, 02:11 PM
yeah i screwed up when i put my 3.73 in cuz i used that calculator thing they have on this web site.cuz i typed in 3.73 and got the figures for 225/60/16's inwhich i had on the car at that time and it said i needed a 22.6 tooth gear so i bought a 23 tooth which is way off.i needed like a 21 tooth.wife got a ticket.but i won in court.proved it was improper equipment and she has perfect record so judge dismissed it.yay i finally won 1.but anyways so i used up those 225/60/16 and i bought a set of 245/50/16 and now my speedometer is right on the money now.so it worked out in the end

mustang5.0racer
06-05-2004, 10:18 AM
Stock speedos are never exactly accurate. They arent made to go to 100 miles an hour. It says it will but the more you accelerate the more its off. The only speedo you can trust is a certified one like out of a cop car. By the way thats a good way to get out of tickets too. WORKS WONDERS lol ;) Just tell the judge that your car didnt come with a certifeid speedo!

93SuperCoupeBoy
06-05-2004, 12:42 PM
I understand that a speedo won't be completly accurate. But there are legal requirements for accurace, which is something like +/-10% or something like that. A speedo shouldn't be about 25% off actual speed for the whole range of it!? No?!

Nick