Speedometer drive

[Star27buck]

I just replaced the gearbox in my '60 S2. The speedometer hasn't worked in years and I took the opportunity to replace the cable as well. I hooked it up to the speedometer which tells me that I was going 65 MPH when my GPS said 42. I surely didn't feel like 65, much as I would like to believe it. The old gearbox had no letter suffix and the new one is "C". Are the speedometer drives different for the transmissions?
The old one was 151118640 and I replaced it with 25241474 C.

[lumpydog]

The mix of tires and speedometer matter. You need to have the correct speedometer paired to the right tire size.

More here.

[TedW]

I just replaced the gearbox in my '60 S2. The speedometer hasn't worked in years and I took the opportunity to replace the cable as well. I hooked it up to the speedometer which tells me that I was going 65 MPH when my GPS said 42. I surely didn't feel like 65, much as I would like to believe it. The old gearbox had no letter suffix and the new one is "C". Are the speedometer drives different for the transmissions?
The old one was 151118640 and I replaced it with 25241474 C.

It's almost like you have a KPH speedo: 42 mph = 68 kph.

Tire size will make a difference, but you need to make a big change to have a noticeable difference: My '70 88 came with 15" tires, which I switched to 235/85/16's. The difference in indicated speed vs. GPS speed is tiny, maybe one or two mph faster at 60 mph indicated.

[cedryck]

It's almost like you have a KPH speedo: 42 mph = 68 kph.

Tire size will make a difference, but you need to make a big change to have a noticeable difference: My '70 88 came with 15" tires, which I switched to 235/85/16's. The difference in indicated speed vs. GPS speed is tiny, maybe one or two mph faster at 60 mph indicated.

They (smiths unit) is inaccurate. Mine with proper size tires tells me I am pushing 70, when traveling like 45, hysterical.

[TedW]

They (smiths unit) is inaccurate. Mine with proper size tires tells me I am pushing 70, when traveling like 45, hysterical.

It makes sense that a 50 year old instrument exposed to heat and dampness might not always work properly.........

[lumpydog]

My 88 was originally shipped with 15 inch tires and a 1536 Speedo. When I put it back on the road (after 18 years of storage), new 16 inch tires were put on her. I swapped the innards from a 1408 speedo (designed for 16 inch tires) and it's dead on accurate.

The link in my prior post explains the math behind the size and speedo gearing. Nisonger can re-gear a Smith's gauge to be accurate to your tires.