i had a friend behind me today on a cell phone, i was in a 66 327 300hp roadster with m21 transmission, 3:36 rear end, my speedometer read 50, the honda behind me read 60, when my speedo read 60, the honda read 67, i assume the problem is in my car, can anybody give me the most likely reason and the fix, i hope its something simple like a speedo cable, thanks joe tassoni, by the way i was driving the vette in a snow storm to bloomfield connecticut to have it soda blasted in prep for a new paint job for the winter!
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Re: speedometer check
you need to change the speedo gears in the trans. most speedo shops can calibrate it for you. -
Re: speedometer check
Since there is no such thing as a stupid question- Did the Honda have OE size tires on it? Also, if you were being real careful about your readings, and assuming the Honda was correct, it seems odd that your reading was almost 17% slow at 60 mph, and down near 10% at 67 mph- at that rate, your speedo should be right when you get to about 80 mph. I believe that the percent error shouldn't change at varying speeds unless there is maybe a friction or slippage issue?Comment
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Re: speedometer check
normal size tires on the honda, the honda speedo is not digital so the error may have been in the driver, but obviously not by 7 to ten miles per hour, i got stopped by a policeman in the vette one time he told me i was going way too fast, ie 60 in a 45 zone, i told him i was only going 50 and i was sorry, he didn't give me a ticket luckily, so i think that the speedometer in my car is really off, is changing the speedo gears on the trans a big job?Comment
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Re: speedometer check
Joe-----
Borrow a hand-held GPS receiver from someone who has one. You can use this to accurately and easily determine what your actual speed is. If you're within 2-3% of actual, then you're not going to get any closer by changing speedometer gears.
Changing speedometer DRIVEN gears is quite simple. If you have to also change the DRIVE gear, it's a LOT more difficult as the transmission has to be removed and partially disassembled. I can't tell you, at this point, what you need to do.
The best bet is to take the car to a speedometer shop as clem suggested. They can quickly and accurately calibrate the speedometer. If necessary, they can avoid the need to change DRIVE gears by installing a speedometer adapter. The adapters are expensive but a LOT cheaper than going into the transmission to chnage DRIVE gears if that would, otherwise, be the only way to get the speedo into calibration. Adapters can also be used to achieve near-perfect speedo calibration. This is how the "certified" speedometers in police cars are brought into very tight tolerances.In Appreciation of John HinckleyComment
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Re: speedometer check
One thing I do if I suspect an unaccurate speedo is drive the car at exactly 60 mph indicated in the suspect vehical for one mile and time it. It should take 60 seconds to cover the mile. The problem is that you have to find a stretch of road that is marked for odometer checking or sectioned off into miles as they often are in the country...farmers fields are often refered to as sections or quarter sections. Remember that you must enter,maintain and exit the mile long run at 60 mph. This is my "back woods" method but it gets you well within the ball park.Comment
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Re: speedometer check
From what I remember,changing the transmission speedo drive gear is simple. You remove the driveshaft,the speedometer cable and drive from the tailshaft and then the four bolts holding the tailshaft to the transmission. Remove the tailshaft and have a pail to catch any oil that comes out. The speedo drive gear will be right there and held in place with a clip. Compress the clip with a small pic or screwdriver and slide it off of the shaft. Reverse the procedure with new seals/gaskets,add oil and you're done. The color of your gear determines the final drive ratio that it is to be mated with and there is a chart for this. I can't find this info right now but maybe someone could post it for you so that you can get the proper color speedometer drive gear for your rear end before you take it apart. Hope this helps.Comment
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Re: speedometer check
It sounds like you have a 3.36 gear in the rear now but had a 3.70 gear once upon a time, or at least your transmission was in a car with a 3.70 gear.
Joe would have you change out the drive gear but usually all you need is the driven gear, the plastic one that goes in the side where the cable attaches.Comment
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Re: speedometer check
Before you change anything, determine if you have a speedometer problem only or a combination of speedometer and odometer problem because the potential causes are different.
Interstate freeways have mile markers that can be used for this test:
Check the odometer accuracy by comparing the odometer reading to a ten mile distance on the freeway as indicated by the mile markers. If the odometer is accurate, do not change any gears because you only have a speedometer problem.
Now, make a ten mile run at exactly 60 miles per hour. This should take exactly 10 minutes. If the speedometer and the odometer have the same percentage of variance, you probably have a gear problem. If only the speedometer is off, the speedometer head should be removed and checked at a speedometer shop. Usually, they can be repaired to correct a weak magnet,damaged speed cup or just need to be recalibrated.Comment
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Re: speedometer check
thank you to all, what kind of auto shop has speedometer repair, i live in amherst massachusetts and i can't find any in the yellow pagesComment
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check with a local over the road trucker because
they get their speedo calibrated/certified so they can go to court if they get a speeding ticket they think is unjust.Comment
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