Bandit60

Presently M-21 transmission. Tires are 225/60R14 changing wheels and tires to 235/60R15. Will I need to change the speedometer driven gear and if so, to what?

Fun71

This site shows the tire diameter difference is 2%, and the speedometer would be off by 1.2 MPH at 60 and 1.8 MPH at 90.

https://tiresize.com/comparison/

As long as your speedo currently reads correctly, I would not change the gears.

joe_padavano

I don't know what site you're looking at, but a 225/60-14 is a Honda-sized 24.63" in diameter. The 235/60-15 is still small at 26.10" diameter. That's about a 6% difference. Given how much smaller the 225/60-14s were than OEM, unless you've changed the speedo gear to match those tiny tires, it was probably off before. The 235/60-15s are significantly closer in diameter to the original tires and likely will be a close match for the original speedo gear.

Fun71

Doh! I inadvertently put in 15" for the 225-60 instead of 14".

So it's actually a 6.1% difference. When the speedo shows 60, actual speed would be 63.7, and when the speedo shows 90, the actual speed would be 95.5.

joe_padavano

Again, this assumes the speedo gear was changed to match the 225/60-14 tires. The original tires on the OP's 1970 442 were G70-14s, which were 26.82" OD. Even the 235/60-15s will be almost 3% smaller than OEM. If the car has the original speedo gear, it will read that much faster (when the speedo reads 60, actual speed will be about 58.)

Bandit60

Gentlemen again a quick succinct answer. My Oldsmobile education continues. Thank you.

joe_padavano

To be honest, 245/60-15 would be even better at 26.5" diameter. I've run 255/60-15s on my 69 H/O and I get only minor rubbing in front when I hit a bump with the wheels turned to the stop.