Another thought on wheels and tires

[hat_man]

I have contemplated a rear gear change and know that I will have to change the speedo gear on the end of the speedo cable to compensate. But I was also exploring the option of new tires for the future and may have to go up a size from 14's to 15's.

So here is my question...is there a "calculator" or formula here to input tire size and rear gearing to get a final drive ratio?

What I am thinking is if I swap from a 3:73 rear gear to a 3:45 I'll change the speedo gear accordingly. But if I also add 15" tires to the equation this would effectively lower my final drive ratio, so for example, could I go 3:45 rear gear and 15" tires but swap for a 3:31 speedo gear (or any other speedo gear) to get it all to even out in the end? And if swapping for an all together different speedo gear would work, how would I "do the math" to find out which one?

It's too many numbers to wrap my head around at this point, but I think you can see what I am getting at.

Thanks for the help!

[2000mc]

3.73 to 3.45 is about a 8% increase, you could get 15" tires larger or smaller than your current size. i dont know if $ wise it'll ever work out, changing gears and wheels, but if you can find a way to do it cheap enough, great. depending on other lowering options, i might look at how small of tires i would want to put on, effectively lowering the vehicle, and then compensate with even taller gears.
i had a 89camaro i swapped a 97firebird rear end and 2000 camaro wheels onto, when i was done, i just drove it on the interstate, cruise control set at a certain speed, timing between mile markers with a stop watch. calculated the % increase or decrease i needed only from speedo reading and timed mile, because my speedo was already slightly off before, and theres just less math to screw up.
know for sure there is a gear to change?

[hat_man]

Lowering is another mod that is in the works. 2"front/4"rear drop for about $20 in material. The drop in the rear gives me the perfect opportunity to swap out the entire rear end with one from a '92 Ranger. The half shafts are 1" shorter on both sides making more room for skirts and it's have a fully functioning e-brake as mine is beginning to stick. I think the 3:45's are a good compromise and any small losses I see in town should be more than negated by the 80% highway miles I drive.

[mcrews]

dont forget to re-align after lower

[CapriRacer]

Tire Size Calculator - tire & wheel plus sizing

BEWARE: Most tire calculators do NOT take into account that the rolling diameter is about 3% smaller than the measured diameter - but this one does.

[niky]

Is that around 3% regardless of pressure? I mean, say, going from 30-40 psi? I have a road speed versus rpm versus force ay wheels graphing sheet and chart generator on Excel, and I simply use calculated diameter. Didn't really think about the difference in rolling diameter when I was programming it.

[hat_man]

Thanks mcerws...I thought I'd have to have it aligned but your post will serve as a reminder when I go back and use this for planning the whole project. I copy out threads and have them in a folder on my desktop for future mods on my little truck.

[CapriRacer]

Originally Posted by niky

Is that around 3% regardless of pressure? I mean, say, going from 30-40 psi? ..........

I KNOW that the difference between the measured diameter and rolling diameter is about 3% - at normal inflation pressures and loads.

I SUSPECT the difference in rolling diameter for different loads and inflation pressures is in the fractions of a percent - which is probably not enough to worry about.

[niky]

Thanks! Time to plug it into the spreadsheets...