Axle Ratio, new truck

dodge guy wrote:
What people fail to understand is that with a 10 speed trans the trans is rarely in 8, 9 or 10th gear even on the highway. So with 3.73's you will be running in 7th, maybe 8 on flat ground with no head wind. With the 4.30 you will use at least 9th. So the lower gearing (numerically higher) you will be better off especially when towing.

As far as mileage. Towing mileage will increase with the 4.30'
S but may be .5 lower on the highway non towing as compared to 3.73's.

When towing heavy always go with the lower gearing. You and he truck will be happier.

The numerical gear of the transmission is largely irrelevant with the new transmissions, with a couple exceptions.

The overall gear ratio from crank shaft to wheel is what counts. Looking up the gear ratios:
- the 4.30 in 8th gear has an overall ratio of 3.66
- the 3.73 in 7th gear has an overall ratio of 3.76

That's within 3% in terms of RPM to maintain the same speed (around 60rpm if in the vicinity of 2000rpm in the gas engine...45RPM if running a diesel at around 1500rpm). So you run in 7th rather than 8th while towing but the engine doesn't know the difference (at least it's close enough not to matter)

Towing mileage isn't going to change as the engine will hold at roughly the same near ideal RPM for the engine.

Non-towing...9th and 10th with the 3.73 is a higher gear (lower numerically) compared to 10th with the 4.30. So when it does get up into those top gears running empty, the higher gears improve efficiency by allowing the engine to keep in the ideal RPM range at higher speeds.

What you describe was true of the older 3-4 speed transmissions where there were big gaps in overall gear ratio between the limited number of transmission gears. It was difficult to get an axle ratio that kept the engine in the ideal RPM range both towing and empty.