Fj62 RPM - Speed - Gearing

[FJ Silver]

Hey Guys,

When I am doing around 2250 RPM I'm at around 65mph in lock up overdrive.
I'm driving a 1990 FJ62 with 31' tires.

Am I driving with stock gearing or did the PO put something else in?

Thank you,
Gerben

 

[robustbambi]

seems like the stock 4.11s to me.

 

[klinetime574]

Using a gear calculator set up for stock FJ62 with 31" tires I found at 2250rpm you should be doing 69.7mph

There is some difference there. But if I change to 4.56 gears or 3.7 gears it is much further from 65 at 2250rpm

I'd assume that factory speedos and tachs just can't be as precise as an excel spreadsheet with numbers.

I'd say you have stock gears.

You can also count the revolutions of the driveshaft and tire I believe. I forget how that works. Someone state it correctly!

Hope that helps!

 

[Benjaman]

When I had 31's on my 62, the speed (by GPS) was about 4 mph higher than indicated.

 

[Randy88FJ62]

Someone correct me if I am wrong but weren't the stock tires for these trucks 29"? This would mean his speedometer is 6.5% off. This means when his speedometer shows 65 mph he is actually going 60.1 mph. Someone check my math but I think I got it right.

 

[elephantsCBR]

Someone correct me if I am wrong but weren't the stock tires for these trucks 29"? This would mean his speedometer is 6.5% off. This means when his speedometer shows 65 mph he is actually going 60.1 mph. Someone check my math but I think I got it right.

I believe your meaning be reversed. One normal rotation of the tire travels x". With larger tires. There is a little bit more rubber to travel over so if it's showing 65. He would actually be doing 68 or what have you. The same revolutions make the vehicle travel further down the road because of the larger circumference.

 

[Randy88FJ62]

I believe your meaning be reversed. One normal rotation of the tire travels x". With larger tires. There is a little bit more rubber to travel over so if it's showing 65. He would actually be doing 68 or what have you. The same revolutions make the vehicle travel further down the road because of the larger circumference.

Guess I had a dyslexic moment.

 

[DFXR]

RPMs are RPMs but since you have larger than stock tires (31s) your speedo is off. You are actually going faster than you think by around 4.5MPH (at 65MPH on the speedo), which confirms what klinetimes calculated and benjaman observed.

The way I have figured out speedo correction is to use as your main factor the percentage difference between stock tires and what you're actually running. I have 31's and stock was 29. That is 2 inches, about a 7% difference. So when my speedo says 55MPH I am actually going 58.5MPH. Using a GPS has confirmed this.

Things to note are that actual tire size may differ from real tire size, so measure if it really matters to you (some 31s are not actually 31 for example), and that your odometer and trip meter will be under-reporting as well. The good news is that you may be getting slightly better gas mileage than you think.

 

[62LostInMn]

I thought that the stock tire size was a 225/75 15? That's what I get if I plug the vehicle into the Goodyear tire finder thingy. Plugged that into the size conversion calculator I get a diameter of 28.3". That's dang small. So 31's are about 9% bigger than stock. When I bought my cruiser it had 235's on it and I thought that was stock (and too small).

 

[FJ Silver]

Hmm thanks allot guys.

I haven't owned this 62 for that long and have to say am pleasantly surprised
by how well accelerates and it's drivability around 75mph. well except going up hill.

I just put some new 31-10.5-15 BFG AT on there which took care of almost all the wandering and other handling problems, as well as the 65mph crazy vibration.

Having so much fun working on this truck!

thanks again,
Gerben

 

[Mark W]

Not rocket science. More like rock simple.

rpm, times final gear ratio (1.17 in the case of the '62 in OD) divided by gear ratio, times tire radius x2 (Or just tire diameter) times 3.14159... this give you your speed in inches per minute... divide by 12 for feet per minute, times 60 and then divide by 5280 for miles per hour.

No need for any magic spread sheet.

OEM tires were 225s.... your speedo should be showing about 60mph at that rpm and you should be doing about 65. I am guessing that you are quoting a speed from a GPS?

Your gears are stock.



Even simpler, jack up a tire, spin it twice and count the driveshaft revolutions.

Only gearing for a Cruiser is 3.70, 4.11 (4.10 front diff in an '80), 4.56, 4.88. 5.29 to 1. Pretty hard to mistake any of these for any other.


Mark...

 

[igniter3]

I am using 32x11.5x15 tires on my fj40
on free way my speedometer show 60mph
I am actually traveling 70mph, my speedometer
indicate 63mph, I actually traveling at 75mph.