Hi. I have a newly acquired 1971 cutlass convertible. All stock 350 auto trans. I was cruising down the road and my speedometer read 34mph. The radar speed device on the road read 30 mph. My guess is the speedo gear is wrong. Does anyone know how to pick the correct speedo gear. I am adding additional miles on the odometer and want to correct that. Thanks for any advice. Just remembered. The previous owner changed the wheels for 14 to 15" rims. That's where the problem is. How do I correct that and keep the 15s

 

There are two gears, the driven gear in the removable housing that is attached to the speedo cable, and the drive gear inside the tail of the transmission. You need the axle ratio and the tire diameter (rim size isn’t important), and you need to know which drive gear you have. If you need to change the drive gear you have to drop the drive line and unbolt the tail shaft of the transmission, actually not too difficult.
https://www.tciauto.com/speedometer-gear-calculator

 

The size of the wheel is irrelevant. The outside diameter of the tire is what matters. The original tires were F78x14 (26.50" outside diameter) or G78x14 (27.06" OD) with A/C. Optional were G70x14, which were halfway between the other two sizes in OD. If your speedo is reading higher than the actual speed, the 15" tires must be smaller in diameter than the OEM tires. As an example, 235/60-15s are only 26.1" in diameter. 245/60-15s would be 26.57" OD, about the same as the F78x14s.

 

I swapped rearend gears multiple times since high school and just made a conversion chart that I cut out and stuck on the dash so I knew how fast I was actually going at the indicated speedometer reading.

You can calculate the % offset using your actual 30 MPH and the indicated 34 MPH and use that to determine the actual speed.

 

I've found the accuracy of those radar speed signs to be all over the map. The best thing to do is to clock yourself against mile markers on the freeway. At 60 MPH you should be doing exactly one mile a minute. Less than a minute means your speedo reads slow, more means it reads fast.

 

That is a good idea. The companies that make speedo recalibration boxes say to compare your odometer to the mile markers over a 5 mile span to determine the offset.


Oh yeah, there are companies that make speedometer calibration boxes that go inline with the speedo cable. Ohio Valley Speedometer is one that I recall.

 

You can possibly also use your Iphone GPS to clock your actual speed - a GPS readout in my Maps app tracks my actual speed as I follow whatever route is set. AND it even warns me of potential speed traps

 

I'm just happy this thread is about a speedometer....I had terrible thoughts of the images I might witness in this thread...

 

Oh now you went and did it.

 

Prior to GPS, I always checked my speedometer against the mile markers as Joe suggested above. I felt the longer the distance used as a check, 10 miles, the more accurate the check!

 

If you use your phone and GPS as a fallback, be sure it’s app you trust too.

I use iPhone waze quite often and found it quite accurate. I also recently started using the gauge feature on my Bluetooth distributor and found that each reported slightly different values at speed.

My trust is more in Waze and I was surprised to find the estimated speeds slightly different.

Chris

 

Thank you for all the help. It is definitely the tire size. A half inch lower profile is the culprit. You folks really know your stuff.