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...

 

Carpetbagger
Snake oil salesman
Master of the obvious
Shameless promoter.

 

you forgot Capt. Obvious

 

How do you calculate revs per mile for different tire sizes?

 

Circumference of tire and divide into one mile.

 

But, he didn't ASK what the RPM's would be at a different speed. I THINK he asked whether it would be better to change tires or rear ends.

He has LARGER tires than your "formula," so.... lower tire rev speed. Given the constant of top gear ratio, he specified his rear end ratio (which was higher.) He didn't even give his RPM's at 60 mph (but you should be able to figure that out.)

Now.... I don't know CHIT about trucks, but it sounds like he has the biggest tires I'm aware of (therefore the lowest tire REV speed,) and a constant top gear ratio (which I WANT to question given the different gear combinations,) but maybe a high rear end ratio.

So.... what would be YOUR answer? A smaller tire will only increase the tire rev's and the RPM's. If they MAKE a bigger tire, would it be cheaper to buy new tires.... or change rear ends?

So, I ask the same question HE did: What rear end ratio do I need to lower my RPM's and therefore increase my fuel mileage?

Aw.... nevermind. If they don't MAKE a bigger tire, ANY reduction in rear end ratio would lower the RPM's and increase fuel economy.

I suppose you O/O's have a "preferred" setup for mostly flatland driving and one for mostly mountain driving. I'd like to know what they are.

Would your setups change if you thought you'd be hauling HEAVIER freight, or lighter freight most of the time?

 

I second that! He was obviously unsable to sleep one night and saw an infomercial on how to be a millionaire quick.

 

Since, i assume, we are talking OTR, and legal loads in USA, i wouldn't pay that much attention to terrain.
First, you suppose to know "sweet spot" (best compromise between power and economy) for the type of engine in your truck. Second, you should determine your "comfortable" speed, in which you would like, and could to stay, most of the time. Than just match RPM at that speed, to your "sweet spot",or plus, minus 25 rpm, depending on your preferences, between power and economy. Don't need any "formula"-just a common sense.
I didn't think so. Only if within above mentioned 25rpm. But that's just me....

 

I use numbers provided by tire manufacturers on their web sites. However actual measurement will be more accurate: Yokohama claims my TY517 tire spins 510 times/mi, when in reality it's more like 504 (based on my RPM at 60 MPH).

 

I spec'ed my truck for heavy loads and big hills found in NC, NY, even KY. In my case, if I wanted to run faster and sacrifice gradability I'd switch to 24.5 tires (473 revs/mi) or keep my current 295s (504 revs/mi) and swap out 3.73 rear ends for 3.58 or even 3.43. (My truck has Meritor diffs and I can only uses these specific ratios below 3.73).