I had my father in law drive is car 60mph and I stuck right next to him.

My speedo read 68-69 mph and my RPM's were 2900-2950.

This was in 4th gear. Mostly stock 95 Z28 M6 on 245/50-16's

I was told that 60mph in 4th gear with 3:42s should be around 2600rpm.

 

Bounce those numbers against the calculator at www.f-body.org/gears/

 

Very cool. However.....would gears make my speedo off by 8-9 mph from a regular car? Like I said his stock Accord was set at 60mph and my speedo read 68-69mph.

 

Just looking at the speedo error due to a gear change, without correcting the PCM:

3.73 / 3.42 = 1.0906
1.0906 X 60 = 65.4 MPH

4.10 / 3.42 = 1.1988
1.1988 X 60 = 71.9 MPH

From those numbers, its a tossup. Have you changed your tire size from the tires that came on your car stock - 235/55-16 on the base V8 package, and 245/50-16 with the Z-rated tire option are two different tire diameters, which will also affect your speedo, if you've changed tire diameter without changing the diameter in the PCM.

Looking at the RPM numbers you quote, using my spreadsheet for the T56:

With stock tire diameter, 4th gear at 60MPH would run:
3.42's - 2,690 RPM
3.73's - 2,934 RPM
4.10's - 3,225 RPM

Looks like you may be running 3.73's.

 

I love the fact that there are so many knowledgable people on this board!

Yes, I did just put 245/50 on the back replacing my 235/55's

How much do you think that will effect the speedo? When I look at that fbody.org calculator should I compare 60mph to the RPM's because thats what HIS car was doing? Sorry about all the questions but im going to have Ion tune my PCM soon.

 

You can also:
- jack yuor car's rear
- mark both rear tires and your drive shaft
- turn your both rear tires by exactly one turn while counting how many times your drive shaft rotated.

The number of turns that your drive shaft made is your gear ratio.

Also, I've heard from many people that manufacturer often make speedometers read a little high, just so nobody can sue them for having speedometer read low and causing them to get into an accident. I've tried my wires Focus that I know for sure is stock, and it reads about 3-4 mph higher than what GPS gives me, regardless of what speed I am drivin at. So, keep that possibility in mind when comparing vehicle speeds with what speedometer shows. Just for that reason, I think that the best way to figure out what gears you have is to actually cound the ratio as explained above.

By the way, I have pretty much the same thing. I bought my camaro used and I was told that the gears were stock. Later on I found that they actually were changed from stock 3.42 to 3.73.

 

Replacing the stock 235/55-16 tires (26.18" diameter) with 245/50-16's (25.65") will also cause the speedo to read higher than actual.

26.18 / 25.65 = 1.0207

Combining the tire size error with the 3.73 gear error:

1.0207 X 1.0906 = 1.1131
1.1131 X 60 = 66.8 MPH

Seems to confirm the 3.73's.

Note also that you are adding 11.3% more miles driven to your odometer. When you drive 1,000 actual miles, the odometer will rack up 1,113 miles.... not good for resale value.

Ive checked my Formula speedo against measured miles with a stopwatch and found it to be dead on (at least when I had the stock rear, stock T56 and stock tires). I don't think manufacturers intentionally screw things up. Also verified my 2001 Silverado as being correct, and can also compare the speedo/odo in my 2003 F150 work truck to the calibrated/certified DMI unit installed for measuring the road work we do, and the F150 speedo is within 1 MPH of the certified unit.

 

I changed my speedometer today and it appears to reveal some new info. At 60mph in 4th gear my RPMs are at 2800. On the Fbody calc it looks more like the stock 3:42. Thoughts?

 

my thoughts, is your PCM set for the 17'' wheels you have on it? it came with 16'' stocks. Also, run your RPO codes (located in the glovebox, its the sticker on the back of the lid) in http://camarosource.ca they have a nifty decoder

 

??????

He has stock 16" wheels. Even if he had 17" wheels, he doesn't have to "set" the PCM, unless he has significantly changed the tire diameter..... e.g. a 245/50-16 is exactly the same diameter at a 275/40-17 and requires no correction in the PCM.

 

Speaking of "set" the PCM. If your car was previously owned you may have had your PCM tire diameter or gear ratio info altered with a hand held tuner like the crane, granatelli or hypertech. Look in your glove box for the GU? code to see what gears the car came through with. Use your calculator to determine tire diameter:

Tire width in millimeters divided by 25.4 times the decimal profile aspect multiplied by two. Add the wheel diameter and you have your overall tire diameter. Compare the solution to the same stock numbers and you will be able to determine a percentage difference and thereby know your speedometer error. For instance, I went from 245/50/16 to 255/45/17 tires; my speedometer error is now 2% which means at 100 miles per hour on the speedometer I am actually traveling 102 giving me slightly taller gearing at the drag strip. I know it seems like a lot just to find out where you are with the speedometer but the challenge can be fun once your get the gist of it all.