How much can wheel size affect RPMs?

I went from 4:11 to 3:25 gears on my Shoebox with a 3 speed 1:1 final drive ratio trans. It still revs a little higher than I want at cruisin speed so my question is I have 15" tires that are 65 series low profile radials that I picked up until I can afford some 75 series wide whites radials. Any thoughts on what a taller tire will do to my RPMs? Think it will be a noticable difference?

Thanks for any thoughts.

 

2000RPMs x tires 28"
______________ = 51 mph.
Gears 3.25 x336

For small tires you need higher gears
I,m running 2.78 gears with 205/70/15s 2000prm =55 mph.

 

Find a tire you think you like. Measure the height and plug it into the calculation. This will give you a good idea what you will achieve. Lots of sites have formulas for this type of stuff. Tall tires do make a big difference. Good luck.

 

Here is a good chart for comparing tires sizes. The other thing to remember is bigger tires effect your Starting Line Ratio, a.k.a. "launch ratio". Basically bigger diameter tires will make it more difficult to get the car moving.

I put 33" diameter tires on my '65 C10, it's noticeable - I have to slip the clutch a bit to get it rolling.

Hope this helps!

John


Yellow = Better Gas Mileage /// Green = Near Factory Ratio /// Red = More Power


More info on starting line ratio: http://www.5speedtransmissions.com/startlineratio.html

 

Thanks for posting that chart. Very helpful.

Ray

 

For what it's worth, the chart above seems to be calculated for 65 mph.

 

Making some assumptions...let's say your current tires are 26" tall. The tires you'll be getting are 28.5" tall. That's 2.5" difference. 2.5 is about 10% of 26. So you'll be increasing tire size by 10%, so you'll decrease engine speed by 10% too.

Assuming you're running about 3000 rpm at 70 mph with the small tires, you'll drop 10% with the big tires. 10% of 3000 is 300, so you'll go down to 2700 rpm.

 

As always, this site is very helpful. Thanks for all the info and the chart. I will wait til I get my taller tires and see how that works before considering working on the rear again.

 

search online: project33.com and click on calculators=tires/trans/rear gears/etc figured in

 

Im running a 3 speed and have 3:73 gears i went to a 7.50x16 and it helped alittle but im going to change gears out sometime.