I've had a magazine before tell me how to calculate the differences in RPM and speed based on tire size, and now that I'm running larger tires, I was wondering what my actual speed is versus my indicated speed on the speedometer. Anyone know the calculation? Also, is there a standard gear ratio on a 1997 4runner? Also, how can I find out my gear ratios too? I'm pretty sure all those items are needed, right?

Chris

 

Very simply...

MPH = (((RPM*(1/(GEAR RATIO))*(TIRE CIRCUMFERENCE))*60)/63360)/4

 

Thanks man! How do I calculate gear ratio? Is this the final drive ratio of the tranny or my rear end ratio, or some combination of both?

Chris

 

Just get the correct ratio from ... http://www.top4runners.com/ja/runhist/1998specs.html

example:

Ratios 1st.........2.804

2nd.........1.531

3rd.........1.000

4th.........0.705

That number is the gear ratio. The formula is not from anywhere though, I just thought it up But I'm pretty sure it works.

 

https://www.yotatech.com/attachment....&postid=108827

The picture above is for the most common upgrade to 285's, here is the link below to do it for yourself:

http://www.csgnetwork.com/lttrucktireinfocalc.html

 

Thanks everyone for the help!

Chris

 

https://www.yotatech.com/~corey/portal/portal.htm
scroll down to tire and gear calculator. It doesn't list the speed changes to the speedometer as well but it does give you gearing info.

 

If you do not have the factory locker and you have an auto you most likely have 4.1:1 diff gears like me.

If you are going to the 285/75R16 tires and want to regear to restore like stock gears then 4.56:1 gears will get you the closest. That will get you 100 RPM more then stock at like 60-65 MPH.

I can't remember off the top of my head right now, but I think the next size down is like 200 RPM under stock.

I am still on the fence about regearing, but if I do, I am going 4.56.

If you are running the same tires as I am your speedo is going to run about 10% low with stock gears. You can hook up with me and I will use a RADAR set and you can make a correction card up for your dash to know you true speed, or you can get one of those speed correction gadget. www.frankenTaco.com has one on his site.

Gadget

www.GadgetOnline.com

 

Thanks guys,

I have an auto with the rear e-locker, so anyone know that rear end ratio?

Chris

 

For simply figuring the difference between tire sizes (no other changes) go HERE .
This Link may be of some help in determining your ratio.

 

Thanks everyone for the help! That's exactly the information I was looking for.

Chris