Packard Forum Index » V-8 (1955-56)

My '55 400 reads 6mph faster than actual. Any suggestions to correct it?

Is that at all speed ranges?

They can be calibrated by a speedo shop.

bills-speedometer.com is owened by Patrick Mescher, a member of this website, and comes recommended.

If your tires are about 26" diameter instead of the 28" the car was built for that will account for 4.7 mph difference changing 66mph to a little over 61.

Oh, and if you read old road tests of most any brand most all of them had errors of 4-7 mph at speed.

My tripometer runs like 10% different than the actual distance.

Mine is 100% accurate when standing still after that it goes downhill.

And if tires or a speedo shop cannot get it close enough there are some street rod type places that make a custom gear box that inserts in the drive cable to correct or fine tune the RPMs the speedo sees. As I recall the boxes are made primarily for those that mixed parts so a brand X speedo would work with a brand Y trans and brand Z rear end but the principle still holds.

To me, a 6 mph difference isn't worth the cost or time to correct since that is in the normal "error" range.

You just have to remember to do the math when looking at your speedo.

And at least it is reading fast. Imagine the speeding tickets you could attract if it was reading 6 mph slow.

The car is running 235/75R15 tires, which have a diameter of 28.9". The speedo reads high pretty much the same across the typical 25-60 driving range.

Do all Ultramatics have the same speedometer drive gear regardless of rear end ratio? I do know the previous owner rebuilt the transmission and had at his disposal a parts donor Clipper. (I had to remove the transmission due to an inoperative DD clutch and discovered a mismatched '55 pump and '56 plate, blocking the oil path.)

There were several different speedometer gears available for Packard rear ends but the parts book does not mention the number of teeth per ratio. Perhaps someone might have a chart with that information.

The Twin Ultra was also used by some 55 Nash and Hudson models and the 56 Stude Golden Hawk. No idea if any of their rear end ratios were different from Packards and if they had any other gear options.

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