Understanding Gear Ratio

[JD522]

My 2015 Ram 1500 has 3.92 gear ratio (window sticker). It came with 275/60/R10 tires, which are effectively 33" tall. I want to lift the truck and put on larger tires. I looked at 'effective gear ratio' charts and the calculator on tiresize.com. Looks like my effective ratios would be:
35" tires -> 3.7
37" tires -> 3.5

I see a 2020 Tradesman 2500 Power Wagon at a local dealer. It comes with 4.1 gears. But, it only comes with 245/70/R17 tires. So, effective gear ratios would be:
35" tires -> 3.57
37" tires -> 3.38

At first glance, this made me think the newer 2500 actually has a worse gear ratio and power. But, I think "effective" ratio was just confusing me. I think the "effective" ratio is really just telling me what the equivalent gear ratio change would be had tire size not changed - to produce the SAME rpms (and therefore power). My truck should be somewhere around 2588 rpms at 65 mph. Me putting on 35" tires would drop my rpms the same amount as keeping my current tires and dropping my gears down to 3.7 gears. But, the Tradesman with 4.10 gears and 30" tires is around 2992 RPMs at 65 mph. With 35" tires, it is still around 2565 RPMs, not much below my truck with 33" tires. So, the 4.10s will always have higher RPMs and more power than 3.92s for a given size tire and speed. Do I understand this correctly now?

Also, I guess towing capability diminishes quickly as you increase tires, particularly if you go bigger than 37".

 

[hunterdan]

You're rpms seem a bit off. A 3.92 truck with the 8 speed would be cruising at around 1750 rpm at 65mph in 8th gear with 33s. With 35s, rpm would drop to 1650. Not a huge drop, but you'd likely be cruising in 7th gear more often due to the extra weight and drag, bringing rpm up to 2100 rpm. Not terrible. The big benefit to the 8 speed is amount of gearing changes. 7th gear is still an overdrive, 6th gear is direct 1:1 drive. Now, a power wagon does t have 30" tall tires, they have 33s as well, 285/70/17. So, they actually have a lower gear ratio than a 1500 with 3.92s. Now, because the 8hp75 in the 5th gens uses different ratios, the power wagon with 33s and 4.10s actually cruises at 1750 rpm at 65 in 8th gear.
8hp70 8th gear =.67:1
8hp75 8th gear =.64:1
The first gear is where the differences are going to be noticeable.
8hp70= 4.71:1
8hp75= 5.0:1
So, let's see what the rpms are at for say 15 mph for the 4th gen 1500 with 3.92 and 5th gen power wagon 4.10
4th gen = 2800 rpm @15 mph
Power wagon = 3100 rpm.

Basically, because the 8 speed has such wide gear spread, gearing changes are quite as necessary. They do help, but changing tire sizes isn't as drastic as it would be on a 4, 5 or 6 speed.
Here's a website that you can plug in tire size, gear ratios, and transmission ratios to play around with numbers.
http://www.csgnetwork.com/multirpmcalc.html
And here's a list of transmission ratios for the 8hp70 and 8hp75.

 

[hunterdan]

Effective ratio would be a comparison to your oem setup. So, if you have 33s and 3.92s, it would be comparable to having the same truck with 35s and 3.73s. It's not a number I would worry about.

 

[Travelin Ram]

Your understanding is correct.