Speedometer correction (switching to 4.10s)

[-=Jeff=-]

I know there are different ways to do this. I am curious to see what everyone else has done and if they have had any issues with their method?

Some use an electronic box to accomplish it while other do the Drive/Driven gear swap in in the trans.

I know the drive/driven set from Marc is listed as a 1% error which is fine, but just curious what everyone else has done

[WVZR-1]

A 4.10 with a 315/17 - tire/wheel there's no doubt that Marc's drive/driven should the "CHOICE"!

When 18" and 19" wheel combinations are applied, the "correction" plans need to be completely "rethought"! Rolling radius/circumference for tire/wheel combination needs to be confirmed, advertised diameters DO NOT work!

I'd think there's NO substitute for a mechanical drive/driven.

Using either Marc's drive (13) or the OE drive (15) a satisfactory driven shouldn't be an issue. The VSS is good for all tooth counts 40+!

[-=Jeff=-]

Well I do have 18s too come to think of it

rears are 335/30R18 Pilot Sports.. on 18x12 wheels

[DaveK]

I initially bought one of the electronic boxes (from Corvette Central) but returned it as I didn't think it was a good solution for several reasons.

First. It needed mounting to the car somewhere, but there were no mounts, just a plastic box and I think that cable ties on their own would be a good long term solution.

Second. You had to cut the wiring to attach the box. This could have been avoided by hunting down the right connections and making it plug and play but again a strike against it.

Third. The wires entering/leaving went through a couple of holes in the box that had no seals/grommets on so being mounted under the car this seemed a perfect way of allowing water/dirt into the electronics. Which clinched it for me.

I went with Marc's gear and have been perfectly happy with it.

Just my experience with that one box. There may be others out there that are better designed and made.

Cheers

Dave

[scottfab]

Not a very satisfactory solution but a solution would be to just use a GPS.
Most read out in MPH so there you go.
It's kinda like other systems that some folks just rip out rather than fix. (tire sensors, secondaries, air bag, anti lock brakes, etc, etc )
You don't really need it.
Personally I fix what breaks and would go with the gear as the most elegant fix.

[WVZR-1]

Originally Posted by -=Jeff=-

Well I do have 18s too come to think of it

rears are 335/30R18 Pilot Sports.. on 18x12 wheels

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.

[WVZR-1]

Originally Posted by scottfab

Not a very satisfactory solution but a solution would be to just use a GPS.
Most read out in MPH so there you go.
It's kinda like other systems that some folks just rip out rather than fix. (tire sensors, secondaries, air bag, anti lock brakes, etc, etc )
You don't really need it.
Personally I fix what breaks and would go with the gear as the most elegant fix.

The speedometer/odometer solution needs to be thought out "long before" the GPS becomes even relative! A mechanical correction is most easily accomplished with the differential removed allowing easier access to the drive if it needs replaced. The GPS will confirm if the drive/driven was a good choice for starters and in this particular application what needs changed regarding a driven after a drive with a GPS. GPS is only a confirmation!

I assumed the 4.10's aren't done yet - you're suggesting driving it for comparison for an install that's not completed?

[mike100]

What does the (wheel) revs per mile value in the ecm control? Can't you just adjust that value by the appropriate percentage? Does it affect the odo and speedometer?


I think it is listed as a road speed constant at 4006 revs per mile. Is this only for the ABS function?

[XfireZ51]

Having just gone through this, I am pretty certain that the speedo gears are for 4.10s. With the 3.45 rear gear and 19" Z06 wheels, my speedo error was 10mph slow at indicated 70mph. Now with the 3.73 gears installed, the speedo error is 5mph at indicated 70mph. In my previous ZR, I had the same 19" Z06 wheels but with 3.45 gear and correct speedo gear. The error was ~3-4mph at an indicated 70+mph which I thought was acceptable.

[WVZR-1]

Originally Posted by XfireZ51

Having just gone through this, I am pretty certain that the speedo gears are for 4.10s. With the 3.45 rear gear and 19" Z06 wheels, my speedo error was 10mph slow at indicated 70mph. Now with the 3.73 gears installed, the speedo error is 5mph at indicated 70mph. In my previous ZR, I had the same 19" Z06 wheels but with 3.45 gear and correct speedo gear. The error was ~3-4mph at an indicated 70+mph which I thought was acceptable.

Jeff's desire here is to have a correct odometer/speedometer after a known ratio change with a specific tire/wheel combination I'm assuming.

Your "conversation" as I recall "danced around" axle ratio changes to arrive at a performance level and "maybe" correct while at it the odo/speedo for a drive/driven you assumed was in the car with a 19" wheel. The wheel has no effect on odo/speedo it's tire size.

Jeff's is easily addressed before hand! There's no similarity between your intent, final install and Jeff's pending install and desire to have a reasonably accurate odo/speedo. An install with Marc's drive (13) and a GPS drive after will confirm odo/speedo error and if further correction with a driven is required it's a piece of cake!