When I dropped my new motor in I had the pcm programed for 4:10. I had a problem with the 4:10 so I now have 3:42 back in there again. If anyone can tell me how many MPH this throws my speedo of, it would be greatly appreciated. It seems like it's off by about 20 or so mph.

 

I think mine was off by 10 mph or so when I went from 3.42's to 4.10's.

 

No, it won't be off by "10 mph" at every speed. It will be off a percentage wise.

Only way it would be off "10" is if you two were talking about the same speed.

You will be going 3.42/4.10-s of what your speedometer says... or else 83.4% of what your current uncorrected speedo says.

So, when it says 100 you are really only going 83.4 mph. Or 70, you're going 58.4 mph.

It's not a set amount you'll be off for every speed. Just thought I'd clear this up.

Ben T.

 

I knew it would be different depending on speed. The MPH you said sounds about right compared to what my MPH looks like. I know from driving on the autobauhn my mph will say 100 and my t-tops whistle like i'm going 120+. I think I need to either get the computer re done or put 4:10S in again when I get back to the states.

 

actually it would be just the opposite...4.10/3.42=119%

100 would be 119 and 70 would be 83.

 

No, you're wrong.

I did a gear swap and I know what my speedometer said before and I know what it said after I fixed it

With the speedometer unadjusted in drive once you're in top gear, speed will be a fuction of rpm. It's linear. The faster you go the more your engine's speed increases. That's simple to understand.

With that said, when you're going 70mph, you engine speed will be whatever it is... if it's 1800... then, 1800 rpm.

Now, what do higher numerical gears do to rpm, they make it increase... So, rpm increases. Now, we didn't make any sort of adjustment to the speedometer, so the function is still the same between mph and rpm. So, after your gear swap your speedo will still say you're doing 70 mph when you're in top gear and when rpm gets to 1800.

But we said that gears increase rpm, so in order for rpm to increase relatively... speed must decrease. So, now you're going slower than what your speedometer says you are doing.

Now multipy whatever speed you're doing by the percentage that your gears changed. Use old ratio/new ratio. In the original poster's case 3.42/4.10 (when I did it, 2.73/3.42). You want this number to be less than one when you gear down.

So the percieved speed times corrrection factor equals your new land speed.

That's really all that there is to it. Now, I might know a lot of things automotively, but I am 100% correct on this.

Ask the original poster, I bet he'd tell you it's really cool to have someone ride along. He can hit the gas and his speedo says he's flying! I am deducting this from what he said.

This means his speedo is TOO HIGH... to make it right you want to multipy by a number less than one (or number less than 100%).

Marshal, does this make sense?

Ben T.

 

STudy Time:

Go back and read what "Guerardsgt" wrote in the original post. His PCM is already programmed for 4.10's. Now he has gone back to 3.42's, but he still has 4.10 programmed into the PCM. His speedo will read LOWER than actual speed. When the speedo says he is going 100mph, he will actually be going 119.9.

With the lower numerical rear axle, the driveshaft is only turning 3.42 times for every tire revolution. So, the PCM, expecting 4.10 DS revs for each tire rev is led to believe the car is moving slower than what it actually is.

 

Could we call this a miscommunication?

I WAS WRONG... Marshal, thanks for trying to point that out.

As everyone knows by now, I interpeted his post to say he had gone to 4.10's. Most people here post about going the other way. You're right.

Make sure you mulitply that speed by a number >1 to make it increase as Marshal said.

Marshal: sorry, I was wrong.

Ben T.

 

no problem

just try to read a little more thorough next time. nothing like typing a big post and it being wrong. well it was right just not for this guy...lol