I was listening to Kevin Rutherford tonight and there's always people calling to ask what rear ends they should have on a truck. Tonight a guy called in saing his rears were 3.90 and even with low profile 24.5 tires his RPM at cruise was high. He wasn't sure whether to swap the tires and change the rear ends, so that he was able to drive at 60-62 MPH while maintaining good fuel economy.
Kevin gave him a vague reply and then the caller asked him what the formula was that allowed to choose the rear end ratio for a given engine speed! And Kevin said ... he didn't remember! He said there was a formula where you have to multiply axle ratio by top gear ratio by tires' revs per mile and something else and then divide by something
and that he wrote this formula down somewhere but he can't remember exactly what it was.
So, my question is: if he doesn't know this very simple basic formula, how can he justify ADVISING callers to his program on axle ratios? Mostly he just says, "Mm... probably 3:55s or "Mmm ... I think 3.73s..."?!
I'm not a host of a radio trucking show and I'm not a mechanic, but here's the SUPER SECRET FORMULA that allows to choose the ratios.
YOUR RPM AT 60 MPH IS ALWAYS EQUAL TO:
TIRES REVS X TOP GEAR RATIO X REAR AXLE RATIO.
So, let's say you have 22.5 tires that spin 495 times per mile, your top gear ratio is the typical 0.73, and you have 3.73 rears. Then at 60 MPH your engine speed will be:
495 x 0.73 x 3.73 = 1,347 RPM
To get RPM for a different road speed, all you have to do is
DIVIDE THE RPM ABOVE BY 60 THEN MULTIPLY BY NEW ROAD SPEED.
RPM @ 60 MPH / 60 x MPH = NEW ENGINE SPEED
Example: your engine speed at 60 is 1,347 but you want to know what your engine speed will be at 58 MPH. Piece of cake:
1347 / 60 x 58 = 1,302 RPM
It surprises me that someone who calls himself an "expert on trucking" doesn't know such simple things.
PS: Actually, I wanted to call in myself and ask his advice on what else I can do to the truck to improve my fuel economy. I guess I won't be calling now...