Speedometer Correction

[Steel Half-Shafts]

A friend of mine just recently installed 3:70 rear end gears in his 1987 Corvette with an automatic transmission. Needless to say, his speedometer is now incorrect.
What can he do to correct the problem? ;help

 

[Ken]

I've got a chart from Richmond for the manual trans cars Gordy, but I can't help you with an automatic. Sorry.

_ken

 

[Mad-Mic]

gotta get this......

where the speedo cable goes in you got to change the speedo gear to compensate for it. any tranny shop can help. it is fairly inexpensive. they can even compensate for tire size.

 

[vms4evr]

send me an email and i'll reply with the speedo gear cross reference chart. it shows all gears for auto and manual trans for all years of C4s.
when i bumped the 2.59 to 3.54 gears in my92 i went through this. it was a PITA. if you search for my gear post you'll see the ones i used. you need a slight adjustment to that,
Graham

 

[vms4evr]

oops i messed that up. with the email in the forum you can't send an enclosure. use it to give me your real email address or post it here. i have the info in a Word doc in tables. so i need to send it that way. it won't post properly in here as a response.
graham

 

[Steel Half-Shafts]

Thanks Guys,
I'll let my friend know what he needs to do to correct the problem.
vms4evr you have mail!

 

[Eagle85C4]

How much hassle is it when you change from 16" to 17" rims?

 

[Mad-Mic]

not much....

my 87 came with 17's but i dont' think they changed the gearing but it's not noticable as when i'm driving seems like it's very close or right on. Remember it's not the size of the rim but the diameter of the overall tire size that changes it. Anyone know what the difference is from 255.50.16's to 245.40.17's? i think thats the sizes that is on mine from stock.

vms can you send me that reference chart?

madmic@cablespeed.com

Thanks!

 

[vms4evr]

madmic,
your 255/50-16 are 26.1" dia.
a 245/45-17 are also 26.1" dia.
so if that is what you swapped then it's dead even.
the 275/40-17 on my92 are 25.7" dia.

with an auto trans at 65mph using 25.7 and 26.1 in tires you get 2167 and 2200rpms respectively with 1.0 final drive. so it would be hardly noticeable. in OD it's about 25rpms difference.

the doc is on the way, check your email
same for you steel half-shafts. just sent it.

Graham