[Carbo]

I recently installed my new TKO 600RR (.82) into my 69 camaro. I have a 3.42 posi with 275x40x18's. According to the RPM calculator I should have been turning 2299 rpms at 65 mph. Now I realize the calcualtors aren't 100% accurate but they should be relatively close. My road test today at 65 showed I was turning 2750 rpms. That's 450 rpms more than I thought I was going to be turning over. Anybody else turning different rpms than expected after doing a TKO swap?

[JimM]

Math is math. If you do it right, it cannot be wrong.

But garbage in = garbage out.

Take her on the highway. time 10 mile markers at a steady fixed rpm. Figure your speed from the time to go 10 miles. Most likely your speedo is off, possibly your tach, maybe even both.

time for 10 miles will tell you exactly how fast you're going.

[onovakind67]

Make a paint mark on the tread of your rear tire and before it dries roll it down the street. Measure the distance between the paint marks and this will tell you how far you go for each revolution of the rear tires. Using this measurement and a little math you can calculate very accurately the rpm required to cover the distance in the time required. There could be cumulative errors in your speedo and tach.

[TMessick]

275/40R18 tyre is ~26.6" tall w/ ~757 revs per mile. At 65mph w/ 3.42's that would be ~2800 RPM..... in 4th. OR 2300 RPM in 5th....

Are you sure you were doing 65? Could be the wrong speedo gear like Jim mentions (and I'm gonna assume you dig grab 5th gear...)

[Steiner]

Best thing you can do is borrow someone's GPS to be sure of your speed.

[Carbo]

All very good suggestions. When I purchased the TKO they sent me the new speedo gear for a 3.42 rearend (the speedo gear was black in color). So I'm assuming that it's the correct one.

As for the GPS, I just thought about that today. I have one from work that I'm going to try out tomorrow. I do have a sneaky suspicion that my speedometer is off but I will let you know.

[JimM]

Originally Posted by Carbo

When I purchased the TKO they sent me the new speedo gear for a 3.42 rearend.

They sent me the "right gear" too. Then I checked my real speed, found the colors and part numbers on a mustang site, and went to a ford dealer for the right gear. Sorry to mention the f word, but well, it was originally a ford trans after all.

[Carbo]

Well you guys were right. After doing a road test with the GPS my speedo is wrong. Here are some of the results:

At 35 mph on my speedo, I was actually doing 42 mph.
At 65 mph on my speedo, I was actually doing 77 mph.

Now I know the GPS is accurate because I have used it in three different vehicles. I'll be calling amp performance tomorrow to get a new speedo gear for it. To give you an idea of the rpms though, at 80 mph I was around 2800 rpm. Pretty happy with that. Thanks for all the help and suggestions.

[Carbo]

Just an update. I talked to AMP Performance and they told me they screwed up. They sent me a black gear when they should have given me the pink gear. I believe he said the black one has 22 teeth and the pink one has 19 teeth. They are sending out a yellow one as well just to be sure.

I don't know if any of you ever dealt with AMP Performance out of Phoenix, AZ but they are very good to deal with. I've had two problems that they have rectified immediately and the customer service is outstanding. I highly recommend these guys.

[yellow heap]

That is customer service

[Carbo]

Just to give an update. I installed a 17 tooth speedo gear (white) and after confirming with my GPS it is dead on, in low mph and high mph.

The interesting thing about this is that the calculation showed I should have had used an 18 tooth gear (yellow). Just letting everybody know in case you run into the same problem.

[Steiner]

Originally Posted by Carbo

Just to give an update. I installed a 17 tooth speedo gear (white) and after confirming with my GPS it is dead on, in low mph and high mph.

The interesting thing about this is that the calculation showed I should have had used an 18 tooth gear (yellow). Just letting everybody know in case you run into the same problem.

Yeah, the calculation can be slightly off because it uses the theoretical ride height of the tire on the rim, not the actual after installed (pressure variance, sidewall stiffness, actual tread depth, etc.)

[GMJim]

Originally Posted by Carbo

Just to give an update. I installed a 17 tooth speedo gear (white) and after confirming with my GPS it is dead on, in low mph and high mph.

The interesting thing about this is that the calculation showed I should have had used an 18 tooth gear (yellow). Just letting everybody know in case you run into the same problem.

I have found the best method to check a speedo is a GPS as Steiner says. a 40+ year old speedo might be accurate at higher speeds and not so accurate at lower speeds, or could be the other way around. There are a few places around the country that still calibrate these but I've found it's cheaper to buy a new one. The bench mark for calibrating speedos is 1000 cable rpm = 60 mph.

Back in the 60's the manufacturers couldn't make a high production speedo that was better than 10% accurate. Todays cars are within 5%. Tell that to the judge if you get a ticket. I worked for a GM dealer that had the contract to fix Highway patrol cars and calibrate their speedos. I once wrote letters to all of the major manufacturers to get something in writing to prove this but only got one reply from Volvo admitting the 5% accuracy. The others wrote back saying this was not consumer information?? Only Police cars get hand calibrated speedos.

[Steiner]

Originally Posted by GMJim

I have found the best method to check a speedo is a GPS as Steiner says. a 40+ year old speedo might be accurate at higher speeds and not so accurate at lower speeds, or could be the other way around. There are a few places around the country that still calibrate these but I've found it's cheaper to buy a new one. The bench mark for calibrating speedos is 1000 cable rpm = 60 mph.

Back in the 60's the manufacturers couldn't make a high production speedo that was better than 10% accurate. Todays cars are within 5%. Tell that to the judge if you get a ticket. I worked for a GM dealer that had the contract to fix Highway patrol cars and calibrate their speedos. I once wrote letters to all of the major manufacturers to get something in writing to prove this but only got one reply from Volvo admitting the 5% accuracy. The others wrote back saying this was not consumer information?? Only Police cars get hand calibrated speedos.

It's just so dang hard to get exact with a cable. With newer cars being a pwm signal it's easy. I changed out the rear end in my S10 and went from a 3.08 open to a 3.42 Torsen and different tires. Using an old Hypertech, I just had to select my gear ratio and it allowed me to change the tire diameter a quarter inch at the time to get dead on. A couple of runs down the road with a GPS was all it took....one to find the error and one more to verify my new tire size input using my error calculation was correct.

[Carbo]

[quote=GMJim;1194882]
Back in the 60's the manufacturers couldn't make a high production speedo that was better than 10% accurate. Todays cars are within 5%. Tell that to the judge if you get a ticket. quote]

That's why I carry the badge every where I go.

As I said earlier, one nice thing with this speedo is that I am right on with the GPS all through the mph range. When I was doing 20 mph, my speedo read 20. When I was at 50, my speedo read 50 and same thing at 80. So I'm pretty happy with that.

Intersting though, I've tried the GPS in other vehicles and in one, 2004 Bonneville, the Bonne is 2 mph faster all the way through the mph range than the GPS. But when I tried in my 99 Sierra and my wifes 07 Tahoe they are dead on with the GPS.