I recently put a lsd rear end on and I wanted to know if it changed my speedo reading or if that can happen? At 4,000rpm in 5th I do like 83-4 mph with a stock engine does this seem correct

 

no ot the lsd, but the gears attached to it might. And that seems close to what i read.

 

i agree it's probably the gearing. i might be wrong though.....

 

There are 2 gear ratios for the stock 1st gens. Both come lsd and non-lsd. The 2 ratios are so close to each other, it's easier to change the speedo readings with tires and wheels, for a noticeable difference, than it is to change the stock rear ends.

 

well then....i guess we have an answer

 

If we knew by how much they were off, we could know what to do to fix it. I am about to perform the same mod. Anybody happen to know this?

 

The stock gears on the 1st gens are 3.909 and 4.1, (this is found in the stickies at the top). If you do the math, i.e., divide each one by the other, both ways, the difference between the speedo reading will be less than 5 mph each way at 100 mph on the speedo. Lsd does not change the speedo, changing to a different gear ratios does. The only way to change the gearing is to switch between gs and se rear gears/rearends. (manual cars only)

On my 2nd gen I upgraded from 14 to 16" tires and wheels, also know as plus 2 sizing. My speedo is within 1%, all due to the tire choice. On my 1st gen I have 15x9's with 245/50/15 tires. I knew the speedo was off due to the combo. On my Christmas road trip I found out how much when I was was stopped and given a 'Holiday' warning for doing 87 in a 75. At the time I was holding the speedo at 78/79. (tach was in the high 3k range) Switching from my 3.909 gear to a 4.33 will bring my speedo back within 1%, if I keep the same aspect ratio on the tires.

If you know how far you are off, most of the good chain tire stores have charts on every tire they sell and can compare revs/mile. By adjusting the aspect ratio of the tire, you can adjust your speedo. The revs/mile is more accurate than the aspect ratio, because they vary between tire brands and models.

 

Trochoid- Can you go more in depth about the changing of the wheels/tire and how they change the speedo because I have been thinking of getting 15' (stock 13's) what sort of set up would I need?

Thanks

 

First, good luck finding 15" wheels. Second. what is it you want to do? Do you want the speedo to stay the same, which will keep the gearing the same? If that's the case, then you need a lower profile tire that has the same revs/mile as the stockers. i.e., if 2 different tire/rim combos make the same number of revolutions to cover a mile, they have the same diameter, and the gearing and speedo remain the same.

My brother-in-law used to manage a Discount Tire store and set me up with the plus 2 sizing on my 2nd gen. He explained all of this to me, unfortunately he has changed jobs and I no longer have access to his data base, nor his discount.

 

It's not the wheel size, it's the diameter of the tire that would effect the spedo. The bigger the tire, the farther you go every revolution. If you go to tirerack.com they have charts that tell you the outside diameter of their tires. If you put REALLY small tires on your car you could fool yourself into thinking you made your car a lot faster and improved your gas mileage at the same time.

 

1/2 right and 1/2 wrong. The effect of shorter or smaller diameter tires would act as a shorter gear, giving you quicker acceleration and limiting top end speed. It would also decrease gas milage due to running at a higher rpm at any given speed.

 

Yeah it is really hard to find wheels so I might just stick with the 13's. I have 185/13r/70 on my car right now not sure if thats stock size. I have been looking at new tires and want to go with a 195/13r/60 tire, the -10 on the profile would change the speedometer a little but how much should I expect? Or should I stick with the stock profile and avoid the potential mess?

 

Talk with your tire supplier, they can give you the exact difference it may or may not make.

 

my bro did this with his first gen...stole an lsd out of an 83 and got a ticket for speeding...lol...so in short...yeah...it CAN change the speedo...by about 7 mph is what my brother's was off....but its not the lsd like was aformentioned...its the gearing

 

Unless your bro was running near 100 mph, most of the difference was in the tire size, not the gears, period. The only way to make that kind of change to the speedo is to go from the SE manual gearset to an SE auto gearset, or any of the other ones. All of the aftermarket ones will make the speedo read higher than actual speed.

Another cause might be that the speedo cable needed lubing and was sticking a bit, but then he should have had a jumpy speedo needle.

 

bkm rx7 did your brother put an SE diff in and then run the 14 inch wheels, or other aftermarket rims, because that's the only way you'd get a 7 mph difference, unless he was just doing the old, blame the equipment deal....