The calculators under 'Articles-Specs', sub 'Automotive Calculators', match the results from the Novak calculator I posted above. It's a known forumula, and the calculator applies the variables. Math being what it is, not much room for error.

Your 2500 at 70 is not far off--for 1:1 third gear. After 3 shifts, you should be in fourth at .67. Something is wrong in your example, but it is not the formula or calculators. It seems a mystery, given your checks of the components. How again did you certify the speedo? Was there a gear change?

 

So, to summerize what your situation is:

Your truck is a 300 six (?)
You have a C 6.
You run 235-75s which are 29 inches in diameter.
You turn 3000 rpm at 70 mph.

As per previous post and the calculator, you are running approx 3.7 gears, probably 3.73s in an 8.8. (the one with the cover on the back, right?)

You can swap to anything around 2.79-3.0 or even 3.08 and make enough of a difference to be worthwhile. You will lose some off-line performance, but the six developes the torque low enough that you'll still be acceptable.

The other alternative is an AOD trans from a V8 car or truck or 6 truck from '88 to '93.

Good luck.

 

The speedometer gear was updated when I changed the tire size. It's been verified using GPS and police radar at several speeds. It's dead on. I'll put a different tach in tomorrow or Thursday...I've done enough cluster swaps at this point that its a ten minute drill. Like I said earlier, thinking about it, the truck doesn't seem like it's running at 2500. It just runs so well that I never gave it a second thought. It certainly isn't straining at 70. It will run much faster then that.

 

I have a 302,AOD,3.55s, and 255-60-15s and I run 2300 at 70.

 

Tires should be 27", and if so:

Calculator shows 3092 at 70 in 1:1 third,
2072 rpm in .67 fourth.

Don't know why your numbers are so far off. 3.91s get you 2282 rpms at 70 with all the rest.....

Beats me......

 

I'm sure that mine is a 4R70W with overdrive and 3.55 and it turns barely over 2,000 RPM at 70 MPH with 255 70 15 tires.

Are you sure that your gears havent been changed to maybe something like 4.10s?

I had 32 inch tires on my truck for a while and it would get about 12 mpg and turned less than 2,000 rpm at 70.

 

This sounds about right.

 

This is interesting though. I used to have a sunpro tach and it read about 1900 at 70. The Lightning instrument cluster I have reads about 2100 at 70. Why does the aftermarket tach read lower or whe OEM tach read higher? I don't know which is the case.

A friend of mine has a nearly identicle truck with a factory tach cluster and his runs about 2100 at 70 as well.

 

Is it possible the the older AOD doesn't have a TC lock up? The larger tires should lower the RPMs.

 

It might sound right, but it doesn't compute out.

 

I don't know how it doesn't compute out. I have 255 70 15 tires and it turns just over 2000 about 2100 at 70.

What calculator are you using?

http://www.1010tires.com/tiresizecalculator.asp

235 75 15 is 28.87 tall
255 60 15 is 27.04 tall
255 70 15 is 29.05 tall

 

If you look at this chart you'll see that 3.55 with a 29 inch tire is around 2674 rpm at 65 mph. This is with a 1:1 ratio not counting overdrive. If you factor in overdrive it would drop to just under 2000 wouldn't it? So at 5 mph faster its up a couple hundred rpm just over 2000. Still its close to what it should be.

With a 27 inch tire and 3.55 in 3rd it would be 2872 rpm at 65 without overdrive factored in.

http://www.4lo.com/calc/geartable.htm

 

If my little sunpro tach was more accurate then the OEM tach reads about 200 rpm high.

 

The Novak calculator (not a table, so it allows different variables) agrees with this number--at 65 mph.

The post you commented on says:

<TABLE width="100%" border=0 itxtvisited="1"><TBODY itxtvisited="1"><TR itxtvisited="1"><TD vAlign=top itxtvisited="1">"I have a 302,AOD,3.55s, and 255-60-15s and I run 2300 at 70." (post 22)

The calculator says and I posted it in post 23:

"Tires should be 27", and if so:

Calculator shows 3092 at 70 in 1:1 third,
2072 rpm in .67 fourth. "

So how can his claim to be running 2300 at 70 be "about right"?

You also mentioned 29 inch tires. Running them through the calculator with 3.55s and 1:1 third shows 2674 at 65 as you posted. With .67 fourth it would be 1791 at 65.

Based on the comparisons of the charts you posted and the various calculators, I take the charts and calculators to be accurate. Tire diameter from charts not so accurate, and if you try to figure the loaded radius of the tire based on tire pressure and load, you'll have a 2 inch + variance easily.

Tachometers are another one. I have one of those mini jobs and you can't really read it within what, 100rpm, especially under driving conditions. Speedos are always a wild card--even with the correct gear, the speedo head can be variable and/or just plain wrong.

Gears: Gears get changed.

If you are trying to solve this problem for a particular vehicle, you just need to use the KNOWN information first. The calculators and charts are not up for arguement. It is what it is, math does not change depending on brand etc. Gears are set, trans ratios the same. Torque converters slip, tachs find the general neighborhood sometimes and speedos are going from lane to lane a times. If it isn't what it "should be", if you want to find the answer, you have to check your data, not the calculators, at least not in this case.


</TD></TR></TBODY></TABLE>

 

I think that the OEM tachs read a couple hundred RPM high. I just figured that if he was running about 2300 and I was running about 2100 with slightly taller tires it makes sense.