56 T-Bird Speedometer Calibration Revolutions/Mile

[Pat_n_Pat]

Hi All:

We recently had our speedometer rebuilt (it hasn't worked in 30 years) on my wife's '56 Bird. On a recent "outing" it appeard that the speedometer was reading about 20% high. We did not check the odometer.

The speedometer was "set" for 1,000 revolutions/mile (pretty much a modern standard). Did Ford use that standard for calibration from the factory? Before we start to get into adapters, etc. I want to make sure our basic set up is correct.

Thanks,

Pat

[1957Birdman]

Hi Pat, I doubt you will get an answer to your question. The early T-Bird speedometers are notoriously inaccurate. Usually at an indicated 70 mph your speed really is 62-63 mph. I just adjust for it in my mind. I doubt you will ever be able to get it exact. Good Luck!

[Pat_n_Pat]

Lew:

I know cars from the era are notorious for speedometer errors. In the 1960's the Auto Club of So Cal actually had a speedometer dyno that would test your speedo for accuracy. I think it was free for members.

This car was my wife's high school car. The Speedometer had not worked since sometime in the 1970's, but it never bothered her (just flow with traffic plus 5 mph).

We ran this car cross country on the Great Race twice. We used a Great Race compliant speedometer that can be adjusted daily (in theory accurate to .001 mph). You are not allowed an odometer on the Great Race. Some rallies we want to run require stock speedometers and odometers.

We want to get the speedo set to factory specs before we start "tweaking" with different gears. First we need to get the odo as close as possible as it is critical for rallying. We can then tweak the speedo needle.

The speedometer "should" have a number that represents revs/mile written somewhere on the the body. I can't find it on ours; perhaps someone has one out of the car and can find the number.

Thanks, Pat

[miker98038]

Back in 67, when I bought my first bird, the speedo with the FOM was as you describe. When I changed to a manual trans, same problem. Likewise with the one I currently own, when stock. I always assumed Ford just made a 150 mph faceplate ( in place of the 120) but never changed the gears. Best I recall, the odo was more accurate than the speedo, but not up to rally standards.

[1957Birdman]

Pat:

Unfortunately you trying to achieve a level of precision that isn't possible in my opinion. Rather than changing gears in the transmission I would do this. Find a five mile measured course and check the odometer against that. Then check the speed on the measured course by timing it while keeping it at the same speed. This will work best on a level stretch of road. If you complete the 5 miles in 5 minutes then you have your 60 MPH calibration. I agree with miker. There is a suspicion that Ford just changed the faceplate and nothing else. I have the parts book for those years and may look to see if I can confirm that. The other kicker in all of this is the size of you tires. If they are anything other than 6.70 X 15 that will also change things, as will tire wear. In the end I think you will not be able to replicate the accuracy of more modern speedometers, at least not through the normal range of driving speeds (25 to 75 mph).

[Guest skygazer]

The dial on the 55-56 Thunderbird speedo goes to 150. The dial on corresponding passenger cars goes 120. Rumor has it that Ford used the same speedo and speedo drive gear in both, even though the transmissions were identical and the rear end ratios were similar.

The speedo in my '55 (ford-o-matic, stock 3.31 axle ratio, "stock" 205/75R15 tires) was reading 20% high according to GPS. It had the stock 17 tooth speedo drive gear. When the cable and gear broke, I replaced it with a 22 tooth gear, and now it reads only a few percent high. This is much more acceptable!