Speedo correction advice needed
Thread Starter
Le Mans Master
Joined: Mar 2002
Posts: 5,889
Likes: 2
From: Castro Valley Calif.
St. Jude Donor '03 thru '26
Speedo correction advice needed
I've gone from my stock 2.73s and 16" wheels on my '87 A4 to 3.07s and 17" wheels. Now, my speedometer is way off, it shows 90-95 or so with the flow of traffic on the freeway. That's like 20% or so I'm guessing.
I called Chris May at Superior, and one of his guys told me I'd need a sensor and the drive gear. He didn't mention anything about the driven gear on the tailshaft. Does this sound right?
I did a search and found a chart of which gears do what, but it was way beyond me........I just couldn't make heads or tails of it. Does anybody have a similar setup they can share with me, or refer me to a page with some info I might be able to understand? TIA guys!!!
I called Chris May at Superior, and one of his guys told me I'd need a sensor and the drive gear. He didn't mention anything about the driven gear on the tailshaft. Does this sound right?
I did a search and found a chart of which gears do what, but it was way beyond me........I just couldn't make heads or tails of it. Does anybody have a similar setup they can share with me, or refer me to a page with some info I might be able to understand? TIA guys!!!
Le Mans Master
Joined: Jun 2003
Posts: 7,857
Likes: 2
From: Ignore this post
St. Jude Donor '03
Re: Speedo correction advice needed (Caboboy)
Acording to the TPIS catalog the GM # for a 42 tooth gear (driven) is 25513049 for 3.07 rear end, as for your tires when I checked the specs for the original 16" tires and the diameter of my new 275/40/17's it was within a tenth of an inch so unless your tires much larger than this you should only have to adjust for the rear end
Le Mans Master
Joined: Jan 2002
Posts: 5,320
Likes: 4
From: Kyle TX (Because Mojo Made me fill it in)
Re: Speedo correction advice needed (Caboboy)
I wouldnt worrk about the difference in wheel/tire size. The difference in wheel/tire size from 255/50-16 to 275/45-17 is 2.7%. According to http://www.miata.net/garage/tirecalc.html When your speedo reads 60 mph, you are actually traveling 61.6 mph. Ideally you want to go with a 285/40-17 tire because it only gives you a .2% difference.
I think the bulk of the descrepency in your speedo reading will be from the gear change. Get that fixed and you should be good to go.
I think the bulk of the descrepency in your speedo reading will be from the gear change. Get that fixed and you should be good to go.
Drifting
Joined: Jan 2002
Posts: 1,606
Likes: 1
From: Las Cruces NM
Re: Speedo correction advice needed (Caboboy)
I swapped to 3.45 gears in my 86. I didn't want to mess with trying to get the correct vss and gears, so I went through Zip and picked up one of their recalibrators. You splice it into one of the wires on the vss, a power and a ground wire. No big deal to install. It has a series of dip switches inside that you use to calibrate your speedo. You just use their formula (nothing complicated) to get a ratio that your speedo is off using your odometer and a known distance, then look up the switch settings. Its a little expensive, but it works well.
Chris
Chris
Racer
Joined: Jul 2002
Posts: 447
Likes: 0
From: Malta Europe
Re: Speedo correction advice needed (okinawa86vette)
I just orderd one like [okinawa86vette] because me too couldn`t bother messing with the trany and gears ,here is a pic of it from the forum mall 
:steering:
:steering:
Elite Torch Red Member
Joined: Sep 1999
Posts: 57,805
Likes: 23
From: Exit 89 GSP,Lakewood,NJ The Land Of Mojo
Cental/South NJ Events Coordinator
CI 1-2-3-4-5-6-7-8-9-10-11-12 Vet
CI-II Burnout & Drag Champ
St. Jude Donor '03-'04-'05-'06-'07
Re: Speedo correction advice needed (Caboboy)
Hey Tim, is this teh chart you saw?
http://www.corvetteforum.net/c4/vett...Gearshtm.shtml
3.07
drive gear-8640517 Red 17 teeth
driven gear-25513049 Green 42 teeth
speed sensor-25007308
Is the overall diameter the same betwwen old and new?
http://www.corvetteforum.net/c4/vett...Gearshtm.shtml
3.07
drive gear-8640517 Red 17 teeth
driven gear-25513049 Green 42 teeth
speed sensor-25007308
Is the overall diameter the same betwwen old and new?
Race Director
Joined: Aug 2002
Posts: 17,367
Likes: 1
From: Kentucky
Re: Speedo correction advice needed (Mr Mojo)
I need help too. While removing my tranny my plastic speedo gear broke. I'm also changing my gears right now while everything is tore down. I'm switching from 2.59s to 3.07s. Can I get by with just the 3.07 speedo gear or do I need the speed sensor too?
Edit to show 86 auto
[Modified by hngacurv, 4:51 PM 7/14/2003]
Edit to show 86 auto
[Modified by hngacurv, 4:51 PM 7/14/2003]
Le Mans Master
Joined: Mar 2001
Posts: 9,464
Likes: 1
From: Modesto CA
Re: Speedo correction advice needed (hngacurv)
Tim, I'm pretty sure your tire size really wouldn't throw anything off. I went from stock 255/50 16's to 275/40 17's (like you) and it's like..MAAYBE 1mph off at 65mph (but with the digital speedometer, it doesn't matter anyway.)
