Is This Normal

[RandomNerd2000]

Okay, so, I have a question about the RPM/MPH ratios in my 01, since I'm halfway concerned there's a problem arising. I'll start by saying the fluid looks decent and the receipt shows it changed in 2014, and it's had almost no miles since then. Basically, at 55, I run about 2300 RPMs, with a 4.0 SOHC, 31s, and 4:10 gears, and get over 20 MPG, however, past that, the MPG goes down, and by 70 I'm turning a good 3K RPM no problem, and getting 16 MPG. I don't normally go on the highways and actually go that fast, but I'm curious what people think because I'm pretty sure that isn't normal.

 

[96eb96]

What happens when you cancel overdrive? My 4.10s rev pretty high in the mid 60s (as fast as I got), but not 3K rpm in overdrive.

 

[RandomNerd2000]

It goes up to about 4K then, it's obvious OD is working when you switch back and forth

 

[RandomNerd2000]

I've also gone from 1st to 2nd to D and counted my shifts, it has all it's gears.

 

[1998Exp]

Okay, so, I have a question about the RPM/MPH ratios in my 01, since I'm halfway concerned there's a problem arising. I'll start by saying the fluid looks decent and the receipt shows it changed in 2014, and it's had almost no miles since then. Basically, at 55, I run about 2300 RPMs, with a 4.0 SOHC, 31s, and 4:10 gears, and get over 20 MPG, however, past that, the MPG goes down, and by 70 I'm turning a good 3K RPM no problem, and getting 16 MPG. I don't normally go on the highways and actually go that fast, but I'm curious what people think because I'm pretty sure that isn't normal.

Theoretically, your RPMs should be about 2000 at 55 and 2500 at 70. Are those 31's original to the vehicle? If it came with smaller tires and the speedometer wasn't re-calibrated, you may be going faster than the indicated speed, and perhaps much faster if there is something wrong with the speedo. I would suggest checking that first, perhaps with a GPS. Barring incorrect speedo reading, it sounds like torque converter slippage due to inoperative TCC (torque converter clutch), but this would almost certainly make the O/D light flash... Is it?

 

[RandomNerd2000]

Nope, the OD light isn't flashing, and it doesn't shift harsh. It came with 255/70R16s, it's been changed to 245/75R16s which are roughly 31s AFAIK. I probably should check to see how off it is, I hadn't thought about that at all, speed sensor could be bad, I know in my 99 the speedo got really off when that went bad. I'll check that and get back with the results

 

[RandomNerd2000]

Okay so the 3K RPM at 70 is perfectly accurate, tried several apps and the live data too, so that's a problem. The thing I noticed is, wouldn't the speedo be wrong with the taller tires? What are the odds it's had the gearing changed? I've given thought to finding a good front 3:73 diff (I have a rear) and just putting something under it I have experience with, but that'd hurt my acceleration in theory, and if it's not a gearing problem, the work would be a waste.

 

[1998Exp]

The tire circumference difference between 255/70R16 and 245/75R16 is about 2%, so that's not your problem. As I said earlier, it would be wise to check your speedometer, possibly against a GPS. If both the speedo and the tachometer are accurate, either your transmission is not in overdrive, or the torque converter is slipping.
Sadly, I could not figure out what you mean by 'finding a good front 3.73 diff'. If you meant installing a front differential in the rear axle, that won't work -- for more reasons than one.

 

[natenkiki2004]

It came with 255/70R16s, it's been changed to 245/75R16s which are roughly 31s AFAIK.

Just to throw this info in; 255/70R16 is 30.1" and 245/75R16 is 30.5".

 

[Prince_Polaris]

This is a dumb question, but are you going uphill or not? Going down the mountains to college at 80 in my Sport SOHC my RPMs are pretty low, but going back up the mountains to get home it's easily around 3k or higher, and that's with overdrive off because I can't get up those grades with overdrive on.. at least not at 80 (or the speed limit: 70)

 

[RandomNerd2000]

So I've done some testing, and I have the results. With OD off, I count 5 shifts as I get progressively faster, and at 55, which was a general testing speed since that was the speed I could repeat 5 shifts, I turned the OD on, and the RPMs dropped maybe 150 at best, and it could be felt but if you weren't paying attention you'd miss it, and the fluid doesn't look burnt, so now what? I'm pretty well lost on it.

@natenkiki2004 That's worth knowing, thanks for that, I know they're tight inside the fender wells, which this car does have the body molding, and I have a set of 18s with 31s coming so that's gonna be a squeeze. Also, for the ones asking, yeah I've done actual research on putting aftermarket lug centric wheels on these things.