Speedometer correction (switching to 4.10s)

[-=Jeff=-]

Originally Posted by WVZR-1

Well - establish (for your car) either a rolling radius (center of rear axle to floor) or rolling circumference (chalk tire and floor 6 o'clock roll one complete rotation ,chalk the floor & measure). Is there a preference? Circumference I'd guess.

Then it's all math - if the OE (15) tooth is still in the car I don't believe you can do just a driven and "be happy"! Not having RR or RC a "WAG" with Marc's (13) would be a driven of 42 - 43. I believe you've got 42 now so just Marc's drive for a start.

Yes planning on Marc's gears.. well his drive at least

according to his site.. I need the 13/43 for the 4.10 (assuming stock tire size)

the 3.45 is a 15/42 so I will buy the 2 parts from Marc and install then compare to a GPS and see if I need to change anything. I am just trying to get all the parts in my possession before the actual swap..

[WVZR-1]

Originally Posted by -=Jeff=-

Yes planning on Marc's gears.. well his drive at least

according to his site.. I need the 13/43 for the 4.10 (assuming stock tire size)

the 3.45 is a 15/42 so I will buy the 2 parts from Marc and install then compare to a GPS and see if I need to change anything. I am just trying to get all the parts in my possession before the actual swap..

I'd just do his drive (13) and try the driven (42) that I believe you'll find is in your car presently. After you confirm the accuracy if there was any reason you found substantial wear on the driven then replace it with new. As easy as it is to do the driven I'd just do the drive first and then ride it for confirmation. I did the math with a rolling "dimension" calculator that was passed on to me by someone many years ago. It doesn't replace measuring but it's been quite accurate for most I've done. I'd be interested in both the RR and the RC if you wouldn't mind measuring yours and posting it.

I ended up with a 43+ but since there's already a 42 there I thought it wise to use "what you got" for a start!

In the "faulty equipment/reckless" days with a mechanical speedometer I was considered the "first stop" for a speedometer adjustment. I accomplished most of those using reduction adapters of various ratios. Drive/driven changes generally wouldn't/couldn't cover the required adjustment. Axle ratio/tire dimensions changes were done pretty regularly.

[Jagdpanzer]

WVZR-1 is the man when it comes to speedo gears, or for that matter, any ZR-1 part you might be looking for. He helped me out when I changed over from 335/30-18 rear tires to 345/30-19 tires. Before the change I was running Marc's 13/43 gear set which was right on for the 335/30-18 tires and 4.10 rear gears. However, with the taller 345/30-19 tires the speedometer error was about 4 mph slow at 60 mph. After doing the math he recommened going with a 41 tooth driven gear against the already installed 13 tooth drive gear. The results were perfect. Now no error at all when running 60 mph according to the speed check app on the Iphone.

[-=Jeff=-]

4.10 has been completed with the 13/43.. at 41mph on the speedo an app on my RAZR shows 41mph..

I did notice the speedo might bounce to 40 and the RAZR app shows 41 still..

I tried my Malibu on the way to work and it was off by 2.. speedo (needle) showed 40.. App showed 38.. the Malibu is 100% stock

[WVZR-1]

Originally Posted by -=Jeff=-

4.10 has been completed with the 13/43.. at 41mph on the speedo an app on my RAZR shows 41mph..

I did notice the speedo might bounce to 40 and the RAZR app shows 41 still..

I tried my Malibu on the way to work and it was off by 2.. speedo (needle) showed 40.. App showed 38.. the Malibu is 100% stock

Malibu is what year?

I got 43+ when I did the math using "random" rolling information so I guess I did well. I suggested trying the 42 that was there first just because it was already there!

[-=Jeff=-]

Malibu is a 2007 MAXX SS.

I went with the 43 based on a later comment with the ties sizes the same as mine. If I swap to 42 what would that do to the Speedo? I can still try it. That is the easy gear to swap.

[WVZR-1]

Originally Posted by -=Jeff=-

Malibu is a 2007 MAXX SS.

I went with the 43 based on a later comment with the ties sizes the same as mine. If I swap to 42 what would that do to the Speedo? I can still try it. That is the easy gear to swap.

You seem to be on the button but a true test would be compare at lets say:

35 - 45 - 55 - 75 - 85 Your error if there were one would be a consistent percentage and would show a deficiency at higher speeds. If you're on at 41 you should be good for all practical purposes. A 42 would "increase" the "speedo" to be higher if yours were accurate presently I'd expect it to go something 1.85 to 2.0 %.

Check your Malibu at the same speeds as I suggested for the ZR-1 and compare.

I thought if you had an older Chevelle Malibu it might make an interesting conversation. An "odometer" in a cable drive can be 100% (nuts on) and the speed be way way way off! The needle in a cable drive runs off a magnetic speed-cup and as the magnets weaken the needle goes crazy. To correct that you increase the magnetism check it with a 1000 RPM source to 60MPH. If it's in error you adjust it by degaussing the magnets, go to far you magnetize again and start over. Troopers used to have this done regularly for their "CERTIFIED" speedometers or a "clocked speeding" ticket could easily be dismissed. I've seen "big tickets" tossed.

A fellow was trying to do a C3 himself and he busted up the neodymium magnets from a hard-drive to add to his speed-cup, he just kept changing the size of the pieces until he hit it. He had a drill that he could control the RPM using an optical tach.

There I ended up telling you anyway. A good read is Eddy-current mechanical speedometer.

[-=Jeff=-]

Originally Posted by WVZR-1

35 - 45 - 55 - 75 - 85 Your error if there were one would be a consistent percentage and would show a deficiency at higher speeds.

Yes this will be the plan when I get a chance..