Now, with that gear swap, THAT would throw you off.
According to http://www.smokemup.com's gear ratio finder, it shows that before, when you were at 1500rpm (in OverDrive) you were going 61mph. Now with your tire/gear change, at the same rpm in OverDrive, you're now going 53mph.
That makes absolutely no sense..but it's mathmatically correct...
Now, with that gear swap, THAT would throw you off.
According to http://www.smokemup.com's gear ratio finder, it shows that before, when you were at 1500rpm (in OverDrive) you were going 61mph. Now with your tire/gear change, at the same rpm in OverDrive, you're now going 53mph.
That makes absolutely no sense..but it's mathmatically correct...
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Elite Torch Red Member
Joined: Sep 1999
Posts: 57,805
Likes: 23
From: Exit 89 GSP,Lakewood,NJ The Land Of Mojo
Cental/South NJ Events Coordinator
CI 1-2-3-4-5-6-7-8-9-10-11-12 Vet
CI-II Burnout & Drag Champ
St. Jude Donor '03-'04-'05-'06-'07
Re: Speedo correction advice needed (hngacurv)
I need help too. While removing my tranny my plastic speedo gear broke. I'm also changing my gears right now while everything is tore down. I'm switching from 2.59s to 3.07s. Can I get by with just the 3.07 speedo gear or do I need the speed sensor too?
Team Owner
Joined: Dec 2000
Posts: 21,094
Likes: 282
From: Phila., PA burbs
Re: Speedo correction advice needed (hngacurv)
I need help too. While removing my tranny my plastic speedo gear broke. I'm also changing my gears right now while everything is tore down. I'm switching from 2.59s to 3.07s. Can I get by with just the 3.07 speedo gear or do I need the speed sensor too?
Thread Starter
Le Mans Master
Joined: Mar 2002
Posts: 5,889
Likes: 2
From: Castro Valley Calif.
St. Jude Donor '03 thru '26
Re: Speedo correction advice needed (Mr Mojo)
Frank, the one I saw seemed to be a little different...... if I recall correctly Grumpy was the guy that posted it. What are those tranny codes in the one you posted?
Tim, my rear tire size is 315/35-17, so the correction will be a bit more than yours. I ran them before the gear swap and they seemed within 3 or 4 MPH at freeway speeds. That was OK to live with, but now with the 3.07s, I'm waaay out of whack. Either that or my car got mysteriously faster :lol:
Tim, my rear tire size is 315/35-17, so the correction will be a bit more than yours. I ran them before the gear swap and they seemed within 3 or 4 MPH at freeway speeds. That was OK to live with, but now with the 3.07s, I'm waaay out of whack. Either that or my car got mysteriously faster :lol:
Thread Starter
Le Mans Master
Joined: Mar 2002
Posts: 5,889
Likes: 2
From: Castro Valley Calif.
St. Jude Donor '03 thru '26
Re: Speedo correction advice needed (TA)
Hey TA I just read your post. Those same parts now are costing about 80 bucks or so from Chris at Superior. The big hit is the sensor which is like 75 dollars. Did those two parts correct your speedo?
Team Owner
Joined: Dec 2000
Posts: 21,094
Likes: 282
From: Phila., PA burbs
Re: Speedo correction advice needed (Caboboy)
I just pulled out my receipt and it looks like either their price has gone up or the speed sensor is different on my car. The speed sensor I needed was part # 10456091 (sensor 4.D50) for $20.57 and the gear I needed was part #25513049 (gear 4.337) for $3.45. The grand total with shipping was $29.97
These two items were all I required to correct the speedo for the 2.59 to 3.07 swap.
I think I remember some mentioning that the speed sensors for some other years were considerably more money.
Hope this helps :cheers:
These two items were all I required to correct the speedo for the 2.59 to 3.07 swap.
I think I remember some mentioning that the speed sensors for some other years were considerably more money.
Hope this helps :cheers:
Thread Starter
Le Mans Master
Joined: Mar 2002
Posts: 5,889
Likes: 2
From: Castro Valley Calif.
St. Jude Donor '03 thru '26
Re: Speedo correction advice needed (TA)
TA, thanks for breaking that down for me :cheers:
I just pulled up Chris' e-mail, and the gear is the same part# for $3.55; but the sensor is part#25007308 for $75.93.......... there's the culprit!
I checked GMPartsDirect and they're in the ballpark for that sensor cost.... just my luck, huh? :lol: I'll give Chris my business since they've done so much for us over the years!
I just pulled up Chris' e-mail, and the gear is the same part# for $3.55; but the sensor is part#25007308 for $75.93.......... there's the culprit!
I checked GMPartsDirect and they're in the ballpark for that sensor cost.... just my luck, huh? :lol: I'll give Chris my business since they've done so much for us over the years!
