MPH Calculator

[Gutling]

Hi, was wanting a little help with something. I have tried searching, but not knowing exactly what to search for has brought up a lot of offtopic/useless information.

I am trying to figure out mph based on rpm/tiresize/rearend/gearratios

I found a bunch of calculators on the internet, but I do not know what values to input.

I have a 89 240sx, open diff, with a sr20det redtop 5speed

my wheels are 245/45/18

I am wanting to do this to see if my speedo is off any and curiosity at what the car does after 110mph

Sorry if this is the third one of these today or anything, but I have been trying to search and appreciate any help!

[Silverbullet]

Go on a dyno. Numbers on paper are always rounded. The more rounded numbers you use, the less accurate your result will be. Things like tirewear could contribute to a slight speed inaccuracy (may or may not, just an example). To know what it actually is, get it physically tested on a dyno.

[cdlong]

the rear end is 4.083, use google to find the SR gear ratios, and use tire rack's spec sheets to find the revs per mile of your tire size. plug that into a calculator or make your own with excel and graph it.

www.xatracing.com/nissanrtSR20DET.html
1st 3.321 2nd 1.902 3rd 1.308 4th 1.000 5th 0.759

[cdlong]

Go on a dyno. Numbers on paper are always rounded. The more rounded numbers you use, the less accurate your result will be. Things like tirewear could contribute to a slight speed inaccuracy (may or may not, just an example). To know what it actually is, get it physically tested on a dyno.

that seems overly elaborate. he isn't looking for speeds carried out to two decimal places, just seeing if his speedo is off.

i guess if that was the point, i can tell you right now. yes, it is, every speedo reads a bit high. but also your tires are 9.26% larger than stock. so in fact it reads low. to maintain the stock diameter you'd need a 245/35. or a 235/35 would be closer yet.