rpms with gear and tire changes

[ivrb]

What rpms are normal with a 4.0 ohv auto tranny 4.10 gears and 32 in tires?
mainly on highway or freeway speeds around 65 mph.

 

[imp]

What rpms are normal with a 4.0 ohv auto tranny 4.10 gears and 32 in tires?
mainly on highway or freeway speeds around 65 mph.

Here's how to figger it out: 1. Find circumference of tires: 32" diam. X pi turns out to be 100.53", or 8.3776 feet. The tire makes 5280 divided by 8.3776 revolutions per mile, or 630 revolutions per mile.

2. Multiply that 630 turns per mile by the axle ratio, 4.10, and you get 2584 as the number of turns the driveshaft (and the engine, if torque converter clutch is locked) make per mile, assuming direct drive, no overdrive gear in use.

So, at 60 mph, the engine is turning 2584 rpm. (without overdrive gear).

WITH overdrive gear, multiply 2584 by the O/D gear ratio (mebbe about 0.8), so with O/D engaged, about 2067 rpm at 60 mph. imp


EDIT: BTW, this all works out at 60 mph, because that is the same as 1 mile per minute, and we want the engine speed in revs per minute.

 

[4x4junkie]

The 4.0L OHV seems happiest around 2500-2600 RPM at 60MPH. 4.10 gears with 32s will likely put you a little under that. I'd say go with 4.56 gears (A4LD overdrive is 0.75:1 IIRC, a little taller than the M5OD manual).

 

[ivrb]

well then I have to correct my speedo, going 65 reads 2500 and I do have an overdrive, its the 5r55e tranny

 

[4x4junkie]

5R55E = same OD ratio (0.75:1)

Yeah you'll need to correct the speedo for the bigger tires (on a '00 I believe speedo adjustments are all done through the PCM)

 

[imp]

well then I have to correct my speedo, going 65 reads 2500 and I do have an overdrive, its the 5r55e tranny

OK, here's where I F. U.! Newer vehicles get their speedo input from the ABS sensors, which read axle speed, not driveshaft speed, as in the old days. The figures above were based on the speedo. reading driveshaft speed, like back in the days of the speedometer cable!

What that means is this: changing gear ratios does NOT change the speedometer calibration, IF the tires remain the same. If we say the old tires were, maybe 235-70-16, they were about 29 inches diameter. The error from the bigger tires is 32 less 29 divided by 29, or about 10%. What that means is that when your speed. now reads 65 mph, the truck is actually going about 72 mph; this sets you up for a "kill" by the radar-cop, unless you are careful to always mentally add 10% to the speedo. reading, or recalibrate the speedo. to read 10% higher. imp

Edit: Hope this all makes sense; I tripped on my tongue.......

 

[ivrb]

well thats the problem about having to recal the car, but how or what options?

 

[nugs]

dont most power programs allow the operator to change stuff like that?

 

[ivrb]

don't know was told dealer

 

[specialized7]

My sport revs at 2500 @120 km/hr with 4.10's and 33's