Help me understand my gear ratio please

I have a 2.7 2015 manual. Supposed to have 4.10s, yes? The door says it is 3:58, but the are notoriously wrong I hear.

Here is the problem, with 245/75r16 at 65 mph in 5th gear I am running roughly 2350 RPMs. Shouldn't it be closer to 2900 with 4.10s - according to tire calc.

Help this poor fool please.

Sean

 

That chart doesn't factor in the actual gears of your diff or your transmission. This link does a better job: http://www.grimmjeeper.com/gears.html
It says your rpm at 65 will be 2498.

Of course that's not factoring in your speedo being off or your tires not being exactly factory specs.

 

OK, and that would be assuming 4.10s, yes?

 

Yes, 4.10s. If your speedo is like mine, it over estimates speed, so you may not really be doing 65mph at 2350 RPMs.

 

THanks! THat helps

 

That would only be true in 4th gear (1:1). That ratio is the overall ratio between the engine and the wheels, not the differential ratio. You need to multiply the differential ratio by the transmission gear ratio. 5th gear is 0.805:1, so the overall ratio between the engine and wheels will be 4.1*0.805:1, which is 3.3:1. Now look THAT up in your chart, and you get the result you were expecting.