I replaced my speedometer gear the other day. I looked up on a chart using my tire size and the rear gear ratio and found I needed a 38. I was 15 mph over with the 33 that was in there. Now I'm still showing 3 mph fast with the 38. Looks to me like maybe just one more tooth will be close. Any experts out there?

car in a parking lot with wheels straight ahead. put a paint mark on a rear tire at 8 o'clock. roll car forward till you have two paint marks. tape measure that straight distance which is "rollout". divide Pi into that number to get the actual "diameter" that the speedo will see. there's a chart that shows the correct pinion teeth for tire diameters down to the tenths of an inch. I have it but it wont post as it keeps telling me it needs to be a jpg extension or some nonsense . jack up the car high on the dr side to minimize the fluid loss. be sure to clock the adapter correctly for the number of teeth your pinion going in will have. find that chart (or show me how to post it). You need correct diameter & rear gear ratio

3 MPH and you are worried?They were not that accurate new.Remember its and old mechanical unit not an electronic one like a new car.Rocky

if my math is right, a 5 tooth change made a 12 MPH reading change so a 39 (your only choice really other than a gear reduction adapter) so 12 divided by 5 is 2.4 (MPH) so a 39 toother (if there is one and you can locate one) would theoretically slow the speedo another 2.4 MPH & put you very near dead on

Use the odometer to verify correct speedo pinion, it's direct-geared and won't pick up errors like the speedo does.

Going off of what master John said, other than a GPS the most accurate test for the odo is the mile markers on the interstate (go five miles) and the most accurate test for the speedo is the temporary roadside lighted digital signs that are set out on occaision. Get the right tooth pinion to get the odo correct THEN if the speedo ain't correct at that point in time right along with it then the speedo head needs to come out & go to a speedometer specialty shop to be recalibrated. EDIT If they ain't equal & I dont want to send out the speedo head then if I can get the right toother in there to get the speedo correct then that is what I prefer (helps with avoiding tickets)

Speedometer - Free - Speed Limit Alert with Map, HUD and GPS Black Box Recorder by Stanislav Dvoychenko
https://appsto.re/us/NmhqH.i

Very handy app

We have always used the mile markers on the interstate and the odometer to pick the gear. But be aware that a family member in law enforcement has radared my car several times and the speedo's accuracy is not the same at all speeds.

Which is why Mopar offered certified speedos on the cop cars. Of course, the certified speedos came with a correction chart in the glove box because even they weren't accurate at every speed.

So I think that you can get the odometer zeroed in, but may have to pick a spot on the speedo to get closer than 3mph while accepting other speeds may still be off.

My 65 the speedo is off about 20%.I just adjust to it being off.Its not hard and if it was 3 MPH Id be happy.Like I said new they werent very accurate.Even back then if you were around 5 plus or minus you were doing good.Rocky

Likely your odometer shows a surplus of 20 percent additional miles than actual. However makes for a great improvement in fuel mileage,,,I can live with that.

Try and find gears for pre 66 trannys.They are rare and pricey.Ill change trans one day so Im not spending big bucks to get the speed close.It is a lot closer then when it had 4.88s in the rear.Rocky

Anybody know where a ratio adapter can be bought? Went from 2:94 to 3:73 in an early ('64 727) car and need to correct speedo.

If I can find mine I will take a pic and post the company name. If I was in a rush I'd Google it/ebay & see if anything pops up

One tooth is good for two MPH.

Imperail Services was making a ratio adapter.They were pricey though.I almost got one from Steve but didnt know what gears I was changing to.3.91s now and either 4.30s or 56s next.Rocky