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Calculating mph using rpm and gear in an 82 NA 280zx

1287 Views 11 Replies 10 Participants Last post by  brazilgoal
Is there a caculation formula for estimating the travel speed knowing the rpm in fifth gear. I was at 4500 rpm in fifth gear on a flat stretch of road. The speedometer had already topped out at 85 mph. Anyway to get an mph estimate? Driving a 1982 280ZX NA 2 seater. Wishing I had a Garmin!
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Calculate it by how many seconds you pass an object.

do a control as an example you pass object X at 1 mile going 60mph in x.x seconds to figure the formula on distance covered X Time =speed.

Have a friend stand at Object at your mile with a stop watch and when you get to your top speed starting the mile "point A" you keep it constant speed and when you pass over the mile "Point B" is when you hit the stop on the stop watch.

If your time is doubled from lets say from 60mph X 1mile= 1 minute - 1 mile X .5 minutes=120mph

then you know you know how fast you went.

That is the long mathamatical nerdy way of it.
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In order to calculate the speed from the RPM, you need to know three things about your car. The gear ratios of the transmission gears, the differential ratio and the circumference of the rear tires in feet.

RPM X tire circ. X 60 / Trans. gear ratio X Diff. ratio X 5280 = Speed in MPH
just pick a couple of points and note speed rpm. Graph it. It is pretty much a straight line progression unless your clutch is slipping
86 mph +. ::)
88mph Marty and we'll be back to the future!
My Garmin Street Pilot has a speedometer feature in it. Great Scott!
JuanitoZcarito Nice site, good for any car if you know the numbers.
D=RT Distance equals rate x time. On the freeway, tell me you were not on some back road, they have the miles marked off. Have someone time it between markers, then it's simple math. Rate = Distance divided by time.

Orrrr...how fast are you going at 2000 rpm...2500 rpm...3000 rpm...get the hang of it? Now all it is, is a simple algebraic graph of a slope. "When will I use this?" Makes you wish you'd paid attention in algebra 1, doesn't it?
Enjoyed the reading the responses. JuanitoZcarito gave a great resource for charting mph and rpm. Thanks for the help guys. When I am traveling that fast I do not pay much attention to road markers to mark the distance. Yes, 115 mph it was.
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