If you want to test your true speed on the road, you could invest in a GPS. You’ll get a speed which is independant of what the engine is doing, and they are about as accurate as you can get short of driving between two measured markers with electronic timing.
Another vote for GPS. It’s fairly cheap and accurate (assuming you get a good satellite lock and keep a fairly steady speed for several seconds). Incidentally, I find that my minivan’s speedometer reads slightly low (about 2-3 mph or so). Of course, my van is about 10 years old at this point, which may have something to do with it.
My favorite way to “calibrate” my speedometer is to drive past one of those locations where the cops have put up one of those “Your speed” signs with the built-in radar gun, that displays my speed. I compare that while I hold my speed real steady, and that lets me know how far off I am.
For the record, my boring Oldsmobile sedan is 2 MPH pessimistic at 55 MPH, that is to say that 55 MPH on the speedomter will be recorded as 57 MPH, consistently, by the roadside radar devices.
Fuel, just for the heck of it, I’d suggest taking your Bitchin’ Firebird to a shop that calibrates speedometers. Last time I did it, I got a report of my calibration for only $35. [I did it to prove to my traffic court judge that my speedo at the time read 0 MPH at all speeds. Got me improper equipment instead of speeding, which I liked.]
Thanks everyone. Are mile markers usually spaced to an exact mile? I wish I had those “your speed” things around. I’m not really really concerned about this persay, i just want to settle this “oh, you weren’t really going that fast” crap once and for all. Now i know what i figured was true, that speedos are off by percentages.
I did the math, if my speedo was off by 5%, then my bro was doing around 180mph, still a little high. Maybe the speedo is ridiculously off… i’ll check to see what size tires my bro put on there.
Thanks again.