Tachometer to Speed Conversion
I have a 1963 roadster with a Speedometer that is suspect. I'm looking for the way to convert RPM's to MPH with a 3:36 posi and a 327/250 HP engine.
Thanks,
Jerry
Thanks,
Jerry
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Re: Tachometer to Speed Conversion
If your installed tires are something close to 760 revs/mile as were the OE 6.70-15s then revs at 60 in top gear = 760 times axle ratio, and you can linearly proportion from there.
Significant speedometer error can be caused by having tires that are significantly different revs/mile than OE or incorrect speedo gearing for the installed axle.
Duke -
Re: Tachometer to Speed Conversion
A real easy way to test the actual speed of a vehicle is by using a hand held GPS. Or, a speeding ticket but they usually cost more than a basic GPS.
If there's an error, there are basically three things that can cause the error. Incorrect speedo drive/driven gears, faulty speedo or rear tire diameter/circumference.
To test each system, first find out if the indicated vehicle speed is accurate. If not, check to see if the ratio of indicated speed to indicated RPM at a certain speed agrees with the factory numbers. For example, a car with a 3.36 differential ratio and 83" circumference tires will be traveling at roughly 58.74 MPH at 2500 RPM.
If your speedo and tachometer don't show these amounts, I suspect the speedo gears or the speedo head itself.
If the Speedo shows 58 MPH and the tach shows 2500 RPM, but the vehicle speed does not match the indicated speed, that would indicate that the speedo head and correct speedo gear set are functioning properly. The problem would then be tire size or non original differential ratio.Last edited by Michael H.; April 30, 2009, 06:49 PM.Comment
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Re: Tachometer to Speed Conversion
To echo what Michael has said, in recent years, I've used a GPS unit. I have a Garmin unit that I use occasionally. Sure beats finding a measured mile course and timing your car with a stopwatch.
My 56 Chevy speedo was WAY off when I bought it. It's a hot rod, and NOTHING is stock/original.
I used the GPS trick to get my true MPH. Compared it with the reading on the speedo.
Pulled the driven gear and counted the teeth. Calculated which new gear I needed, ordered it, and then installed it later on.
Problem solved.
Chuck1963 Corvette Conv. 327/360 NCRS Top Flight
2006 Corvette Conv. Velocity Yellow NCRS Top Flight
1956 Chevy Sedan. 350/4 Speed Hot RodComment
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Re: Tachometer to Speed Conversion
Thanks for the ideas everyone. I'll get a GPS and go from there.
JerryComment
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Re: Tachometer to Speed Conversion
You'll need to know the number of teeth on your driven gear first.
Lets say you're speedo shows 50 MPH, and your GPS shows 60 MPH.
50/60 = .833, or lets round it to .8 or 80%
Doing it the other way, 60/50 = 1.20.
So.....you have a 20% error. In my example, you want to "speed up" the speedometer. To do that, you need a driven gear with FEWER teeth.
Lets say you have a 24 tooth gear in there now. 20% of 24 = 4.8.
Lets round that to 5. In this example, you would need a gear with 5 fewer teeth, or a 19 tooth gear.
You can never get it "dead on" at every MPH, so 60 is a good compromise.
Now, there are also different "drive" gears on the tail shaft of the trans. LETS not get into that now.
Pull your driven gear, look at the color, count the number of teeth on it, and post that info here.
Chuck1963 Corvette Conv. 327/360 NCRS Top Flight
2006 Corvette Conv. Velocity Yellow NCRS Top Flight
1956 Chevy Sedan. 350/4 Speed Hot RodComment
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Re: Tachometer to Speed Conversion
Jerry,
If my memory serves me correctly, when you have a 3.36 and are in 4th gear the tach and speedo move parallel to one another. Or at least so close you can't tell the difference.
As Duke pointed out, that with the original 6.70's tire circumference on the ground.
JRComment
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Re: Tachometer to Speed Conversion
No, I didn't say that. I said "760 revs per mile."
DukeComment
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Re: Tachometer to Speed Conversion
Jerry,
I see you live in Conn, is it a small town? If so, stop in your P.D. and explain to the O.I.C. that you want to verify your speedo on your old car. We use to do this all the time and would check with the guys on the road who were working a low volume area and then send the citizen to that officer. He would check you on radar and let you know your speed. We would verify each patrol car speedo once a month first thing on a early Sunday dayshift. Stop in and ask, they might drool on your car though.
Rick
P.S. They might even laser you which is better yet!!Comment
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Re: Tachometer to Speed Conversion
Most of the Colleges around here have radar controlled speed signs that show the speed that you are going. See if you can find one of these in your area. Takes all of the guess work out of the equation. The only down fall is every once and awhile a cop is next to the speed reading sign with his own radar gun.Comment
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Re: Tachometer to Speed Conversion
Go to the Randys Ring and Pinion website and they have speed calculators which require you enter rear end gear ratio, tire diameter and final drive ratio.
Works great and is free.Comment
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Re: Tachometer to Speed Conversion
My vote is for the GPS. I just returned from a 900 mile road trip with our 63 Fuely and I used the Garmin for the first tank of gas and then I knew what the speedometer error was so it was the radar detector that went into the lighter space. We got as high as 22.05 MPG @ 78 MPH. Gotta love it. Haven't driven the car for 21 years.It's a good life!
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Re: Tachometer to Speed Conversion
I agree with the tach and speedo needle match. I too have a 63 w/3.36 final. When I ran 670x15's, back in the day, it was a near perfect match. I now run radials (P205-75Rx15's) and it's very close, perhaps a little off. I have not measured the circumference lately, but will and check them against my still new spare U.S. Royal in the tub, next time I drop it. For cruising, I always tried to keep it below 3000 rpm for best mileage. Back in the day, I always expected a minimum of 21 mpg. That is not possible any more as I tend to flog it too much on short trips.
Stu FoxComment
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