rpm+gear = wrong speed?
Thread Starter
Staging Lane
Joined: Jun 2014
Posts: 98
Likes: 0
From: brainerd mn
rpm+gear = wrong speed? kinda the right sport but this is for my truck 82 chev with a built 383 doing a gear ratio calculator and in 2nd gear at 6k i should be doing 111mph
38in tires 4:10 gears 1st 2.48 2nd 1.48 3rd 1.00 3k stall
i know i was in 2nd gear tonight and gps said i was doing 65ish but thats what 1st should be doing at 6k not 2nd this is the calculator im using
http://www.planet4x4.net/forums/calculator.php
thanks for any help
38in tires 4:10 gears 1st 2.48 2nd 1.48 3rd 1.00 3k stall
i know i was in 2nd gear tonight and gps said i was doing 65ish but thats what 1st should be doing at 6k not 2nd this is the calculator im using
http://www.planet4x4.net/forums/calculator.php
thanks for any help
Moderator
Joined: Aug 2010
Posts: 7,323
Likes: 529
From: Ann Arbor, MI
I tried another calculator and got the same results as you.
So, one of your assumptions has to be wrong:
1. 4L80E with 2.48 and 1.48 gear ratios
2. 4.11 rear end
3. You know what gear you are in
4. Your tachometer is accurate
5. Your truck doesn't have a "Low range" underdrive unit
You would not be the first person here who thought they had one rear ratio but turned out to have a completely different ratio. You can easily test this with the wheels off the ground and the trans in neutral. Mark a tire and count how many driveshaft turns it takes to turn the tire one revolution.
Have you considered the tach accuracy?
So, one of your assumptions has to be wrong:
1. 4L80E with 2.48 and 1.48 gear ratios
2. 4.11 rear end
3. You know what gear you are in
4. Your tachometer is accurate
5. Your truck doesn't have a "Low range" underdrive unit
You would not be the first person here who thought they had one rear ratio but turned out to have a completely different ratio. You can easily test this with the wheels off the ground and the trans in neutral. Mark a tire and count how many driveshaft turns it takes to turn the tire one revolution.
Have you considered the tach accuracy?
Thread Starter
Staging Lane
Joined: Jun 2014
Posts: 98
Likes: 0
From: brainerd mn
the rear end gears say 4:10 right on them iv had the pumpkin cover off and looked its a th400 not 4l80e but same gears i know i was in 2nd put it there manually and i dont have 3rd it slips out i dont have an under drive unit i put this truck together my self tcase wasnt in 4 lo i tested that the next day just to make absolute sure even tho i knew were 4 lo was its not the highest quality tach but i couldnt see it being off more then a couple hundred rpms i guess is there any other way to check rpm exept with a different tach
otherthing i was talking about with a buddy tonight since i have a 3k stall it just doesnt lock up and has that much slip?
otherthing i was talking about with a buddy tonight since i have a 3k stall it just doesnt lock up and has that much slip?
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Staging Lane
Joined: Sep 2012
Posts: 53
Likes: 0
I know a small difference in tire size won't make a huge difference that's why I said it could be some of it. Some tires are way worse then 1" some "40's" are actually closer to 37" when measured.
Thread Starter
Staging Lane
Joined: Jun 2014
Posts: 98
Likes: 0
From: brainerd mn
like tonight went for a long drive to warm up my truck every time i tested it at 3400-3500 in 2nd i was doing 45 mph but after the drive and using reverse more i may be thinking that it may be sliping a lil cuz i noticed reverse slipping a lil bit
