Hoping someone could give me some ideas on what to do next. Went out and bought a hypertech programmer for my 1998 Camaro Z28 with 350 V8 as my speedo was out by as much as 15-20 Kph. When the speedo is showing 120 Kph my GPS says 110 Kph +/-. Measured the tires to a height od 24.75 Inches high and put it in the programmer but made no difference. Tires are 235/55 R16. This one is a 6 speed standard as well. Programmed it with an extra 1.25 inches to the tire size to see if it made any difference and it jumped to being out close to 20 Kph! Doesn't make a lot of sense to me as the stock size on this car is 245/50 R16. As far as I know the grearing in the rearend is stock and has not been touched. The speedometer needle seems to be stiiting down as low as it can go when the vehicle is turned off, there is no pin for it to rest on so it almost looks like it is sitting on the console.


Any help would be much appreciated.

 

Does the scan tool MPH reading match the Gage?

All the computer knows is how fast the output shaft is going. If you tell the computer you have bigger tires it thinks you are going faster. Also don't think in Kph think in percentages. If the speedo is off 5% too fast then reduce you tire size 5%

 

Would not the radius be diameter be reduced by 1.3%?
MPH = distance covered (tire circumference) by time.
Unless the programmer formula takes this into account, forget I asked.

 

has your gauge cluster ever been apart? i am wondering if someone took the speedo arrow/pointer off and didnt get it back on right.

 

The car is actually my brother in laws. He bought it from a buddy. I don't think it was ever apart but I will take a pic and post later today. I thought about changing the gear ratio in the programmer, but as far as we know the rear end has never been touched, and was afraid if we did we might do damage.

 

Just to be clear. If you made the computer think it had a larger tire it will make the speedometer read that the car is going faster. Bigger tire means a faster car, You went the wrong way. Try -1.25 to start. If your 120Kph actual 110Kph is accurate you have a 9% error. 24.75" X 9% You should end up around 22.5" but I would sneak up on it.

 

tire a couple inches smaller than stock would cause speedo to show faster than actual speed. stock tire is 26 inches, 235/55 R16 shows as 26.1 inch height 245/50 16 shows 25.6 so you are bigger by .5. i believe measurements are taken with tire off car, what air pressure you running.

 

Not sure what the tire pressure is set at, I will have to check. We measured from the ground to the top of the tire, same way I did it with my dodge ram and my superchip programmer - and it was dead on doing it that way. Is this one that finiky? I tried changing it from 20 up to 26 inches on the programmer and really saw minimal difference. Is it possible the speed sensor is screwed?

 

Smaller tire would show faster? I think that is backwards. If you program in a smaller tire then the computer assumes less distance is traveled everything time the tire rotates. You are think about if you put a smaller tire and don't change the computer? All the computer knows is how fast the output shaft is turning. So if you do not change the tire and tell the computer you put a smaller tire on the car it will calculate a slower speed. If you don't change tire and tell the computer you put on a taller tire it calculates you are going faster. He added 1.25" to his tire his car is showing him going faster.


Measure tire diameter left to right. If you cut the tire and laid the tread flat that is the distance you are going to get every revolution regardless of air level. Computer will use Pie*D to get the circumference. Since the radial is steal belts the length of tread does not change with air pressure. (think tank tread). If the distance around the tire is 10 feet every rotation make you move ten feet. So if you put a heavy chalk mark on the tire then rolled the car forward in a straight line you could measure the distance from one chalk make on the road to the next and divide that by 3.1416 and get the most accurate tire diameter.

 

That would make sense, when I had my ram I went up in tire size with my lift causing the speedo to show slower than my actual speed. So if this car came with the 235/55 from factory and the previous owner changed to 245/50 than going smaller tire than stock should have the opposite effect that my Ram had - Speedo showing more than actual. This is without any change in the programming.