huey_33

Haven't really drove my car on the highway since I had the 4.10's installed until today. Got on the highway and noticed my car was at 3000 rpm's while cruising at 67 mph. Is that normal or is that too high? Also have to add that I have yet to figure out how to install my speedcal without flipping 7 CEL's so my speedometer was sitting around 78 mph. Had my GPS on so I knew how fast I was actually going. Will my RPM's go down once my speedometer reads the correct speed or is it just the nature of the beast for having 4.10's installed? I knew my RPM's were going to be high but didn't think they would be this high. Thanks.

cliffyk

3k rpm in 5th with a T-45 (you do have a 5-speed?) and 4.10s should be up around 80mph.

If the rear tires are standard outside diameter (25" or so), then you'd have to have 4.90 (actually 4.88) gears to only be going 67 in 5th gear.

Here's my How Fast Can I Go in each Gear calculator.

Have the gauge faces been changed, or any thing else done that might have messed up the gauge needles?

WannaBeGearHead

What?

You're saying: 3k rpm in 5th with a T-45 and 4.10 gears would put him around 80mph BUT him seeing 67mph at 3k rpm in 5th with a T-45 would give him 4.88 gears?

Is that correct?

cliffyk

The 67 mph is, at this time, only what the speedometer says--it may be accurate in which case the rear-end gears have to be 4.88:1 (assuming stock size tires), or it may be inaccurate and he has 4.10 gears--at this point we do not know the truth of either possibility.

If the speedometer and tach are correct, a rather large assumption, to only be going 67mph, in 5th with a T-45, you would have to have 4.88:1 gears.

If he had 4.10s, normal size tires, and an accurate speedometer and tachometer, then 3k RPM would be around 80--actual vehicle speed, not necessarily what the speedometer might say.

So either the speedometer and/or tach are inaccurate, or gears are not 4.10s--again, at this time we do not know either of those facts.

However the tach is likely accurate, so by checking the actual vehicle speed in 4th gear (a 1:1 ratio), at "x" MPH (via GPS, pace car, or timing a measured mile), and recording the engine RPMs at that speed, and tire size--the rear-end ratio can be calculated with reasonable accuracy--the math is trivial...

WannaBeGearHead

So just to clarify what you're saying...Assuming both are T-45s, both in 5th gear, both going 3k rpms, one has 4.10 gears going 80mph and the other one has 4.88 gears going 67mph?

If he is going 80mph in 5th gear with 4.10 gears at 3k rpm wouldn't he be at a higher rpm with 4.88 gears all else being equal?

cliffyk

Yes, but all else is not equal--forget the speedometer readings for this exercise and concentrate on actual vehicle speed (which again we do not know at this time).

You are comparing 80 with 4.10s, to 67 with 4.88s (both assumed to be actual vehicle speeds).

With 4.88s at 80 the engine would be spinning 3500+ rpm.

You can map it all out here...

huey_33

Sorry it took so long to reply, I was at practice. It's an automatic and I did change the gauge faces a while ago but everything was reading correct before the 4.10's were installed. And I did go up in tire size over a year ago. I went from stock 17's to 18's. Like stated before, the speedometer reads 78-80 mph when the RPM's are at 3000 but I had my GPS on and it was saying I was doing 67 mph. If my speedcal got installed correctly and fixed the speedometer, would that make everything read correctly including RPM's?

BraMas

Your RPM's are correct no matter what your Speed or speedcal are set at. They have Nothing to do with each other. Your Tach is getting it's info from the ECM.

cliffyk

What size are the tires?

I cannot come up with any combination of the auto transmission and 4.10 gears that should result in an actual vehicle speed of 67mph, at 3k rpm, in any gear.

3 grand in 3rd with the auto and 4.10s should be around 54mph, 3k rpm in 4th should be near 77 mph (actual speed). Is the SpeedCal installed at all, or completely disconnected?

Let me know the tire size (complete size spec, not just the rim diameter), and drive the car locked into 3rd gear (overdrive disengaged) at 45 to 50 mph, actual mph via the GPS, and note the actual speed and engine rpm.

Post all that back here and we can do some math...

huey_33

Tires are 245 40R18 and are approximately 25.8 inches in diameter. Can't really do the overdrive thing cause my overdrive button wires ripped off the button when I changed shifter ***** a while back. I tried installing the speedcal again the other day but just like the last time, I get it all wired up and the car throws 7 CEL's so I took it off and it's now sitting in the backseat of my car. So I have no idea what else I can do. I think it's just gonna end up with me living with high RPM's.