I just bought an 89 Firebird Formula. When I was driving around everyone seemed to be going under the speed limit. So I had my dad drive around and we compared speeds. We found out that my speedo is reading 6 MPH slower than what I am really going. I am 99% sure they didn't change the rear gears and it has the stock rims with the stock size tire. The car has 181,000 miles on it with the original engine and tranny. My dad said that the tranny might be to blame so I need your help here. Thanks.

 

There were a few different ratio's available for your car, all dependent on engine and trans combo. List your engine and trans so we can determine your rear gear ratio. Some ratios are hard to get the speedo exact. Some ratios from the factory were off a small amount stock.

Just because you have the stock size tires doesn't mean they are the true stock size. A 245/50r16 tire made by 3 different companies can be 3 different measured sizes. Are they the stock brand and model tire?

How do you know your dad's car is truly accurate? Not all new cars are exact either.

You stated it was off 6 MPH... Was this measured at 30 MPH, 60 MPH, 75 MPH? Makes a big difference.

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1985 Z28, Jasper Class 1 350, Edelbrock Performer intake, Dual snorkel air cleaner w/K&N, SLP 1 3/4, Catco cat, Hooker cat-back, built 700r4 w/ Darrell Young 2400 stall TC, Jet stage 2 chip, SSM subframes, Lakewood LCA's, Powertrax locker w/stock 3.23's, Centerline Auto Drag's

 

I have a 305 TBI with an auto tranny. My dad has a 92 RS and it might be a bit off but I don't think by that much. We checked the speed at 40 and 50 MPH. Also I have 215/65/15 Firestone Firehawks. It says on the car door that it came from the fact. with 215/65/15. I am replaceing the tires next week with the same size Kuhmo's. Thanks

I forgot to add that my dads RS has the same engine and tranny combo as mine.

[This message has been edited by Tang (edited August 25, 2002).]

 

TTT

 

What speedo does it have? TBI formulas are sposed to have 120 MPH speedometers, but mine has a 140. I figure the guy just switched the speedo out to lower the odometer reading (i know the odometer is definately wrong), and he didnt recalibrate it. its off about 7 MPH at 60 MPH.

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1988 Pontiac Firebird Formula
TBI 305 A4 (boy do I want a manual..)
It ain't too fast, but it makes for a nice first car

 

It has a 120MPH speedo on it. I know the last 2 owners of the car and I have all the service records for it. It does have the original speedo in it with the orignal engine and tranny. I was talking to some of my coworkers about the speedo and they said that the speedo head might be screwed up. Thanks