Rear end ratio

[thetex]

May be a dumb question but I have an 82 Fiat spider 5 speed that runs 4,000 rpms at 70 mph. Have there been any changes in recent years for a better highway ring/pinion gear ratio? Thanks, Tom

This may not be clear- I want to know if you can buy a ring/pinion gear set that will not run such high rpms on the highway.

[Daniel]

if it's the stock/original ring and pinion then you have 390s to lower the ratio you want to find 358s for the automatic
rear end.

[So Cal Mark]

there is nothing wrong with 4k at 70mph

[FiatMac]

May be a dumb question but I have an 82 Fiat spider 5 speed that runs 4,000 rpms at 70 mph. Have there been any changes in recent years for a better highway ring/pinion gear ratio? Thanks, Tom

This may not be clear- I want to know if you can buy a ring/pinion gear set that will not run such high rpms on the highway.

A search of this topic using the Search Function will show that this question has been raised many times. We really need to have a Frequently Asked Questions section for the site.

I set up an Excel Spreadsheet calculating the engine speed vs. car speed for the various rear end ratios supplied (with the exception of the 4.10 which is apparently very rare to find). The charts below are the results for my '82 Spider running 195 60 tires. A 185 60 tire would yield a 98% lower speed in each gear for the same rpm. Also, in my experience speedometers typically register 2-3 miles/hr faster than actual speed.

I screwed up the image sizing so you need to right click on the image and then click "view image" to see the whole chart





Thetex,
The '82 Spider was originally supplied with a 3.9 rear ratio, but it appears that a previous owner installed a 4.3 ratio rear end in your car. From the charts above a 4.3 rear end would give a speed of 73 mph at 4000 rpm, whereas the standard 3.9 rear end would give a speed of 80.4 mph. If it is indeed an older style 4.3 rear end, it will not have the removable cover on the back of the differential that the 3.9 rear end had. Instead the front section of the differential case (pumpkin) is removable. Vick Autosports has a new 3.9 pumpkin that can be installed in the older rear end. The easiest option might be to source a complete used axle or pumpkin with the desired ratio.

If you do a search using the site search feature on "rear end ratio speed" you will find a lot of information.

[supplyguy]

So if I get a 3:58 an redline it in 5th I can hit 160mph? Where do I sign up? Just kidding - enjoyed your work!

[FiatMac]

So if I get a 3:58 an redline it in 5th I can hit 160mph? Where do I sign up? Just kidding - enjoyed your work!

Yes, after I posted these charts I thought I should have limited the top gears to achievable speeds. I don't think I would even want to drive one of these at that speed! Glad you enjoyed it.

[fiat218]

So if I get a 3:58 an redline it in 5th I can hit 160mph? Where do I sign up? Just kidding - enjoyed your work!

Yes, after I posted these charts I thought I should have limited the top gears to achievable speeds. I don't think I would even want to drive one of these at that speed! Glad you enjoyed it.

I would, I like to see what the top speeds are, not fast let me tell u ,lol. 105 in the 72 and 92 in my 80, I believe something not working right in the 80, gotta go faster then 92 for a 2liter! But compressions are low, but still a broken down engine can hit 100. Wth

[thetex]

Thanks for all the helpful info. Tom

[narfire]

I would, I like to see what the top speeds are, not fast let me tell u ,lol. 105 in the 72 and 92 in my 80, I believe something not working right in the 80, gotta go faster then 92 for a 2liter! But compressions are low, but still a broken down engine can hit 100. Wth

Just kinda thinking here but the 80 carb'd had a horsepower rating (stock)of around 85 I thought, the 72 with a 1608 was rated around 105 hp? that 20 hp difference is the 15 mph difference. Do those bolt on upgrades and then see what the top speed and acceleration is like

[fiat218]

I would, I like to see what the top speeds are, not fast let me tell u ,lol. 105 in the 72 and 92 in my 80, I believe something not working right in the 80, gotta go faster then 92 for a 2liter! But compressions are low, but still a broken down engine can hit 100. Wth

Just kinda thinking here but the 80 carb'd had a horsepower rating (stock)of around 85 I thought, the 72 with a 1608 was rated around 105 hp? that 20 hp difference is the 15 mph difference. Do those bolt on upgrades and then see what the top speed and acceleration is like

2liter in both and to top that off, the 79 I got suppose to have a 2liter with a mild cam ans dual carbs, well it was not running right and I did not get to 90 lol it was missing, got a new unit coming from so cal mark , so we see what happens after that

[FiatMac]

May be a dumb question but I have an 82 Fiat spider 5 speed that runs 4,000 rpms at 70 mph. Have there been any changes in recent years for a better highway ring/pinion gear ratio? Thanks, Tom

Tom,
I just had a thought last night. Have you checked your actual speed with a gps unit, with another car, or by timing between mileposts? It occurred to me that if the previous owner replaced the transmission with one from a car that had a 4.3 rear end you may be running at 80 mph when the speedometer reads 70.

Others with experience can chime in here, but I believe the speedometer output from the transmission is geared to match the speedometer to the rear end ratio that is in the car. If the output unit on the car is not replaced with one that is for the rear end in the car then the speedometer will be off.

It may be difficult or impossible to replace the transmission speedometer output with the transmission in the car. Not much room to access it.

[RRoller123]

Just an FYI: my 80 FI runs almost exactly 10% slower than the speedometer reading, measured with radar. So when I think I am going 50, it is actually going about 45 mph.

[fiat218]

I check all my cars with gps, my 80 is actually 3 mile slower then what it says that I'm going, so that great

[RRoller123]

Is it uniform across all speed ranges? As an example, mine is 10% off, not a specific amount all the time, e.g. so when it says 35mph, I am actually going about 32, when it says 60, I am actually going about 54 mph. (Maybe the rear end was changed somewhere along the line?)

Pete

[baltobernie]

Most mechanical speedometers "back in the day" were slightly optimistic. Winter tires were popular in cold regions, and they were often slightly larger. Are you running a stock tire size? The 10% you're seeing is not unusual. My '73 is about the same. Ironically, the odometer did not have the fudge-factor built-in, and mine is spot-on, less than 0.1 miles over 15.