Was getting passed steadily by ALL other vehicles when I was cruising at 65mph (65mph zone). I increased to 75mph and started passing the semis and it took longer for automobiles to overtake me.

I have:
700R4
27.05" tire diameter (P255/60R15)
3.54 gears

According to the Bow-Tie Overdrives site, with my 700R4 I should be cruising at 65mph at 2000rpm. However, at 2000rpm my speedo says 75. I installed the Bow-Tie specified speedo gear in the trans (after they had sent the gear for the 3.07 that I replaced with the 3.54 Spicers) back in 2001 when I installed the transmission.

So, given I was passed less often at 75mph, I deduce that my mph is not accurate. If it were the tach in error, I'd be able to sustain 65mph and just have a faulty rpm reading. I replaced the tach filter with a used one from a mid-80's Monte Carlo back when mine failed and my hand-made filters (2) failed after use.

I may research the gear installed in the transmission just to verify I didn't go and install the speedo gear they sent for the 3.07. And, it COULD be the speedo... it has over 133K miles. Could the cruise transducer be the problem?

From Bow-Tie:

MPH 700R4 Trans rpm
45 mph 1,386
50 mph 1,540
55 mph 1,694
60 mph 1,848
65 mph 2,002 * my RPM
70 mph 2,155
75 mph 2,309 * my Speedometer reading
80 mph 2,463

 

use a gps to verify true speed and go from there, larger or smaller on the driven gear...if the transducer is ok that is....good luck

 

Your 700R4 has a speedo DRIVE gear and a DRIVEN gear. The driven gear is accessible where the cable enters the trans (at least it is on my 200-4R) but the DRIVE gear is DEEP within the trans.

Easiest and cheapest option is to change the driven gear...OR go by a speedo shop and get them to make you up a ratio adapter to correct the error.. It is around $60 here in Tacoma... plumbs in the speedo cable. Takes about 15 minutes for them to make it, and they have to know your ACTUAL speed when your speedo is reading 60 mph...(GPS or a pace car)

 

You could also send your speedo put to have recalibrated Willcox did mine they did a great job.

 

When Willcox recalibrates, is that essentially a rebuild? Did you have to specify the displayed speed vs. actual speed at different rpm's?

 

Suggest that you purchase a replacement speedo [driven] gear in your tranny; selection should be dependent on differential gear ratio...with a 'fudge' factor included for non-standard tire diameter, if necessary. The square recess in that driven gear can wear till the cable end starts to slip in it; that fault will result in the same symptoms you are describing. The cruise transducer will NOT cause that kind of problem, unless the gear teeth inside are damaged; and then the speedo starts jumping around, too. It is possible that the number of teeth on that driven gear needs to be revised...but if it worked correctly with that setup before, I doubt the number of teeth is your problem.

 

Low tech way to calibrate your speedo is to use the measured mile markers on many highways. Keep it at 60mph for the mile and time the seconds (should be 60sec). Do the math to figure your speedo error.

 

Garmin/GPS is the way to fly, all else including those tables is

and from personal experiences, I can say it's true....

for instance MY speedo is NOW correct at all speeds up to 80 which is 78, at 100 it's only really 94, I ran out of room for any further experiments....but it proved my point that the speedo is not worth flip at anything like decent speeds.....

 

No i did not have to specify anything. I was doing a face swap of the 85 mph speedo to a 140 mph. but i had to keep the original speedo because of the VSS. so willcox changed the faces and calibrated the speedo. when i got it i installed it and with my GPS took the car out for a spin. it was right on.

 

I used my Tom Tom GPS to find out what I had suspected, my car seemed to be going slower than what my speedo was reading. Sure enough, when my speedo is reading 70 I am actually going 61 MPH, off by 9 MPH. I thought about changing the gear in the trans to make it read correctly, then I said the heck with it. I know now actually what speed I'm driving at and hopefully an OD trans is in the near future and I'll worry about calibrating it then.