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

[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.

[-=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.

[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 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?