Axle Ratios

[SwiftRunner]

Having two vehicles to use, I have been jumping from a coupe ('89 Riv) to my '94 4-Runner lately. I find the difference in low end driving remarkable. Consequently, I have been doing web searches on this topic. Of course my current experience would be comparing front wheel drive to rear, but since I once was a valet parking attendant in the biggest hotel in Boston, salesman for Jeeps and worked as a driver at an ADESA auction, I have quite a bit of experience driving different vehicles. The 4-runners rear end action stands out. I found that the '09 Jeep Grand Cherokee Rocky Mountain has an 3.07 Axle Ratio while my vehicle has a 4.875. What advantages does that give me?

 

[CJ3Flyr]

Generally speaking the higher the number (referred to as lower gearing) the better the low-speed driving. It'll accelerate better, pull harder etc. While at cruise speed the RPM will be higher and the MPG may be lower.

The lower (numerically) axle ratio tends to be better for highway cruising and high speed, light weight, flatland driving and may give better MPG as the engine turns slower at cruise speed.

Many other factors go into axle ratio selection: 1) The rpm range/torque curve of the engine. What's that engines "sweet spot" on the tach? 2) The ratios within the transmission; example: A Jeep may have a 1st gear with a reduction of 4.2/1 and a 3.07/1 axle. That means in 1st gear, hi-range you have a reduction of about 12.9/1 or the engine will spin 12.9 times for each rotation of the rear wheel. The Toyota may do it the other way, a 1st gear with a 3/1 reduction and the axle ratio may be 4.3/1. The total reduction in 1st is still 12.9/1.

To compare axle ratios without noting tire diameter and the ratios within the transmission (particularly 1st and OD) is of little value IMHO. What you read about here is people going to a 4.88 from a 4.1 or 4.3 to compensate for large tires. The goal being getting the engine rpm back into the power range wihile at normal cruise speed.

Hot Rodders will ditch the 3.07 or 3.55 in favor of a 4.1 to improve acceleration. The price they pay is high rpm during highway cruise.
:cheers:

 

[SwiftRunner]

Thanks for the reply. Sticking to the 4-runner, I bought mine for local use. However, that doesn't exclude a long trip on the hiway. Wouldn't keeping your speed down effectively achieve the same results as lowering your axle ratio? Although the Riv remains a great hiway vehicle yielding 25+ m.p.g. at speed limits and a good snow car (front wheel drive, low center of gravity, wide wheel base), I want to garage it in the winter as it has so little body rust, it will become a show car in a few years. When I worked for AMC-Jeep so many years ago in their biggest dealership in New England (Boston), we were told at hiring that something like 13% of the CJ/Wrangler owners actually took their vehicles off road at any time during their period of ownership. The Cherokee's had an even poorer record. Consequently I see this >>Rocky Mtn.<< Grand Cherokee as being designed for Rt. 70 and other hiways like it.