rpm and speed

[Zigmont M.]

can anyone tell me at what rpm is fiftyfive mph with a 3:70 rear and close ratio 4 speed trans for a midyear?

[Edward B.]

Re: rpm and speed

You need to know the tire diameter as well, but here's a simple calculator for you:

http://www.advanced-ev.com/Calculators/TireSize/index.html

Ed

[Zigmont M.]

Re: rpm and speed

Thanks! I need to get the speedometer fixed and have had the car for ten years and was always curious as to how fast I was traveling.

[Duke W.]

Re: rpm and speed

Forget tire diameter. It doesn't result in accurate calculations because of tire deflection under load.

You need loaded tire revs per mile, which you can find at The Tire Rack or manufacturers Web sites. If you can't find your manufacturer and model, just use the size for any other manufacturer/model. For any given size the revs/mile is in a narrow range based on standards promulgated by the Tire and Rim Assc.

One you have tire revs per mile, use the following relationship:

Revs @ 60 = Tire revs per mile x axle ratio (For 1:1 transmission ratio)

Then you can linearly proportion the above answer to any speed.

All you need it a two-dollar pocket calculator. I bought a couple at Walmart last month for a $1.47 each.

Duke

[Stuart F.]

Re: rpm and speed

I like to go through the portable lighted speed indicating signs that you find placed in communities where speeders are a problem, and then compare the reading with what speed is showing on my speedometer. On my Sunday morning drive route there are several, and after a number of pass throughs, I found that mine reads 4 mph slower than actual (assuming they are right).

Stu Fox