I get a call about once a day that goes like this:

       Caller: "I wanna put some bigger tires on my rig and I need some lower gears;
       what ratio should I use”

       JE: "What size tires are you running and what is your current gear ratio?”

       Caller: “I have 37” tires and 3.73 gears”

       JE: "4.88’s will work fine, but I would like some more info like what kind of
       engine you have and how is the rig driven, street, trail or competition?"

       Caller: “I know a guy running 44’s with 4:11 gears, aren’t 4:88 gonna make me
       over rev my engine?

At this point I pull my hair out and try to explain that the only reason he’s running 4.11s is because that’s what he had, and things could be better. Then I do some math and prove what gear ratio should be used, but for some reason people still want to use those 4.11’s - so with let’s do some math and see where it leads us.

We start with a theory that says for the speedometer to be correct, the mathematical ratio of the old tire size to the old gear ratio must be the same as the new tire size to the new gear ratio. If these ratios are Identical then we will have the equivalent to factory gearing with the new tires and viola - the speedometer will be right on.

Also, let’s assume the rig had a 29” tall tires from the factory.

So: old gear ratio/ old tire size = new gear ratio / new tire size

Now we know 3 out of 4 of these variables and you will just have to trust me that rearranging the formula as follows will give you the correct answer.

New gear ratio = (old gear ratio/ old tire size) x new tire size

Plugging in our numbers:

New gear ratio = (3.73 / 29) x 37 = 4.76

Now since there are no 4.76 ratios available the next closest thing is 4.88’s.

So now we have a mathematical solution, but is it best for the customer? That depends on more factors. If you are looking for best gas mileage then I would go for 4.56 gears, this will lower the rpms a bit but still have fair power, and if you are looking for performance then go with the next ratio up which are 5.13’s or even 5.38. Now about then my caller says “5.38’s! I have to drive this thing down the highway, won’t I over rev the engine?” So now we need to know how high the engine will rev. Well looking a most small blocks and foreign built 4x4s you will see a factory red line (if they have a built in tach) of 6000 rpms . (For stock big blocks figure 5000 rpms for safety) Now you will just have to believe me that the following rpms are true.

And if you want to know others please follow the link below, and remember anything they sell there I can meet or beat their prices!

Gear Ratio Calculators



And here is a chart of rpms for gear ratios with a few different tire sizes tire at 60 miles per hour.