i know someone here has the math formula to figure this out
tire size P 225/50R15
getrag 5spd. idk what the ratios are.
3.4 v6
when crusing at about 2500 rpms what MPH should i be going? my tach is a tick or two off. my speedo says im doing 80 but it doesnt feel like 80. and im just keeping up with traffic.

I would guess you reached plaid

http://www.youtube.com/watch?v=NP6DXoNKITc

[This message has been edited by ALJR (edited 05-30-2011).]

Get a GPS that has MPH on it. I recently discovered that my very accurate speedometer starts to veer off after hitting 68 mph. It will say Im doing 71mph and incremently gets worse the faster I go. So 75mph it says Im doing 81. Try that out.

When I plug those numbers into my spreadsheet it say the speed would be about 68 mph.

So how would one adjust the speed? Different gear? If so which one? I'm going to get a chip burnt soon then maybe in a year or so do a 7730 swap.

Try this calculator. It lets you plug in your specific criteria.

http://www.gafiero.org/speedcalc.shtml

so what would you suggest i do in order to fix it? sould i try the gear off my 4spd vss? can it be adjusted in the ecm tune? i have to get a chip burnt to solve some drivability issues. eventually im going to swap to a 7730 if i can get my hands on one with the pigtails. need one from a manual car.

The ECM really has nothing to do with the speedometer in a Fiero, unlike an OBD II car. The ECM does have vehicle speed as a data input, but the ECM does not output the data to the speedo.

The best you could do gear-wise would be to pull the VSS and look at the gear. On the top surface, right out at the edge of the teeth it will be marked with the number of teeth, such as 29 T or 30 T. If you have a 29 tooth, you can go to the 30 tooth, which will slow your speedometer reading at any given rpm. If it has the 30 tooth gear, you could only change tire diameters. If you're still after the wide footprint of the 50 series tires, you could go up to a larger diameter 50 series, providing you have the clearance on the rear. Getting the diameter of the tire back near the original of course will bring your speedometer closer to being correct.

quote Originally posted by fierofool: The best you could do gear-wise would be to pull the VSS and look at the gear. On the top surface, right out at the edge of the teeth it will be marked with the number of teeth, such as 29 T or 30 T. If you have a 29 tooth, you can go to the 30 tooth, which will slow your speedometer reading at any given rpm. If it has the 30 tooth gear, you could only change tire diameters. If you're still after the wide footprint of the 50 series tires, you could go up to a larger diameter 50 series, providing you have the clearance on the rear. Getting the diameter of the tire back near the original of course will bring your speedometer closer to being correct.

???? 50 series tires the 50 is the profile of the tire itself isnt it? as the 225 is the footprint and 50 its the aspect ratio or something like that. i know what you meant just sounded like some things got switched. i like the wider tires. and the profile seems all to perfect. i guess i get to pull my vss tomorrow. oh joy.

I have changed the tire size on all my vehicles including by Bus so all my speedos read wrong. I watch the speed on my GPS and never look at the dash other than to check the rest of the gauges. Works for me.
At the least check your speed with a GPS to make sure you have a problem before trying to fix it.
Harry

[This message has been edited by HarryT (edited 06-01-2011).]

i dont like using GPS not to shoot you idea down. i usually turn off everything but my radio......i like it being just me and the car. kinda out of respect for the car. idk why i guess im just nit picky and want to get this thing right the first time.

quote Originally posted by sricka01: Get a GPS that has MPH on it. I recently discovered that my very accurate speedometer starts to veer off after hitting 68 mph. It will say Im doing 71mph and incremently gets worse the faster I go. So 75mph it says Im doing 81. Try that out.

Hate to say this, but iphones have a mph app for free.

I don't have an iphone...... But my wife does :P
We used it on the dart train and reached 76 mph hardy har har!

quote Originally posted by outlawfiero17: i dont like using GPS not to shoot you idea down. i usually turn off everything but my radio......i like it being just me and the car. kinda out of respect for the car. idk why i guess im just nit picky and want to get this thing right the first time.

Use the GPS to confirm accuracy of the speedometer.

The stock size rear tire is 215/60-R15, which has a calculated circumference of 79.0 inches. The size you listed, 225/50R15 has a circumference of 75.0 inches, about 5% smaller. This will make your speedometer read 5% higher, all else considered. That being said, if your speedometer is off the problem is likely with the driver circuitry for the speedo itself. The needle is driven by a motor that in turn is driven by some chips and other bits of electronics. There's no adjustment available.

quote Originally posted by outlawfiero17: i know someone here has the math formula to figure this out tire size P 225/50R15 getrag 5spd. idk what the ratios are. 3.4 v6 when crusing at about 2500 rpms what MPH should i be going? my tach is a tick or two off. my speedo says im doing 80 but it doesnt feel like 80. and im just keeping up with traffic.

quote Originally posted by Rick Vanderpool: When I plug those numbers into my spreadsheet it say the speed would be about 68 mph.

For 2500 RPM, I come up with 68 MPH as well (instead of the 80 MPH at 2500 which outlawfiero17 observed on his speedo and tach).

quote Originally posted by fierofool: Getting the diameter of the tire back near the original of course will bring your speedometer closer to being correct.

I understand what you're saying, but the tire change outlawfiero17 made wasn't particularly radical. His 225/50R15 tires are a little under 24" tall, but the tallest tire GM originally offered with any Fiero was only a little over 25" tall. For outlawfiero17's car to actually be going 80 MPH at the 2500 RPM he observed, he'd need a 28" tall tire!

For that reason, I suspect there is more behind this problem than just the tire change you mentioned, outlawfiero17 -- like maybe a transmission and final drive ratio change as well --- or some faulty speedo and/or tach electronics. (The engine's 3.4L displacement is irrelevant to the math underlying this particular problem.)

I went from a 4 spd to a 5spd. I used the vss that was already on the getrag. I gonna switch the gears out next chance I get.

B.t.w. wouldn't going to a 5spd increase mpg's? My mpg's suck. Granted I put a bigger motor but I expected at least to break even. Used to take 1/3 tank for a 98 mile drive. Now its a half tank. And in town milage sucks (but that's my lead foot)

quote Originally posted by outlawfiero17: I went from a 4 spd to a 5spd. I used the vss that was already on the getrag. I gonna switch the gears out next chance I get. B.t.w. wouldn't going to a 5spd increase mpg's? My mpg's suck. Granted I put a bigger motor but I expected at least to break even. Used to take 1/3 tank for a 98 mile drive. Now its a half tank. And in town milage sucks (but that's my lead foot)

Gas mileage calculations require an accurate measurement of both fuel and miles. Right now your speedo/odometer is inaccurate so it's not possible to calculate accurate mileage.

Well I know its 98.6 miles from my front door to my reserve unit. Like I said I used to do ok. A tank would get me there and back. So far looks like ill have to stop on my way home.

For reference, when I drive my 2.8 that's been bored and stroked to 3.2 liters connected to a Getrag I get in the high 20's on the highway at 75+ and in the low to mid-20's in city/stop-n-go traffic.

I never trust math or computer for this type of info. There are too many variables in a formula to make anything accurate. Best, like already mentioned is to use a GPS that gives a speed readout then drive accordingly. Myself, I used a GPS to go 65 mph and pushed the needle on the speedo at the 65 mark. Then the major error will be at less than 10mph and over 120mph which really dont make a difference anyway.