at 65mph my tach is reading 3000rpm. at 70 it reads 3500.I have used the calculators at differant sites. They say I have something like a 430 gear. I jacked the car up, rotated the tire, counted the revolutions of the driveshaft in comparison to the wheel,it shows that I have a 3:08. That I can believe. also at say 65mph. a lot of cars seem to be passing me, which makes me believe I have a problem with the speedo. I plan on changeing the rearend but Im not sure where to go from here. Is the speedo wrong,the tach, or both? Here are the numbers if you want to crunch them. a 350 trans. 27" tire.

You can go to www.ringpinion.com, they have an online calculator that can get you really close on rpm calculations with tire sizes, rear ratios, and transmission types. 3.08's were typical of all C3's or most of them anyway. As far as being passed with your speedo reading 65mph, the only way I know to verify the speed is by time and distance. It seems to be easiest for me to figure out. I map out a long stretch of HWY and do it late at night so you can maintain the speed without any traffic. Map 30 miles and drive it at 60 mph and you should be able to be at the finish point in 30 minutes. Maybe someone else has a better idea.

On the trip to BG I used the mile post markers on the interstate. Did several 1 mile checks and then a couple of 10 mile checks. I found it easier to maintain a set speed for the shorter distances. Discovered the odometer was off by 10 percent (1 marked mile registered 1.1 on the odometer at 60 mph indicated) so that explained why I'm always getting passed by so many folks when running at the posted limit.

But then on the way home from BG the odometer quit working altogether so now I don't know how accurate/inaccurate it really is (or was).
____________________________________

Jim O.
Save the Wave...!!!

Those RPMs are similar to mine with 3.08 gears and 235/60 tires, I don't know how you came up with 4.30:1. {edit - i got mixed up - 3000 rpm is about 75 mph}
I know my tires are a little smaller than stock, so when people pass me while I'm doing 70, it's not just fast drivers (although with a 65MPH speed limit traffic moves at about 75 most places). Joe

|UPDATED|7/30/2003 10:02:43 AM|/UPDATED|

You might want to find someone who has a GPS unit. When I bought my car the guy told me the speedometer was off and I borrowed my brothers unit. It really worked god.

After following all the good advice here, and still finding the speedometer inaccurate, it is a fairly simple matter to order a new speedometer gear (the one in the transmission)from any of the major Corvette part suppliers. They offer them in number of tooth variations
that allow you to increase or reduce your speedometer reading. Please let us all know how it turns out. Hope this helps.

c3

If all you need is a driven gear, just go to GM. They are less than $10 and you won't have to pay shipping. You do need a counter guy with some time/experience to help you. Find out how much too fast or slow, at 60 mph for instance. For example, say you are register 60 at actual 66 mph which is 10% too slow, and also that you have a 20 tooth driven gear. Go to GM and get an 18 tooth driven gear (10% of 20 is 2, you want the speedo to go faster so subtract 2 from 20 and you get 18). Good luck with it.

That's pretty close to the same readings I get in mine at the same speeds. To really get a accurate reading, find someone with a GPS and compare the speed with it.