Turbo Buick Forums banner
1 - 12 of 12 Posts

·
Registered
Joined
·
1,752 Posts
Discussion Starter · #1 ·
I just went from 275/50-15 Nittos (25.6") to 275/60-15 BFGs (28"). Anyone know what percent of speedometer error I now have? I used to have a web site bookmarked that computed this percent, but I must have deleted it. Thanks.

------------------

Jim Anson, 87 GN with: TA-49/PTE .63; 50# injectors; 62mm Hemco; 3" DP; Walbro 340; CAS V-4; 3" MAF Translator with 21* Extender chip; Nittos; and SMC alcohol system. Stock long block, transmission, and converter. Best run of [email protected] with JL 110 chip, 110 octane and 23# boost.
 

·
Premium Member
Joined
·
43,735 Posts
Somebody answer this!
I was just fixing to calculate it for myself but I would rather someone else have a formula to do it so I don't have to think!
 

·
Registered
Joined
·
507 Posts
I've never done this, but a friend read it somewhere and it sounded good to me:

1. Go to a highway with markers.
2. Record the odo at a marker.
3. Drive a good distance, the longer the drive, the more chance if accuracy.
4. Record the odo and compare to marker distance.
Ex. If you went 10.0 miles and the odo reads 9.0, then it is 10% off. (I would do more than 10). Then get a new speedo gear that has 10% fewer teeth than the current one. (OR is it 10% more teeth??) I have never touched my speedo cable on this car, but I assume it has a removable gear like my previous car.
Also, somewhere on gnttype.org, there is a reference to rolling distance per revolution for these tires. You can estimate the % change here too.
 

·
Registered
Joined
·
743 Posts
(28*pi-25.6*pi)*(100%)/25.6*pi

approx. 9.4%



[This message has been edited by shaunchrisc (edited February 01, 2002).]
 

·
Fuhgeddaboudit
Joined
·
18,613 Posts

·
Premium Member
Joined
·
1,652 Posts
I get 8.57%

with the 25.6" tire and 3.42, you would be doing approx 2760RPMs at 60mph and with the 28", 2523. You now have the equivalent of a 3.12:1 final drive.

------------------

Scottie's GNZ

71 240Z/GN Power/Corvette IRS
 

·
Registered
Joined
·
1,752 Posts
Discussion Starter · #7 ·
Thanks everyone...I really appreciate all the info. Now I need to go to the track next week and see if the taller tire gets me better ET/mph.

------------------

Jim Anson, 87 GN with: TA-49/PTE .63; 50# injectors; 62mm Hemco; 3" DP; Walbro 340; CAS V-4; 3" MAF Translator with 21* Extender chip; Nittos; and SMC alcohol system. Stock long block, transmission, and converter. Best run of [email protected] with JL 110 chip, 110 octane and 23# boost.
 

·
Registered
Joined
·
1,752 Posts
Discussion Starter · #9 ·
Thanks Steve.

------------------

Jim: 87 GN with: TA-49/PTE .63 housing; stock motor, transmission and converter; MSD 50#; CAS V-4; 3" DP; ATR 2.5" SS exhaust; 62 mm Hemco; Translator w/3" MAF; Bigmouth air system; SMC alcohol; Walbro 340; BFG DRs; and Extender 100 chip with 21* timing. 11.961 @115.19 w/93 octane + alcohol.
 

·
Registered
Joined
·
1,752 Posts
Discussion Starter · #11 ·
Thanks toady. That is a real good chart.
 

·
Registered
Joined
·
244 Posts
You're welcome, GNCOL. That's the best tire size app I've seen.
 
1 - 12 of 12 Posts
Top