Some where i think i saw a chart that converted rpms into mph
my speedo is reading high, i have the wrong reducer on the transfer case. Id like to know how fast im going using my tach

5 speed 4:10 gears

i think its reading 80 mph @ 2,000 rpm if i remember correctly

btw true 80mph is the absolute maximun speed my truck is capable of

 

The difference must be the rear end, I get 90 or 91 with the 354s and rag. I would just buy a cheap gps unit and no more guessing on your mph. You could also borrow one and note the speed a various rpms and make your own chart...Mark

 

With 4.10's and stock size 235 85 16's 80 mph is 2675 rpms. 2000 rpm's is 60mph.

 

I have 245 75 16's a little wide but i thought the 75 were stock
what do you do at 1500 rpm ?

 

1500 in 5th is 45 mph.

 

Is the speed taken off the stock speedo? If so all those numbers are relative as there is a factory underspeed built into speedos. This is for liability reasons, they want to see you going faster than you are. The higher the speed the greater the variance. One speedo is different than the next. Mark

 

I'm using a on line gear ratio calculator chart for these. On my 93 the speedo is within 1 mph with the stock size tires. Checked by local police radar boards. I have never checked my truck rpm's against these figures but they are pretty close. Of coarse these only work with a lock up tranny or standard.

 

This is where I'm confused about my truck running 235/85/16s. I know I have a 3.55 (or 3.54 whatever) rear end, I've counted rotations. In overdrive (6th) which is a .73 ratio, my tach shows mine turning 2400RPM at 75mph (GPS verified). My tach has a little sensor that straps to the alternator so I can't imagine it being highly accurate. Although, here is a possibility that my tach is off in the higher RPM range which was noticed when I dyno'd last August. When idling, it matched perfect though.

 

At what speed? 1 mph @ 35 can be 6 or 7 at 70mph...Mark

 

I understand the percentage deal. As I say, the figures I stated above are mathmatical facts that have nothing to do with my speedo. I have checked my speedo from 25 to 55 on radar boards and its real close in that range. I also have a GPS.

 

Ok, so you calculated the speed on line, then you confirmed that they are correct with your gps? Not arguing, just trying to understand...Mark

 

I did not confirm. Just giving some ball park figures. I now have bigger tires so that throws off the speedo.

 

Today's semi-worthless trivia information:
Those radar highway display signs never correspond with my GPS or my speedometer. I have noticed that the further from the edge of the road that the radar signs are posted, the lower they will read in comparison to my GPS. Maybe this is because they are reading the long side of a triangle instead of the closest side where the truck is traveling. Likely the same reason that a passing train looks like it's going faster while standing next to it instead of viewing it from a 100 feet away.

 

I changer the little gear box on the speed sensor to what I think is the right one, marked 1.0400 down from 1.0200 and if all is working right it looks like 2000 rpm is about 67 mph for me. My tires are 245 75r 16

 

just get a garman gps. it shows your speed exactly. no guessing or conversion and the map is a benefit